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MED II-B GROUP 6 NOTES

Level II Block V Module 2 – URINARY SYSTEM

Case 2: “Happy Honey”

1. Recall the anatomic and histologic features of the urinary bladder.

Anatomy of the Urinary Bladder

It is situated immediately behind the pelvic bones within the pelvis. Receptacle for the storage of urine (adult-max capacity 500mL). It has a strong muscular wall. Its shape and relations vary according to the amount of urine that it contains.

Surfaces and Position of the Urinary Bladder

Adult-empty bladder lies entirely within the pelvis; as the bladder fills, its superior wall rises up into the hypogastric region

Young child-empty bladder projects above the pelvic inlet; later when the pelvic cavity enlarges, the bladder sinks into the pelvis to take up the adult position.

Empty bladder is pyramidal in shape (fig.7-5), having an apex, a base, and a superior and two inferolateral surfaces; it has also a neck.

MALES

Apex of the Bladder

It points anteriorly and lies behind the upper margin of the symphysis pubis. It is connected to the umbilicus by the median umbilical ligament (remains of

urachus).

Base or posterior surface of the Bladder

It faces posteriorly and is triangular. The superolateral angles are joined by the ureters, and the inferior angle gives rise

to the urethra. The two vasa deferentia lie side by side on the posterior surface of the bladder and

separate the seminal vesicles from each other. The upper part of the posterior surface of the bladder is covered by peritoneum,

which forms the anterior wall of the rectovesical pouch. The lower surface of the posterior surface is separated from the rectum by the vasa

deferentia, the seminal vesicles, and the rectovesical fascia.

Superior surface of the Bladder

It is covered with peritoneum and is related to coils of ileum and sigmoid colon. Along the lateral margins of the surface, the peritoneum is reflected onto the lateral

pelvic walls. As the bladder fills it becomes ovoid and the superior surface bulges upward into the

abdominal cavity. The peritoneal covering is peeled off the lower part of the anterior abdominal wall so

that the bladder comes into direct contact with the anterior abdominal wall.

Inferolateral Surfaces

They are related in front to the retropubic pad of fat and the pubic bones. More posteriorly, they lie in contact with the obturator internus muscle above and the

levator ani muscle below.

Neck of the Bladder

It lies inferiorly and rests on the upper surface of the prostate (fig 7-5). Here, the smooth muscle fibers of the bladder wall are continuous with those of the

prostate. The neck of the bladder is held in position by the puboprostatic ligaments (male) and

pubovesical ligaments (female). These ligaments are thickenings of the pelvic fascia.

FEMALES

Bladder lies at a lower level than in the male pelvis because of the absence of the prostate and the neck rests directly on the upper surface of the urogenital diaphragm.

Apex of the bladder

It lies behind the symphysis pubis.

Base or Posterior Surface

It is separated by the vagina from the rectum.

Superior Surface

It is related to the uterovesical pouch of peritoneum and to the body of the uterus.

Inferolateral sSurfaces

It is related in front to the retropubic pad of fat and to the pubic bones. It is more posteriorly they lie in contact with the obturator internus muscle above and

the levator ani muscle below.

Neck of the Bladder

It rests on the upper surface of the urogenital diaphragm. When the bladder fills, the posterior surface and neck remain more or less

unchanged in position, but the superior surface rises into the abdomen.

Structure of the Urinary Bladder

Is a smooth muscle chamber composed of two main parts: body - major part of the bladder in which urine collects neck - funnel -shaped extension of the body, passing inferiorly and anteriorly

into the urogenital triangle and connecting with the urethra The lower part of the bladder neck is also called the posterior urethra because of its

relation to the urethra. The smooth muscle of the bladder is called the detrusor muscle. Its muscle fibers extend in all directions and when contracted can increase the

pressure in the bladder to 40 to 60mmHg. Thus, contraction of the detrusor muscle is a major step in emptying the bladder.

The mucous membrane of the greater part of the empty bladder is thrown into folds that disappear when the bladder is full.

The area of mucous membrane covering the internal surface of the base of the bladder is referred to as the trigone.

Here, mucous membrane is always smooth, even when the viscus is empty (fig 7-6), because the mucous membrane over the trigone is firmly adherent to the underlying muscular coat.

The superior angles of the trigone correspond to the openings of the ureters, and the inferior angle to the internal urethral orifice (fig 7-6).

The ureters pierce the bladder wall obliquely, and this provides a valvelike action, which prevents a reverse flow of urine toward the kidneys as the bladder fills.

The trigone is limited above by a muscular ridge, which runs from the opening of one ureter to that of the other and is known as the interureteric ridge.

The uvula vesicae is a small elevation situated immediately behind the urethral orifice that is produced by the underlying median lobe of the prostate.

The muscular coat of the bladder is composed of a smooth muscle and is arranged as three layers of interlacing bundles known as the detrusor muscle. At the neck of the bladder, the circular component of the muscle coat is thickened to form the sphincter vesicae.

Vascular supply of the Urinary Bladder

Arteries

The superior and inferior vesical arteries which are branches of the internal iliac arteries supply the bladder.

Veins

The veins form the vesical venous plexus which communicates below with the prostatic plexus and it is drained into the internal iliac vein.

Innervation of the Urinary Bladder

The principal nerve supply of the bladder is by way of the pelvic nerves, which connect with the spinal cord through the sacral plexus, mainly connecting with cord segments S-2 and S-3.

Coursing through the pelvic nerves are both sensory nerve fibers and motor nerve fibers.

The sensory fibers detect the degree of stretch in the bladder wall. Stretch signals from the posterior urethra are especially strong and are mainly responsible for initiating the reflexes that cause bladder emptying.

The motor nerves transmitted in the pelvic nerves are parasympathetic fibers. These terminate on ganglion cells located in the wall of the bladder. Short

postganglionic nerves then innervate the detrusor muscle. In addition to the pelvic nerves, two other types of innervation are important in

bladder function. Most important are the skeletal motor fibers transmitted through the pudendal nerve

to the external bladder sphincter. These are somatic nerve fibers that innervate and control the voluntary skeletal

muscle of the sphincter. Also, the bladder receives sympathetic innervation from the sympathetic chain

through the hypogastric nerves, connecting mainly with the L-2 segment of the spinal cord.

These sympathetic fibers stimulate mainly the blood vessels and have little to do with bladder contraction.

Some sensory nerve fibers also pass by way of the sympathetic nerves and may be important in the sensation of fullness and, in some instances, pain.

(Snell’s Clinical Anatomy 7th Edition)

Histology

Layers of the Bladder Wall

It is essentially an organ for storing urine until the pressure becomes sufficient to induce the urge for micturition or voiding.

Its mucosa also acts as an osmotic barrier between the urine and the lamina propia. The mucosa of the bladder is arranged in numerous folds which disappear when the

bladder becomes distended with urine. During distention, the large, round, dome-shaped cells of the transitional epithelium

become stretched and change their morphology to become flattened. The accommodation of cell shape is performed by a unique feature of the transitional

epithelial cell plasmalemma which is composed of a mosaic of specialized, rigid,

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thickened regions, plaques, interspersed by normal cell membrane, interplaque regions.

When the bladder is empty, the plaque regions are folded into irregular, angular contours which disappear when the cell becomes stretched.

These rigid plaque regions, anchored to intracytoplasmic filaments resemble gap junctions, but this similarity is only superficial.

Plaques appear to be impermeable to water and salts; thus, these cells act as an osmotic barriers between the urine and the underlying lamina propia.

The superficial cells of the transitional epithelium are held together by desmosomes and possibly by the tight junctions which also aid in the establishment of the osmotic barrier by preventing the passage of fluid between the cells.

The triangular region of the bladder, whose apices are the orifices of the 2 urethra, is known as the trigone.

The mucosa of the trigone is always smooth and is never thrown into folds. The lamina propia of the bladder may be subdivided into 2 layers: a more superficial,

dense, irregular collagenous connective tissue and a deeper, loose layer of connective tissue composed of a mixture of collagen and elastic fibers.

The lamina propia contains no glands except at the region surrounding the urethral orifice where mucous glands may be found.

Usually, these glands extend only into the superficial layer of the lamina propia. They secrete a clear viscous fluid that apparently lubricates the urethral orifice. The muscular coat of the urinary bladder is composed of 3 interlaced layers of

smooth muscle which can be separated only in the region of the neck of the bladder. They are arranged as a thin inner longitudinal layer, a thick middle circular layer and

a thin outer longitudinal layer. The middle circular layer forms the internal sphincter muscle around the internal

orifice of the urethra. The adventitia of the bladder is composed of a dense, irregular collagenous type of

connective tissue containing a generous amount of elastic fibers. Certain regions of the adventitia are covered by a serosa, a peritoneal reflection onto

the wall of the bladder, whereas other regions may be surrounded by flat.

Additional Information:

In an empty bladder the superficial cells of the transitional epithelium are low cuboidal or columnar.

When the bladder is full and the transitional epithelium is stretched, the cells exhibit a squamous appearance.

The deeper layers of cells in the epithelium are round and basal cells are more columnar.

In the lamina propia are two zones, as in the ureter but more pronounced. The subepithelial region is denser with fine fibers and numerous fibroblasts. The deeper zone typically contains loose or moderately dense irregular connective tissue, which extends between the muscle fibers as in interstitial connective tissue.

The lumen of the bladder is lined by transitional epithelium, which allows the organ to stretch or enlarge (change in shape) as it fills in urine.

(Gartner and Hiatt’s Textbook of Histology 2nd Edition)

2. Review the characteristics and morphology of Escherichia coli.

Morphology

The Enterobacteriaceae are small (0.5x3.0µm), gram (-), non-spore-forming rods. They may be motile or nonmotile. When motile, locomotion is by means of peritrichous flagella, a property that aids in

differentiating the enteric from the polar flagellated Pseudomonadaceae and Vibrionaceae.

Two genera, Shigella and Klebsiella are characteristically non-motile. Enteric bacilli may possess a well-defined capsule as seen with the genus Klebsiella

or a loose, ill-defined coating referred to a slime layer or they may lack either structure.

Fimbriae or pili are present in most species and are responsible for the attachment of the bacterial cells to other bacteria, host cells and bacteriophages.

The cell wall is composed of murein, lipoCHON, phospholipid, CHON and lipopolysaccharides (LPS) and is arranged in layers.

The murein-lipoCHON layer constitute approximately 20% of the cell wall and is responsible for cellular rigidity.

The remaining 80% of the cell wall is joined to the lipid of the lipoCHON to form a lipid bilayer.

LPS contains the specific polysaccharide side chains that determine the antigenicity of the various species and is the portion of the cell responsible for endotoxicity.

Physiology

Biochemical and Cultural Characteristics

Enterobacteriaceae are biochemically diverse, facultative organisms. When grown in anaerobic or low-oxygen atmospheres, they ferment CHOs; but

when given oxygen, they utilize the tricarboxylic acid cycle and the electron transport system for energy production.

By definition, all members of the family, ferment glucose, reduce nitrates to nitrites but do not liquefy alginate and are oxidase negative.

Most enteric ferment glucose by the mixed acid pathway but members of the genera Klebsiella, Enterobacter and Serratia utilize the butanediol fermentative pathway.

Various species differ in the CHOs they ferment and these differences together with the variations in end-product production and substrate utilization form the basis for speciation within this family.

On non-differential or non-selective media, such as blood agar or infusion agars, the various species cannot be distinguished from each other and appear moist, smooth, gray colonies.

Smooth to rough variations can occur. Some strains of certain genera are β-hemolytic.

A variety of selective and differential media has been devised for the isolation and differentiation of this family.

The selective media originally were designed to isolate the enteric pathogens of the genera Salmonella and Shigella from fecal material, whereas the differential media were designed to selectively inhibit gram (+) organisms and to separate the enteric into broad catergories by their ability to ferment selected CHOs such as lactose.

Genetics

The enterobacteriaceae are useful tools for geneticists and are the organisms most often used in the developing recombinant DNA industry.

This transfer of genetic material occasionally gives rise to hybrids with altered biochemical or structural properties.

Such hybrids are not only products of laboratory experimentation but are also observed in the hospital environment.

For example, most Escherichia coli do not produce H2S but some strains have acquired plasmids from Salmonella that enable the E. coli to produce this gas.

Changes in antimicrobial susceptibility also occur when a resistant organism possessing the resistance transfer factor (RTF) transfers the genes encoding for the antimicrobial resistance to a previously sensitive organism.

The pathogenicity of an organism can also be altered with the acquisition of genetic material (e.g. the plasmids’ encoding for enterotoxin production).

A number of enteric harbour plasmids that encode for the production of antibacterial substances known as bacteriocins.

Different bacteriocins attack different molecular sites such as sites of nucleic acid or CHON synthesis or ATP formation.

Bacteriocins generally are active only against susceptible organisms of the same species.

This selectivity provides a useful epidemiologic tool for subdividing or typing a species.

For example, if all the Proteus mirabilis strains isolated from wounds of patients on a particular hospital ward had the same bacteriocin type, they probably originated from the same source or had a common means of transmission.

Resistance

Since the enteric bacilli do not produce spores, they are destroyed relatively easily by heat and low concentrations of common germicides and disinfectants.

Phenolics, formaldehyde, β-glutaraldehyde and halogen compounds are bactericidal but quaternary ammonium compounds may be only bacteriostatic, depending on the particular formulation and the situation in which they are used.

Chlorination of water has been effective in controlling the dissemination of intestinal pathogens such as agent of typhoid fever.

The enteric are also relatively sensitive to drying but can survive for long periods of time if provided adequate moisture.

Moisture-laden respiratory care and anesthesia equipment have been sources of enterobacterial infections in the hospital setting and organisms have been isolated from snow and ice after several months providing a mechanism of contaminating water supplies during spring thaws.

Control of these organisms in foods can be achieved by pasteurization, thorough cooking and proper refrigeration.

Antigenic structure:

Serologic typing of E. coli is based primarily on the determination of the O antigen type, H antigen type, and when applicable, the K antigen type.

There are more than 164 O antigens, 100 K antigens and 50 H antigens described for E. coli.

H antigens can be further subdivided in the L, A and B subgroups. Determination of the antigenic profile of the various strains is useful in epidemiologic

studies, and several studies have linked particular antigenic types to various diarrheal diseases.

For example, serotype O157:H7 produces Shiga-like toxin that is responsible for hemorrhagic colitis, and nearly all O78:H11 and O78:H12 isolates are enterotoxigenic.

Other antigenic types such as O111a, 111b:H2 have been associated with infantile diarrhea, and O124:H30 strains are enteroinvasive and cause bacilliary dysentery similar to that caused by Shigella.

Many other antigenic types in addition to the ones listed above are linked to the various diseases.

Determinants of Pathogenicity

The descriptive term E. coli encompasses a divers group of organisms that can infect any host system and produce a vast number of virulence factors, ranging from structural features to excreted toxins. The relative importance of each of these factors depends not only on the genetics of a particular strain of organism but also on the site of infection and the underlying condition of the host.

Surface Factors:

In both the United States and Europe, E. coli and group B streptococci are the primary causes of neonatal meningitis, and 80% of all E. coli isolated from individuals with this disease produce a polysialic acid capsule termed K1.

Organisms possessing this capsular type also are more likely to cause neonatal sepsis.

Interestingly, this capsule is identical to the group B polysaccharide capsule of Neisseria meningitides.

The K1 capsule is unique among the capsular antigens of E. coli in that it enables the organism to resist killing by both human neutrophils and normal serum in various in vitro assays.

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Other capsular types inhibit the killing power of normal serum, particularly when associated with smooth LPS, but fail to protect organisms from phagocytic death.

The K1 capsule may also aid in the survival of the organism in the blood and spinal fluid of neonates because of its similarity to the embryonic form of the polysialic acid of the neural cell adhesion molecule (N-CAM).

The type of O antigen of the infecting organism also appears to be important, as does production of S fimbriae, which have a predilection for binding receptors present on the vascular endothelium and the epithelial lining of the choroid plexus and the ventricules of the brains of neonatal mice.

In addition to the S type fimbriae, E. coli produce a number of different types of fimbriae that enable the organism to attach to various host tissues.

These fimbriae have been divided into two large groups termed mannose resistant and mannose sensitive.

The mannose sensitive fimbriae bind to mannose-containing receptors in host cells, and their ability bind to these receptors is reduced when the bacterial cells are pretreated with D-mannose.

Mannose-sensitive or type I fimbriae are also called common pili because they are found on most E. coli.

Although type I fimbriae attach to a wide variety of eukaryotic cells, no pathogenic function has been defined for them, since there is no correlation between the presence or absence of type I fimbriae and disease.

However, some authors believe that these fimbriae are important in the colonization of the bladder in the absence of other fimbriae.

Furthermore, type I fimbriae are believed to play a major role in colonization by binding the organism to the mucosa of the large intestines, buccal cavity, and vaginal tract.

The surface factors involved with mannose-resistant adhesion are more complex than type I fimbriae.

Included in this group of surface-adherent factors are fimbriae and other surface properties termed adhesins.

Regardless of the nature of these adhesive factors, they all appear to be important in the establishment of pathogenic strains of E. coli in various host tissues, and the genetic information for a number of them is found closely associated with other virulence factors.

One such mannose-resistant adherence factor is the type S fimbriae described above as a cofactor with K1-encapsulated E. coli.

Another major mannose-resistant fimbriae group is that of the P fimbriae, so-named because of their ability to bind to the human P blood group antigens.

These antigens contain a Gal(α1-4)Galβ moiety, which is also found in a number of other human cells, including those of the kidney and bladder.

Uropathogenic E. coli frequently contain the P fimbriae that bind to these sites, and organism containing P fimbriae are more likely to be associated with complicated urinary tract infections.

Approximately 70% of E. coli strains isolated from patients with pyelonephritis possess P fimbriae, compared to only 36% of strains from cystitis patients and 19% of fecal strains.

Another group of heterologous adhesins, termed X factors, may also be important in the uropathogenicity of E. coli, although their exact role has not yet been determined.

They appear to bind sites other than P blood group antigen and mannose-containing moieties.

They include the adhesins that appear to bind to the Dr blood group antigen, the adhesin associated with the M blood group antigen, the nonfimbrial protein antigens NFA1 and NFA2, and the F1C and G fimbriae.

Mannose-resistant fimbriae and adhesins are also important adherence factors in intestinal infections caused by E. coli.

Enterotoxins:

Various E. coli strains play a significant role in gastrointestinal disease, and the pathogenic mechanisms of E. coli diarrhea are varied and complex.

One of these pathogenic mechanisms involves the production of a wide variety of enterotoxins, some of which are associated with human disease, whereas others are primarily associated with animal infections.

Regardless of the host system, the target organ of E. coli enterotoxins is the small intestine, and the result is a watery diarrhea caused by the outpouring of fluids and electrolytes.

The ability to produce the majority of these toxins is dependent on the presence of plasmids encoding for toxin production. E. coli strains possessing the necessary plasmid produce a heat-labile enterotoxin (LT) that is similar to the enterotoxin of Vibrio cholerae.

As with cholera toxin (CT), LT stimulates adenyl cyclase activity in the epithelial cells of the small intestinal mucosa, which in turn increases the permeability of the intestinal lining, resulting in a loss of fluids and electrolytes.

The B subunits of both the CT and LT bind to the ganglioside of GM1 of intestinal cells.

The A subunit is then hydrolyzed and the A1 fragment enters the host cell and enzymatically catalyzes the transfer of adenosine diphosphate (ADP)-ribose from nicotinamide adenine dinucleotide (NAD) to the regulatory subunit of adenyl cyclase, thereby increasing the level of cyclic AMP.

The increase in cyclic AMP causes a loss of electrolytes and fluid from the cells. Although CT and LT are structurally similar and produce the same effects in tissue

culture and animal models, they have slightly different antigenic structures, and the potency of the LT is about 100-fold less than the cholera toxin in animal models.

The overall amino acid and nucleotide homology of the two toxins is approximately 80%.

A second class of LT, LT-II, which does not share immunologic reactivity or nucleotide homology with either CT or LT, has been identified.

The LT-II class contains at least two different toxins, LT-IIa and LT-IIb. These genes encoding for the B subunits of LT-IIa and LT-IIb are 66% homologous with each other, but they share no significant homology with LT-I or CT.

The genes encoding for the A subunit of both the LT-IIa and LT-IIb are 71% homologous with each other and 57% homologous with the genes of LT-I and CT, with most of the homology occurring in the region encoding for the A1 subunit.

Although the mode of action of LT-IIa and LT-IIb is similar to that of CT and LT, they do not cause fluid accumulation in the ligated adult rabbit intestine model, nor do they bind to the ganglioside GM1.

Furthermore, the genetic information for these toxins is encoded on the bacterial chromosome, not on plasmids.

The importance of these toxins in human disease is not known. In addition to the LT, E. coli can also produce two heat-stable (ST) enterotoxins, STa (ST-I) and STb (ST-II).

STa is a polypeptide with a molecular weight of 1500-2000 Da, is methanol soluble, and is active in suckling mice and neonatal pigs. STa has a tightly coiled secondary structure that appears to be required for activity as evidenced by its high content of cysteine, its inactivation by reducing agents, and its alkaline pH.

STb is not methanol soluble and is active only in weaned pigs. The two toxins are also different in amino acid sequence. STa binds tightly to specific intestinal receptors and then rapidly activates a

particulate guanylate cyclase in intestinal mucosa cells, causing a secretory response primarily by inhibiting sodium and chloride absorption by the brush border membrane.

The mechanism of action of STb is not known, but it does not involve cyclic nucleotide production.

Although STa-producing E. coli strains do not cause diarrhea in humans, one study did not report the detection of any STb-producing E. coli in stool of patients.

The ability to produce STa is encoded on two plasmids, one of which also encodes for LT and the other for ST only, and probe studies have revealed that there are at least two distinct STa genes, STa-I and STa-II.

Verotoxins (Shigalike Toxins):

E. coli produces at least two human-derived and one porcine-derived cytotoxin, termed verotoxins because of their irreversible cytotoxic effect on Vero tissue culture cells, a cell line developed from African green monkey kidney cells.

VTEC have been associated with three human syndromes: diarrhea, hemorrhagic colitis and hemolytic uremic syndrome (HUS).

Because of the similarities of verotoxins of Shiga toxin, these toxins have also been referred to as Shigalike toxins; the term SLT-I is interchangeable with VT1, and VT2 is referred to as SLT-II by other authors.

Both VT1 and VT2 inhibit protein synthesis in eukaryotic cells in the same manner as Shiga toxin, but they differ from each other and Shiga toxin in immunologic reactivity and biologic activities in animal and tissue culture models.

VT1 is almost identical to Shiga toxin, both in structure and mode of action, but it differs in molecular weight, and the two toxins differ in their activities in animal models.

VT2 has similar biologic properties to VT1 but is not neutralized by Shiga-toxin antibody.

The two verotoxins share a 58% homology in amino acid composition. VT2 appears to differ from VT1 in the spacing of the subunits and in the DNA restriction patter.

The level of toxin production is important in development of disease. High-level VTEC produce large amounts of toxin in supernatant fluids of cultures and

have been linked to hemorrhagic colitis, diarrhea, and HUS. Low- and trace-level-producing VTEC do not have easily detected amounts of toxin

in supernatant fluids and do not appear to be associated with disease production. VTEC are infected with either one or both bacteriophages that encode for the

production of either VT1 or VT2 or both VT1 and VT2. Although the number of E. coli strains can become infected with these

bacteriophages and thus produce verotoxins, the majority of VTEC isolates in outbreaks have been attributed to the O157:H7 serotype.0

Colonization Factors

The cellular surface properties of certain enteric are important in the establishment of the organisms in the host.

The capsule of Klebsiella pneumoniae functions in a manner similar to that of the pneumococcal capsule to prevent phagocytosis.

Although another type of K antigen, the “Vi” antigen of S. typhi does not prevent phagocytosis of the organism, it may function in some protective manner to prevent intracellular destruction of the bacterial cell.

Fimbriae such as the CFA antigens of animal strains of E. coli are necessary for the attachement of the organism to target tissues.

The O antigen may also bind the organism to certain tissue receptor sites. Experiments with Salmonella typhimurium show that the loss of O-specific side

chains is associated with an increase of the lethal dose (LD50) in mice.

Other Factors

Members of the genus Shigella and certain enterpathogenic E. coli penetrate the epithelial lining of the intestinal tract.

Whether these organisms elaborate a toxin or possess unique surface characteristics has not been clearly determined.

However, tissue penetration is an important feature of shigellosis and occurs even with enterotoxin-producing strains.

Similarly, Salmonella are able to penetrate the epithelial lining of the large intestines and they also can invade tissues beyond the epithelium and survive intracellularly in a variety of host cells.

A number of enteric produce additional toxins, enzymes and hemolysins that produce a variety of effects in various experimental systems.

(Zinsser Microbiology 20th Edition)

3. Describe the two general anatomic categories of urinary tract infection.

Many different microorganisms can infect the urinary tract, but by far the most common agents are the gram-negative bacilli.

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Escherichia coli causes ~80% of acute infections in patients without catheters, urologic abnormalities, or calculi.

Other gram-negative rods, especially Proteus and Klebsiella and occasionally Enterobacter, account for a smaller proportion of uncomplicated infections.

These organisms, plus Serratia and Pseudomonas, assume increasing importance in recurrent infections and in infections associated with urologic manipulation, calculi, or obstruction.

They play a major role in nosocomial, catheter-associated infections. Proteus spp., by virtue of urease production, and Klebsiella spp., through the

production of extracellular slime and polysaccharides, predispose to stone formation and are isolated more frequently from patients with calculi.

Gram-positive cocci play a lesser role in UTIs. However, Staphylococcus saprophyticus—a novobiocin-resistant, coagulase-

negative species—accounts for 10 to 15% of acute symptomatic UTIs in young females.

Enterococci occasionally cause acute uncomplicated cystitis in women. More commonly, enterococci and Staphylococcus aureus cause infections in

patients with renal stones or previous instrumentation or surgery. Isolation of S. aureus from the urine should arouse suspicion of bacteremic infection

of the kidney. About one-third of women with dysuria and frequency have either an insignificant

number of bacteria in midstream urine cultures or completely sterile cultures and have been previously defined as having the urethral syndrome.

About three-quarters of these women have pyuria, while one-quarter have no pyuria and little objective evidence of infection.

In the women with pyuria, two groups of pathogens account for most infections. Low counts (102 to 104/mL) of typical bacterial uropathogens such as E. coli, S.

saprophyticus, Klebsiella, or Proteus are found in midstream urine specimens from most of these women.

These bacteria are probably the causative agents in these infections because they can usually be isolated from a suprapubic aspirate, are associated with pyuria, and respond to appropriate antimicrobial therapy.

In other women with acute urinary symptoms, pyuria, and urine that is sterile (even when obtained by suprapubic aspiration), sexually transmitted urethritis-producing agents such as Chlamydia trachomatis, Neisseria gonorrhoeae, and herpes simplex virus are etiologically important.

These agents are found most frequently in young, sexually active women with new sexual partners.

The causative role of several more unusual bacterial and nonbacterial pathogens in UTIs remains poorly defined.

Ureaplasma urealyticum has frequently been isolated from the urethra and urine of patients with acute dysuria and frequency but is also found in specimens from many patients without urinary symptoms.

Ureaplasmas probably account for some cases of urethritis and cystitis. U. urealyticum and Mycoplasma hominis have been isolated from prostatic and renal

tissues of patients with acute prostatitis and pyelonephritis, respectively, and are probably responsible for some of these infections as well.

Adenoviruses cause acute hemorrhagic cystitis in children and in some young adults, often in epidemics.

Although other viruses can be isolated from urine (e.g., cytomegalovirus), they are thought not to cause acute UTI.

Colonization of the urine of catheterized or diabetic patients by Candida and other fungal species is common and sometimes progresses to symptomatic invasive infection.

(Harrison’s Principles of Internal Medicine 16th Edition)

Acute infections of the urinary tract fall into two general anatomic categories: lower tract infection (urethritis and cystitis) and upper tract infection (acute pyelonephritis, prostatitis, and intrarenal and perinephric abscesses).

Infections at various sites may occur together or independently and may either be asymptomatic or present as one of the clinical syndromes described in this chapter. Infections of the urethra and bladder are often considered superficial (or mucosal) infections, while prostatitis, pyelonephritis, and renal suppuration signify tissue invasion.

From a microbiologic perspective, urinary tract infection (UTI) exists when pathogenic microorganisms are detected in the urine, urethra, bladder, kidney, or prostate.

In most instances, growth of ≥105 organisms per milliliter from a properly collected midstream "clean-catch" urine sample indicates infection. However, significant bacteriuria is lacking in some cases of true UTI.

Especially in symptomatic patients, fewer bacteria (102–104/mL) may signify infection. In urine specimens obtained by suprapubic aspiration or "in-and-out" catheterization and in samples from a patient with an indwelling catheter, colony counts of 102–104/mL generally indicate infection.

Conversely, colony counts of >105/mL in midstream urine are occasionally due to specimen contamination, which is especially likely when multiple bacterial species are found.

Infections that recur after antibiotic therapy can be due to the persistence of the originally infecting strain (as judged by species, antibiogram, serotype, and molecular type) or to reinfection with a new strain.

"Same-strain" recurrent infections that become evident within 2 weeks of cessation of therapy can be the result of unresolved renal or prostatic infection (termed relapse) or of persistent vaginal or intestinal colonization leading to rapid reinfection of the bladder.

Symptoms of dysuria, urgency, and frequency that are unaccompanied by significant bacteriuria have been termed the acute urethral syndrome.

Although widely used, this term lacks anatomic precision because many cases so designated are actually bladder infections.

Moreover, since the pathogen can usually be identified, the term syndrome—implying unknown causation—is inappropriate.

Chronic pyelonephritis refers to chronic interstitial nephritis believed to result from bacterial infection of the kidney.

Many noninfectious diseases also cause an interstitial nephritis that is indistinguishable pathologically from chronic pyelonephritis.

(Harrison’s Principles of Internal Medicine 17th Edition)

4. Identify the etiologies of acute cystitis as to:

4.1 Morphology

Most cases of cystitis take the form of nonspecific acute or chronic inflammation of the bladder.

In gross appearance, there is hyperemia of the mucosa, sometimes associated with exudate. When there is a hemorrhagic component, the cystitis is designated hemorrhagic cystitis.

This form of cystitis sometimes follows radiation injury or antitumor chemotherapy and is often accompanied by epithelial atypia.

Adenovirus infection also causes a hemorrhagic cystitis. The accumulation of large amounts of suppurative exudate may merit the

designation of suppurative cystitis. When there is ulceration of large areas of the mucosa, or sometimes the

entire bladder mucosa, this is known as ulcerative cystitis. Persistence of the infection leads to chronic cystitis, which differs from the

acute form only in the character of the inflammatory infiltrate. There is more extreme heaping up of the epithelium with the formation of a

red, friable, granular, sometimes ulcerated surface. Chronicity of the infection gives rise to fibrous thickening in the tunica propria

and consequent thickening and inelasticity of the bladder wall. Histologic variants include follicular cystitis, characterized by the aggregation

of lymphocytes into lymphoid follicles within the bladder mucosa and underlying wall, and eosinophilic cystitis, manifested by infiltration with submucosal eosinophils together with fibrosis and occasionally giant cells.

All forms of cystitis are characterized by a triad of symptoms: 1. Frequency, which in acute cases may necessitate urination every 15

to 20 minutes.2. Lower abdominal pain localized over the bladder region or in the

suprapubic region.3. Dysuria; pain or burning on urination.

Associated with these localized changes, there may be systemic signs of inflammation such as elevation of temperature, chills, and general malaise.

In the usual case, the bladder infection does not give rise to such a constitutional reaction.

The local symptoms of cystitis may be disturbing, but these infections are also important as antecedents to pyelonephritis.

Cystitis is sometimes a secondary complication of some underlying disorder such as prostatic enlargement, cystocele of the bladder, calculi, or tumors. These primary diseases must be corrected before the cystitis can be relieved.

(Robbins and Cotran Pathologic Basis of Disease 7th Edition)

4.2 Conditions affecting the pathogenesis of acute cystitis.

4.2.1 Gender and sexual activity

The female urethra appears to be particularly prone to colonization with colonic gram-negative bacilli because of its proximity to the anus, its short length (~4 cm), and its termination beneath the labia.

Sexual intercourse causes the introduction of bacteria into the bladder and is temporally associated with the onset of cystitis; it thus appears to be important in the pathogenesis of UTIs in both pre- and postmenopausal women.

Voiding after intercourse reduces the risk of cystitis, probably because it promotes the clearance of bacteria introduced during intercourse.

Use of spermicidal compounds with a diaphragm or cervical cap or use of spermicide-coated condoms dramatically alters the normal introital bacterial flora and has been associated with marked increases in vaginal colonization with E. coli and in the risk of both cystitis and acute pyelonephritis.

In healthy, community-dwelling postmenopausal women, the risk of UTI (both cystitis and pyelonephritis) is increased by a history of recent sexual activity, recent UTI, diabetes mellitus, and incontinence.

In male patients who are <50 years old and who have no history of heterosexual or homosexual insertive rectal intercourse, UTI is exceedingly uncommon, and this diagnosis should be questioned in the absence of clear documentation.

An important factor predisposing to bacteriuria in men is urethral obstruction due to prostatic hypertrophy.

Insertive rectal intercourse is also associated with an increased risk of cystitis in men. Men (and women) who are infected with HIV and who have CD4+ T cell counts of <200/L are at increased risk of both bacteriuria and symptomatic UTI.

Finally, lack of circumcision has been identified as a risk factor for UTI in both male neonates and young men.

(Harrison’s Principles of Internal Medicine 17th Edition)

4.2.2 Pregnancy

UTIs are detected in 2–8% of pregnant women. Symptomatic upper tract infections, in particular, are unusually

common during pregnancy; fully 20–30% of pregnant women

5

with asymptomatic bacteriuria subsequently develop pyelonephritis.

This predisposition to upper tract infection during pregnancy results from decreased ureteral tone, decreased ureteral peristalsis, and temporary incompetence of the vesicoureteral valves.

Bladder catheterization during or after delivery causes additional infections.

Increased incidences of low birth weight, premature delivery, and neonatal death result from UTIs (particularly upper tract infections) during pregnancy.

(Harrison’s Principles of Internal Medicine 17th Edition)

4.2.3 Obstruction

Any impediment to the free flow of urine—tumor, stricture, stone, or prostatic hypertrophy—results in hydronephrosis and a greatly increased frequency of UTI.

Infection superimposed on urinary tract obstruction may lead to rapid destruction of renal tissue.

It is of utmost importance, therefore, when infection is present, to identify and repair obstructive lesions.

On the other hand, when an obstruction is minor and is not progressive or associated with infection, great caution should be exercised in attempting surgical correction.

The introduction of infection in such cases may be more damaging than an uncorrected minor obstruction that does not significantly impair renal function.

(Harrison’s Principles of Internal Medicine 17th Edition)

4.2.4 Neurogenic bladder dysfunction

Interference with bladder enervation, as in spinal cord injury, tabes dorsalis, multiple sclerosis, diabetes, and other diseases, may be associated with UTI.

The infection may be initiated by the use of catheters for bladder drainage and is favored by the prolonged stasis of urine in the bladder.

An additional factor often operative in these cases is bone demineralization due to immobilization, which causes hypercalciuria, calculus formation, and obstructive uropathy.

(Harrison’s Principles of Internal Medicine 17th Edition)

4.2.5 Vesico-ureteral reflux

Defined as reflux of urine from the bladder cavity up into the ureters and sometimes into the renal pelvis, vesicoureteral reflux occurs during voiding or with elevation of pressure in the bladder.

In practice, this condition is detected as retrograde movement of radiopaque or radioactive material during a voiding cystourethrogram.

An anatomically impaired vesicoureteral junction facilitates reflux of bacteria and thus upper tract infection.

However, since—even in the healthy urinary system—a fluid connection between the bladder and the kidneys always exists, some retrograde movement of bacteria probably takes place during infection but is not detected by radiologic techniques.

Vesicoureteral reflux is common among children with anatomic abnormalities of the urinary tract or with anatomically normal but infected urinary tracts. In the latter group, reflux disappears with advancing age and is probably attributable to factors other than UTI.

Long-term follow-up of children with UTI who have reflux has established that renal damage correlates with marked reflux, not with infection. Thus, it appears reasonable to search for reflux in children with unexplained failure of renal growth or with renal scarring, because UTI per se is an insufficient explanation for these abnormalities.

On the other hand, it is doubtful that all children who have recurrent UTIs but whose urinary tract appears normal on pyelography should be subjected to voiding cystoureterography merely for the detection of the rare patient with marked reflux not revealed by intravenous pyelography.

(Harrison’s Principles of Internal Medicine 17th Edition)

4.2.6 Bacterial virulence factor

Not all strains of E. coli are equally capable of infecting the intact urinary tract.

Bacterial virulence factors markedly influence the likelihood that a given strain, once introduced into the bladder, will cause UTI.

Most E. coli strains that cause symptomatic UTIs in noncatheterized patients belong to a small number of specific O, K, and H serogroups. These uropathogenic clones have accumulated a number of virulence genes that are often closely linked on the bacterial chromosome in "pathogenicity islands."

Bacterial adherence to uroepithelial cells is a critical first step in the initiation of infection.

For both E. coli and Proteus spp., fimbriae (hairlike proteinaceous surface appendages) mediate bacterial attachment to specific receptors on epithelial cells, which in turn initiates important events in the mucosal epithelial cell, including secretion of IL-6 and IL-8 (with subsequent chemotaxis of leukocytes to the bladder mucosa) and induction of apoptosis and epithelial cell desquamation.

Besides fimbriae, uropathogenic E. coli strains usually produce cytotoxins, hemolysin, and aerobactin (a siderophore for scavenging iron) and are resistant to the bactericidal action of human serum.

Nearly all E. coli strains causing acute pyelonephritis and most of those causing acute cystitis are uropathogenic strains possessing pathogenicity islands.

In contrast, infections in patients with structural or functional abnormalities of the urinary tract are generally caused by bacterial strains that lack these uropathogenic properties; the implication is that these properties are not needed for infection of the compromised urinary tract.

(Harrison’s Principles of Internal Medicine 17th Edition)

4.2.7 Genetic factor

Increasing evidence suggests that host genetic factors influence susceptibility to UTI.

A maternal history of UTI is more often found among women who have experienced recurrent UTIs than among controls.

The number and type of receptors on uroepithelial cells to which bacteria may attach are, at least in part, genetically determined.

Many of these structures are components of blood group antigens and are present on both erythrocytes and uroepithelial cells.

For example, P fimbriae mediate attachment of E. coli to P-positive erythrocytes and are found on nearly all strains causing acute uncomplicated pyelonephritis.

Conversely, P blood group–negative individuals, who lack these receptors, are at decreased risk of pyelonephritis.

Furthermore, nonsecretors of blood group antigens are at increased risk of recurrent UTI; this predisposition may relate to a different profile of genetically determined glycolipids on uroepithelial cells.

Mutations in host genes integral to the immune response (e.g., Toll-like receptors, interferon receptors) may also affect susceptibility to UTI.

(Harrison’s Principles of Internal Medicine 17th Edition)

5. Describe/Identify the diagnostic test of acute cystitis.

5.1 Review the normal values of urinalysis.

Normal Values of Urinalysis

Gross Appearance:

Color = yellow Clarity = clear Specific gravity = 1.003 – 1.035 (random urine specimen) [using

refractometer]

Chemical examination of urine:

pH = chemstrip - 5.0 - 6.0 (1st morning specimen) or 4.5 – 8.0 (random urine specimen)

Protein = chemstrip – negative [<10 mg/dL (random sample) or 100 mg (per 24 hour urine)]

Glucose = chemstrip – negative [<200 mg/dL (renal threshold is 160-180mg/dL)]

Ketones = chemstrip – negative [<9mg/dL acetoacetic acid; <70 mg/dL acetone]

Blood = chemstrip – negative Bilirubin = chemstrip – negative [<0.5 mg/dL] Urobilinogen = chemstrip – negative [0.4 m/dL (normal: 0.2 Erlich units)] Nitrite = chemstrip – negative [<0.005 mg/dL]

Microscopic exam:

RBC = 0-2/hpf or 0-3/hpf WBC = 0-5/hpf Epithelial cells = few squamous eptithelial cells/lpf Mucous threads = frequently seen in female specimens (no clinical

significance) Bacteria = few Yeast = should be negative in a normal urine Parasites = should be negative in a normal urine Spermatozoa = normal in males; report to medico-legal if found in females

<18 years old Casts = hyaline cast: 0-2/lpf Crystals = normal urinary crystals in urine depending on pH:

6

Acid Acid/ Neutral (alkaline)

Alkaline (Neutral)

Alkaline

Uric acid Calcium oxalate

Amorphous phosphates

Triple phosphate

Amorphous urates

Calcium phosphate

Ammonium biurate

Calcium carbonate

(Urinalysis and Body Fluids 4th Edition by Strasinger)

General Characteristics and Measurements

Chemical Determinations

DeterminationsMicroscopic Examination of

SedimentColor: pale yellow to amber Glucose: negative Casts negative: occasional

hyaline casts

Appearance: clear to slightly hazy

Protein: negative Red blood cells: negative or rare

Specific gravity: 1.005–1.025 with a normal fluid intake

Ketones: negative Crystals: negative (none)

pH: 4.5–8.0; average person has a pH of about 5 to 6

Bilirubin: negative White blood cells: negative or rare

Volume: 600–2,500 mL/24 h; average 1200 mL/24 h

Urobilinogen: 0.5–4.0 mg/d

Epithelial cells: few; hyaline casts 0–1/lpf (low-power field)

Nitrate for bacteria: negative

Leukocyte esterase: negative

Blood: negative

Urine Sediment Component and its Clinical Significance:

Bacteria - Urinary tract infection Casts - Tubular or glomerular disorders Broad casts - Formation occurs in collecting tubules; serious kidney disorder,

extreme stasis of flow Epithelial (renal) casts - Tubular degeneration Fatty casts - Nephrotic syndrome Granular - Renal parenchymal disease Waxy - Stasis of flow Hyaline casts - Chronic renal failure, chronic renal disease, congestive heart

failure; stress or exercise Red blood cell casts - Acute glomerulonephritis White blood cell casts - Pyelonephritis, acute interstitial nephritis Epithelial cells - Damage to various parts of urinary tract Renal cells - Tubular damage Squamous cells - Normal or contamination Erythrocytes - Most renal disorders, menstruation; strenuous exercise Fat bodies (oval) - Nephrotic syndrome Leukocytes - Most renal disorders; urinary tract infection; pyelonephritis

(Fischback: A Manual of Laboratory and Diagnostic Tests)

5.2 Identify the methods of detection of bacteruria.

Diagnostic Testing

Determination of the number and type of bacteria in the urine is an extremely important diagnostic procedure.

In symptomatic patients, bacteria are usually present in the urine in large numbers (≥105/mL).

In asymptomatic patients, two consecutive urine specimens should be examined bacteriologically before therapy is instituted, and ≥105 bacteria of a single species per milliliter should be demonstrable in both specimens.

Since the large number of bacteria in the bladder urine is due in part to bacterial multiplication in the bladder cavity, samples of urine from the ureters or renal pelvis may contain <105 bacteria per milliliter and yet indicate infection.

Similarly, the presence of bacteriuria of any degree in suprapubic aspirates or of ≥102 bacteria per milliliter of urine obtained by catheterization usually indicates infection.

In some circumstances (antibiotic treatment, high urea concentration, high osmolarity, low pH), urine inhibits bacterial multiplication, resulting in relatively low bacterial colony counts despite infection.

For this reason, antiseptic solutions should not be used to wash the periurethral area before collection of the urine specimen.

Water diuresis or recent voiding also reduces bacterial counts in urine. Microscopy of urine from symptomatic patients can be of great diagnostic

value. Microscopic bacteriuria, which is best assessed with Gram-stained

uncentrifuged urine, is found in >90% of specimens from patients whose infections are associated with colony counts of at least 105/mL, and this finding is very specific.

However, bacteria cannot usually be detected microscopically in infections with lower colony counts (102–104/mL).

The detection of bacteria by urinary microscopy thus constitutes firm evidence of infection, but the absence of microscopically detectable bacteria does not exclude the diagnosis.

When carefully sought by chamber-count microscopy, pyuria is a highly sensitive indicator of UTI in symptomatic patients.

Pyuria is demonstrated in nearly all acute bacterial UTIs, and its absence calls the diagnosis into question.

The leukocyte esterase "dipstick" method is less sensitive than microscopy in identifying pyuria but is a useful alternative when microscopy is not feasible. Pyuria in the absence of bacteriuria (sterile pyuria) may indicate infection with unusual agents such as C. trachomatis, U. urealyticum, or Mycobacterium tuberculosis or with fungi.

Alternatively, sterile pyuria may be documented in noninfectious urologic conditions such as calculi, anatomic abnormality, nephrocalcinosis, vesicoureteral reflux, interstitial nephritis, or polycystic disease.

Although many authorities have recommended that urine culture and antimicrobial susceptibility testing be performed for any patient with a suspected UTI, it is more practical and cost-effective to manage women who have symptoms characteristic of acute uncomplicated cystitis without an initial urine culture. Two approaches to presumptive therapy have generally been used.

In the first, treatment is initiated solely on the basis of a typical history and/or typical findings on physical examination.

In the second, women with symptoms and signs of acute cystitis and without complicating factors are managed with urinary microscopy (or, alternatively, with a leukocyte esterase test).

A positive result for pyuria and/or bacteriuria provides enough evidence of infection to omit urine culture and susceptibility testing and treat the patient empirically.

Urine should be cultured, however, when a woman's symptoms and urine-examination findings leave the diagnosis of cystitis in question.

Pretherapy cultures and susceptibility testing are also essential in the management of all patients with suspected upper tract infections and of those with complicating factors (including all men).

In these situations, any of a variety of pathogens may be involved, and antibiotic therapy is best tailored to the individual organism.

Urologic Evaluation

Very few women with recurrent UTIs have correctable lesions discovered at cystoscopy or upon IV pyelography, and these procedures should not be undertaken routinely in such cases.

Urologic evaluation should be performed for selected female patients—namely, women with relapsing infection, a history of childhood infections, stones or painless hematuria, or recurrent pyelonephritis.

Most male patients with UTI should be considered to have complicated infection and thus should be evaluated urologically.

Possible exceptions include young men who have cystitis associated with sexual activity, who are uncircumcised, or who have AIDS.

Men or women presenting with acute infection and signs or symptoms suggestive of an obstruction or stones should undergo prompt urologic evaluation, generally by means of ultrasound.

(Harrison’s Principles of Internal Medicine 17th Edition)

6. Describe the morphology and clinical course of acute pyelonephritis.

Acute Pyelonephritis

Acute pyelonephritis is an acute suppurative inflammation of the kidney caused by bacterial and sometimes viral (e.g., polyoma virus) infection, whether hematogenous and induced by septicemic spread or ascending and associated with vesicoureteral reflux.

Morphology

The hallmarks of acute pyelonephritis are patchy interstitial suppurative inflammation, intratubular aggregates of neutrophils, and tubular necrosis.

The suppuration may occur as discrete focal abscesses involving one or both kidneys, which can extend to large wedge-shaped areas of suppuration.

The distribution of these lesions is unpredictable and haphazard, but in pyelonephritis associated with reflux, damage occurs most commonly in the lower and upper poles.

In the early stages, the neutrophilic infiltration is limited to the interstitial tissue. Soon, however, the reaction involves tubules and produces a characteristic abscess

with the destruction of the engulfed tubules. Since the tubular lumens present a ready pathway for the extension of the infection,

large masses of intraluminal neutrophils frequently extend along the involved nephron into the collecting tubules.

7

Characteristically, the glomeruli appear to be resistant to the infection. Large areas of severe necrosis, however, eventually destroy the glomeruli, and

fungal pyelonephritis (e.g., Candida) often affects glomeruli. Three complications of acute pyelonephritis are encountered in special

circumstances. Papillary necrosis is seen mainly in diabetics and in those with urinary tract

obstruction. Papillary necrosis is usually bilateral but may be unilateral. One or all of the pyramids of the affected kidney may be involved. On cut section, the tips or distal two-thirds of the pyramids have areas of gray-white to yellow necrosis. On microscopic examination, the necrotic tissue shows characteristic coagulative necrosis, with preservation of outlines of tubules. The leukocytic response is limited to the junctions between preserved and destroyed tissue.

Pyonephrosis is seen when there is total or almost complete obstruction, particularly when it is high in the urinary tract. The suppurative exudate is unable to drain and thus fills the renal pelvis, calyces, and ureter, producing pyonephrosis.

Perinephric abscess implies extension of suppurative inflammation through the renal capsule into the perinephric tissue.

After the acute phase of pyelonephritis, healing occurs. The neutrophilic infiltrate is replaced by one that is predominantly mononuclear, with

macrophages, plasma cells, and (later) lymphocytes. The inflammatory foci are eventually replaced by scars that can be seen on the

cortical surface as fibrous depressions. Such scars are characterized microscopically by atrophy of tubules, interstitial

fibrosis, and lymphocyte infiltrate and may resemble scars produced by ischemic or other types of injury to the kidney.

However, the pyelonephritic scar is almost always associated with inflammation, fibrosis, and deformation of the underlying calyx and pelvis, reflecting the role of ascending infection and vesicoureteral reflux in the pathogenesis of the disease.

Clinical Course

Acute pyelonephritis is often associated with predisposing conditions, some of which were mentioned in the discussion of pathogenetic mechanisms. These include the following: Urinary tract obstruction, either congenital or acquired. Instrumentation of the urinary tract, most commonly catheterization. Vesicoureteral reflux. Pregnancy → 4% to 6% of pregnant women develop bacteriuria sometime

during pregnancy, and 20% to 40% of these eventually develop symptomatic urinary infection if not treated.

Patient's sex and age. After the first year of life (when congenital anomalies in males commonly become evident) and up to around age 40 years, infections are much more frequent in females. With increasing age, the incidence in males rises owing to the development of prostatic hypertrophy and frequent instrumentation.

Preexisting renal lesions, causing intrarenal scarring and obstruction. Diabetes mellitus, in which acute pyelonephritis is caused by more frequent

instrumentation, the general susceptibility to infection, and the neurogenic bladder dysfunction exhibited by patients.

Immunosuppression and immunodeficiency. When acute pyelonephritis is clinically apparent, the onset is usually sudden, with

pain at the costovertebral angle and systemic evidence of infection, such as fever and malaise.

There are usually indications of bladder and urethral irritation, such as dysuria, frequency, and urgency.

The urine contains many leukocytes (pyuria) derived from the inflammatory infiltrate, but pyuria does not differentiate upper from lower urinary tract infection.

The finding of leukocyte casts, typically filled with neutrophils (pus casts), indicates renal involvement, because casts are formed only in tubules.

The diagnosis of infection is established by quantitative urine culture. Uncomplicated acute pyelonephritis usually follows a benign course, and the

symptoms disappear within a few days after the institution of appropriate antibiotic therapy.

Bacteria, however, may persist in the urine, or there may be recurrence of infection with new serologic types of E. coli or other organisms.

Such bacteriuria then either disappears or may persist, sometimes for years. In the presence of unrelieved urinary obstruction, diabetes mellitus, or

immunodeficiency, acute pyelonephritis may be more serious, leading to repeated septicemic episodes.

The superimposition of papillary necrosis may lead to acute renal failure. An emerging viral pathogen causing pyelonephritis in kidney allografts is polyoma

virus. Latent infection with polyoma virus is widespread in the general population, but

immunosuppression of the allograft recipient can lead to reactivation of latent infection and the development of a nephropathy resulting in allograft failure in up to 1% to 5% of kidney transplant recipients.

This form of pyelonephritis is characterized by viral infection of tubular epithelial cell nuclei, leading to nuclear enlargement and intranuclear inclusions visible by light microscopy (viral cytopathic effect).

The inclusions are composed of viral structures arrayed in distinctive crystalline-like lattices when visualized by electron microscopy.

An interstitial inflammatory response is invariably present. Treatment is reduction in immunosuppression.

(Robbins and Cotran Pathologic Basis of Disease 7th Edition)

7. Describe/Identify the different the pharmacodynamics and pharmacokinetics and toxicology of the different pharmacologic agents used against acute cystitis and pyelonephritis.

7.1 Sulfonamides

Introduction

The sulfonamide drugs were the first effective chemotherapeutic agents to be employed systemically for the prevention and cure of bacterial infections in humans.

The considerable medical and public health importance of their discovery and their subsequent widespread use were quickly reflected in the sharp decline in morbidity and mortality figures for treatable infectious diseases.

The advent of penicillin and subsequently of other antibiotics has diminished the usefulness of the sulfonamides, and they presently occupy a relatively small place in the therapeutic armamentarium of the physician.

However, the introduction in the mid-1970s of the combination of trimethoprim and sulfamethoxazole has increased the use of sulfonamides for the prophylaxis and treatment of specific microbial infections.

Chemistry

The term sulfonamide is employed herein as a generic name for derivatives of para-aminobenzenesulfonamide (sulphanilamide).

Most of them are relatively insoluble in water, but their sodium salts are readily soluble.

The minimal structural prerequisites for antibacterial action are all embodied in sulfanilamide itself.

The ¾ SO2NH2 group is not essential as such, but the important feature is that the sulfur is linked directly to the benzene ring.

The para-NH2 group (the N of which has been designated as N4) is essential and can be replaced only by moieties that can be converted in vivo to a free amino group.

Substitutions made in the amide NH2 group (the N of which has been designated as N1) have variable effects on antibacterial activity of the molecule. However, substitution of heterocyclic aromatic nuclei at N1 yields highly potent compounds.

Effects on Microbes

Sulfonamides have a wide range of antimicrobial activity against both gram-positive and gram-negative bacteria.

However, resistant strains have become common, and the usefulness of these agents has diminished correspondingly.

In general, the sulfonamides exert only a bacteriostatic effect, and cellular and humoral defense mechanisms of the host are essential for final eradication of the infection.

Antibacterial Spectrum

Resistance to sulfonamides is increasingly a problem. Microorganisms that may be susceptible in vitro to sulfonamides include

Streptococcus pyogenes, Streptococcus pneumoniae, Haemophilus influenzae, Haemophilus ducreyi, Nocardia, Actinomyces, Calymmatobacterium granulomatis, and Chlamydia trachomatis.

Minimal inhibitory concentrations (MICs) range from 0.1 mg/ml for C. trachomatis to 4 to 64 mg/ml for Escherichia coli.

Peak plasma drug concentrations achievable in vivo are approximately 100 to 200 mg/ml.

Although sulfonamides were used successfully for the management of meningococcal infections for many years, the majority of isolates of Neisseria meningitidis of serogroups B and C in the United States and group A isolates from other countries are now resistant.

A similar situation prevails with respect to Shigella. Strains of E. coli isolated from patients with urinary tract infections

(community-acquired) often are resistant to sulfonamides, which are no longer the therapy of choice for such infections.

Mechanism of Action

Sulfonamides, structural analogs and competitive antagonists of para-aminobenzoic acid (PABA), prevent normal bacterial utilization of PABA for the synthesis of folic acid (pteroylglutamic acid).

More specifically, sulfonamides are competitive inhibitors of dihydropteroate synthase, the bacterial enzyme responsible for the incorporation of PABA into dihydropteroic acid, the immediate precursor of folic acid.

Sensitive microorganisms are those that must synthesize their own folic acid; bacteria that can use preformed folate are not affected.

Bacteriostasis induced by sulfonamides is counteracted by PABA competitively.

Sulfonamides do not affect mammalian cells by this mechanism because they require preformed folic acid and cannot synthesize it.

Thus, mammalian cells are comparable to sulfonamide-insensitive bacteria that use preformed folate.

Synergists of Sulfonamides

One of the most active agents that exerts a synergistic effect when used with a sulfonamide is trimethoprim (see Bushby and Hitchings, 1968).

This compound is a potent and selective competitive inhibitor of microbial dihydrofolate reductase, the enzyme that reduces dihydrofolate to tetrahydrofolate.

It is this reduced form of folic acid that is required for one-carbon transfer reactions.

The simultaneous administration of a sulfonamide and trimethoprim thus introduces sequential blocks in the pathway by which microorganisms synthesize tetrahydrofolate from precursor molecules.

8

The expectation that such a combination would yield synergistic antimicrobial effects has been realized both in vitro and in vivo.

Acquired Bacterial Resistance to Sulfonamides

Bacteria resistant to sulfonamides is presumed to originate by random mutation and selection or by transfer of resistance by plasmids (see Chapter 42). Such resistance, once it is maximally developed, usually is persistent and irreversible, particularly when produced in vivo.

Acquired resistance to sulfonamide usually does not involve cross-resistance to antimicrobial agents of other classes.

The in vivo acquisition of resistance has little or no effect on either virulence or antigenic characteristics of microorganisms.

Resistance to sulfonamide probably is the consequence of an altered enzymatic constitution of the bacterial cell; the alteration may be characterized by 1. A lower affinity for sulfonamides by dihydropteroate synthase.2. Decreased bacterial permeability or active efflux of the drug. 3. An alternative metabolic pathway for synthesis of an essential

metabolite.4. An increased production of an essential metabolite or drug antagonist.

For example, some resistant staphylococci may synthesize 70 times as much PABA as do the susceptible parent strains.

Nevertheless, an increased production of PABA is not a constant finding in sulfonamide-resistant bacteria, and resistant mutants may possess enzymes for folate biosynthesis that are less readily inhibited by sulfonamides.

Plasmid-mediated resistance is due to plasmid-encoded drug-resistant dihydropteroate synthetase.

Absorption, Fate, and Excretion

Except for sulfonamides especially designed for their local effects in the bowel, this class of drugs is absorbed rapidly from the gastrointestinal tract.

Approximately 70% to 100% of an oral dose is absorbed, and sulfonamide can be found in the urine within 30 minutes of ingestion.

Peak plasma levels are achieved in 2 to 6 hours, depending on the drug. The small intestine is the major site of absorption, but some of the drug is

absorbed from the stomach. Absorption from other sites, such as the vagina, respiratory tract, or abraded

skin, is variable and unreliable, but a sufficient amount may enter the body to cause toxic reactions in susceptible persons or to produce sensitization.

All sulfonamides are bound in varying degree to plasma proteins, particularly to albumin.

The extent to which this occurs is determined by the hydrophobicity of a particular drug and its pKa; at physiological pH, drugs with a high pKa exhibit a low degree of protein binding, and vice versa.

Sulfonamides are distributed throughout all tissues of the body. The diffusible fraction of sulfadiazine is distributed uniformly throughout the

total-body water, whereas sulfisoxazole is confined largely to the extracellular space.

The sulfonamides readily enter pleural, peritoneal, synovial, ocular, and similar body fluids and may reach concentrations therein that are 50% to 80% of the simultaneously determined concentration in blood.

Since the protein content of such fluids usually is low, the drug is present in the unbound active form.

After systemic administration of adequate doses, sulfadiazine and sulfisoxazole attain concentrations in cerebrospinal fluid that may be effective in meningeal infections.

At steady state, the concentration ranges between 10% and 80% of that in the blood.

However, because of the emergence of sulfonamide-resistant microorganisms, these drugs are used rarely for the treatment of meningitis.

Sulfonamides pass readily through the placenta and reach the fetal circulation.

The concentrations attained in the fetal tissues are sufficient to cause both antibacterial and toxic effects.

The sulfonamides undergo metabolic alterations in vivo, especially in the liver. The major metabolic derivative is the N4-acetylated sulfonamide. Acetylation, which occurs to a different extent with each agent, is

disadvantageous because the resulting products have no antibacterial activity and yet retain the toxic potential of the parent substance.

Sulfonamides are eliminated from the body partly as the unchanged drug and partly as metabolic products.

The largest fraction is excreted in the urine, and the half-life of sulfonamides in the body thus depends on renal function. In acid urine, the older sulfonamides are insoluble and may precipitate, forming crystalline deposits that can cause urinary obstruction.

Small amounts are eliminated in the feces, bile, milk, and other secretions.

Pharmacological Properties of Individual Sulfonamides

The sulfonamides may be classified into three groups on the basis of the rapidity with which they are absorbed and excreted: 1. Agents that are absorbed and excreted rapidly, such as sulfisoxazole

and sulfadiazine.2. Agents that are absorbed very poorly when administered orally and

hence are active in the bowel lumen, such as sulfasalazine.3. Agents that are used mainly topically, such as sulfacetamide,

mafenide, and silver sulfadiazine.4. Long-acting sulfonamides, such as sulfadoxine, that are absorbed

rapidly but excreted slowly.

Rapidly Absorbed and Eliminated Sulfonamides

Sulfisoxazole

Sulfisoxazole (GANTRISIN, others) is a rapidly absorbed and excreted sulfonamide with excellent antibacterial activity.

Since its high solubility eliminates much of the renal toxicity inherent in the use of older sulfonamides, it has essentially replaced the less-soluble agents.

Sulfisoxazole is bound extensively to plasma proteins. Following an oral dose of 2 to 4 g, peak concentrations in plasma of 110 to

250 mg/ml are found in 2 to 4 hours. From 28% to 35% of sulfisoxazole in the blood and about 30% in the urine is

in the acetylated form. Approximately 95% of a single dose is excreted by the kidney in 24 hours. Concentrations of the drug in urine thus greatly exceed those in blood and

may be bactericidal. The concentration in cerebrospinal fluid averages about a third of that in the

blood. Sulfisoxazole acetyl is tasteless and hence preferred for oral use in children. Sulfisoxazole acetyl is marketed in combination with erythromycin

ethylsuccinate (PEDIAZOLE, others) for use in children with otitis media. The urine becomes orange-red soon after ingestion of this mixture because of

the presence of phenazopyridine, an orange-red dye. Fewer than 0.1% of patients receiving sulfisoxazole suffer serious toxic

reactions. The untoward effects produced by this agent are similar to those which follow

the administration of other sulfonamides, as discussed below. Because of its relatively high solubility in the urine as compared with

sulfadiazine, sulfisoxazole only infrequently produces hematuria or crystalluria (0.2% to 0.3%).

Despite this, patients taking this drug should ingest an adequate quantity of water.

Sulfisoxazole and all sulfonamides that are absorbed must be used with caution in patients with impaired renal function.

Like all sulfonamides, sulfisoxazole may produce hypersensitivity reactions, some of which are potentially lethal.

Sulfisoxazole currently is preferred over other sulfonamides by most clinicians when a rapidly absorbed and rapidly excreted sulfonamide is indicated.

Sulfamethoxazole

Sulfamethoxazole is a close congener of sulfisoxazole, but its rates of enteric absorption and urinary excretion are slower.

It is administered orally and employed for both systemic and urinary tract infections.

Precautions must be observed to avoid sulfamethoxazole crystalluria because of the high percentage of the acetylated, relatively insoluble form of the drug in the urine.

The clinical uses of sulfamethoxazole are the same as those for sulfisoxazole. It also is marketed in fixed-dose combinations with trimethoprim (see below).

Sulfadiazine

Sulfadiazine given orally is absorbed rapidly from the GI tract, and peak blood concentrations are reached within 3 to 6 hours after a single dose. Following an oral dose of 3 g, peak concentrations in plasma are 50 mg/ml.

About 55% of the drug is bound to plasma protein at a concentration of 100 mg/ml when plasma protein levels are normal.

Therapeutic concentrations are attained in cerebrospinal fluid within 4 hours of a single oral dose of 60 mg/kg.

Sulfadiazine is excreted quite readily by the kidney in both the free and acetylated forms, rapidly at first and then more slowly over a period of 2 to 3 days. It can be detected in the urine within 30 minutes of oral ingestion.

About 15% to 40% of the excreted sulfadiazine is in acetylated form. This form of the drug is excreted more readily than the free fraction, and the

administration of alkali accelerates the renal clearance of both forms by further diminishing their tubular reabsorption.

In adults and children who are being treated with sulfadiazine, every precaution must be taken to ensure fluid intake adequate to produce a urine output of at least 1200 ml in adults and a corresponding quantity in children.

If this cannot be accomplished, sodium bicarbonate may be given to reduce the risk of crystalluria.

Poorly Absorbed Sulfonamides

Sulfasalazine (AZULFIDINE) is very poorly absorbed from the GI tract. It is used in the therapy of ulcerative colitis and regional enteritis, but relapses

tend to occur in about one-third of patients who experience a satisfactory initial response.

Corticosteroids are more effective in treating acute attacks, but sulfasalazine is preferred to corticosteroids for the treatment of patients who are mildly or moderately ill with ulcerative colitis.

The drug also is being employed as the first approach to treatment of relatively mild cases of regional enteritis and granulomatous colitis.

Sulfasalazine is broken down by intestinal bacteria to sulfapyridine, an active sulfonamide that is absorbed and eventually excreted in the urine, and 5-aminosalicylate, which reaches high levels in the feces.

5-Aminosalicylate is the effective agent in inflammatory bowel disease, whereas sulfapyridine is responsible for most of the toxicity.

Toxic reactions include Heinz-body anemia, acute hemolysis in patients with glucose-6-phosphate dehydrogenase deficiency, and agranulocytosis. Nausea, fever, arthralgias, and rashes occur in up to 20% of patients treated with the drug; desensitization has been an effective treatment.

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Sulfasalazine can cause a reversible infertility in males owing to changes in sperm number and morphology.

There is no evidence that the compound alters the intestinal microflora of patients with ulcerative colitis.

Sulfonamides for Topical Use

Sulfacetamide

Sulfacetamide is the N1-acetyl-substituted derivative of sulfanilamide. Its aqueous solubility (1:140) is approximately 90 times that of sulfadiazine. Solutions of the sodium salt of the drug (ISOPTO-CETAMIDE, others) are

employed extensively in the management of ophthalmic infections. Although topical sulfonamide for most purposes is discouraged because of

lack of efficacy and a high risk of sensitization, sulfacetamide has certain advantages.

Very high aqueous concentrations are not irritating to the eye and are effective against susceptible microorganisms.

A 30% solution of the sodium salt has a pH of 7.4, whereas the solutions of sodium salts of other sulfonamides are highly alkaline.

The drug penetrates into ocular fluids and tissues in high concentration. Sensitivity reactions to sulfacetamide are rare, but the drug should not be

used in patients with known hypersensitivity to sulfonamides.

Silver Sulfadiazine

Silver sulfadiazine (SILVADENE, others) inhibits the growth in vitro of nearly all pathogenic bacteria and fungi, including some species resistant to sulfonamides.

The compound is used topically to reduce microbial colonization and the incidence of infections of wounds from burns.

It should not be used to treat an established deep infection. Silver is released slowly from the preparation in concentrations that are

selectively toxic to the microorganisms. However, bacteria may develop resistance to silver sulfadiazine. Although little silver is absorbed, the plasma concentration of sulfadiazine

may approach therapeutic levels if a large surface area is involved. Adverse reactions burning, rash, and itching are infrequent. Silver sulfadiazine is considered by most authorities to be one of the agents

of choice for the prevention of burn infection.

Mafenide

This sulfonamide (a-amino-p-toluene-sulfonamide) is marketed as mafenide acetate (SULFAMYLON).

When applied topically, it is effective for the prevention of colonization of burns by a large variety of gram-negative and gram-positive bacteria.

It should not be used in treatment of an established deep infection. Superinfection with Candida occasionally may be a problem. The cream is applied once or twice daily to a thickness of 1 to 2 mm over the

burned skin. Cleansing of the wound and removal of debris should be carried out before

each application of the drug. Therapy is continued until skin grafting is possible. Mafenide is rapidly absorbed systemically and converted to para-

carboxybenzenesulfonamide. Studies of absorption from the burn surface indicate that peak plasma

concentrations are reached in 2 to 4 hours. Adverse effects include intense pain at sites of application, allergic reactions,

and loss of fluid by evaporation from the burn surface because occlusive dressings are not used.

The drug and its primary metabolite inhibit carbonic anhydrase, and the urine becomes alkaline.

Metabolic acidosis with compensatory tachypnea and hyperventilation may ensue; these effects limit the usefulness of mafenide.

Long-Acting Sulfonamides

Sulfadoxine (N1-[5,6-dimethoxy-4-pyrimidinyl] sulfanilamide) has a particularly long half-life (7 to 9 days).

It is used in combination with pyrimethamine (500 mg sulfadoxine plus 25 mg pyrimethamine as FANSIDAR) for the prophylaxis and treatment of malaria caused by mefloquine -resistant strains of Plasmodium falciparum.

Because of severe and sometimes fatal reactions, including the Stevens-Johnson syndrome, the drug should be used for prophylaxis only where the risk of resistant malaria is high.

Sulfonamide Therapy

The number of conditions for which the sulfonamides are therapeutically useful and constitute drugs of first choice has been reduced sharply by the development of more effective antimicrobial agents and by the gradual increase in the resistance of a number of bacterial species to this class of drugs.

However, introduction of the combination of trimethoprim and sulfamethoxazole has revived the use of sulfonamides.

Urinary Tract Infections

Since a significant percentage of urinary tract infections in many parts of the world are caused by sulfonamide-resistant microorganisms, sulfonamides are no longer a therapy of first choice.

Trimethoprim-sulfamethoxazole, a quinolone, trimethoprim, fosfomycin, or ampicillin are the preferred agents.

However, sulfisoxazole may be used effectively in areas where the prevalence of resistance is not high or when the organism is known to be sensitive. The usual dosage is 2 to 4 g initially followed by 1 to 2 g, orally four times a day for 5 to 10 days.

Patients with acute pyelonephritis with high fever and other severe constitutional manifestations are at risk of bacteremia and shock and should not be treated with a sulfonamide.

Use of Sulfonamides for Prophylaxis

The sulfonamides are as efficacious as oral penicillin in preventing streptococcal infections and recurrences of rheumatic fever among susceptible subjects.

Despite the efficacy of sulfonamides for long-term prophylaxis of rheumatic fever, their toxicity and the possibility of infection by drug-resistant streptococci make sulfonamides less desirable than penicillin for this purpose.

They should be used, however, without hesitation in patients who are hypersensitive to penicillin.

If untoward responses occur, they usually do so during the first 8 weeks of therapy; serious reactions after this time are rare.

White blood cell counts should be carried out once weekly during the first 8 weeks.

Untoward Reactions to Sulfonamides

The untoward effects that follow the administration of sulfonamides are numerous and varied; the overall incidence of reactions is about 5%.

Certain forms of toxicity may be related to individual differences in sulfonamide metabolism.

Disturbances of the Urinary Tract

Although the risk of crystalluria was relatively high with the older, less soluble sulfonamides, the incidence of this problem is very low with more soluble agents such as sulfisoxazole.

Crystalluria has occurred in dehydrated patients with the acquired immune deficiency syndrome (AIDS) who were receiving sulfadiazine for Toxoplasma encephalitis.

Fluid intake should be sufficient to ensure a daily urine volume of at least 1200 ml (in adults).

Alkalinization of the urine may be desirable if urine volume or pH is unusually low because the solubility of sulfisoxazole increases greatly with slight elevations of pH.

Hypersensitivity Reactions

The incidence of other hypersensitivity reactions to sulfonamides is quite variable.

Among the skin and mucous membrane manifestations attributed to sensitization to sulfonamide are morbilliform, scarlatinal, urticarial, erysipeloid, pemphigoid, purpuric, and petechial rashes, as well as erythema nodosum, erythema multiforme of the Stevens-Johnson type, Behcet's syndrome, exfoliative dermatitis, and photosensitivity.

These hypersensitivity reactions occur most often after the first week of therapy but may appear earlier in previously sensitized individuals.

Fever, malaise, and pruritus frequently are present simultaneously. The incidence of untoward dermal effects is about 2% with sulfisoxazole,

although patients with AIDS manifest a higher frequency of rashes with sulfonamide treatment than do other individuals.

A syndrome similar to serum sickness may appear after several days of sulfonamide therapy.

Drug fever is a common untoward manifestation of sulfonamide treatment; the incidence approximates 3% with sulfisoxazole.

Focal or diffuse necrosis of the liver owing to direct drug toxicity or sensitization occurs in fewer than 0.1% of patients.

Headache, nausea, vomiting, fever, hepatomegaly, jaundice, and laboratory evidence of hepatocellular dysfunction usually appear 3 to 5 days after sulfonamide administration is started, and the syndrome may progress to acute yellow atrophy and death.

Miscellaneous Reactions

Anorexia, nausea, and vomiting occur in 1% to 2% of persons receiving sulfonamides, and these manifestations probably are central in origin.

The administration of sulfonamides to newborn infants, especially if premature, may lead to the displacement of bilirubin from plasma albumin.

In newborn infants, free bilirubin can become deposited in the basal ganglia and subthalamic nuclei of the brain, causing an encephalopathy called kernicterus.

Sulfonamides should not be given to pregnant women near term because these drugs pass through the placenta and are secreted in milk.

Drug Interactions

The most important interactions of the sulfonamides involve those with the oral anticoagulants, the sulfonylurea hypoglycemic agents, and the hydantoin anticonvulsants.

In each case, sulfonamides can potentiate the effects of the other drug by mechanisms that appear to involve primarily inhibition of metabolism and, possibly, displacement from albumin.

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Dosage adjustment may be necessary when a sulfonamide is given concurrently.

(Goodman and Gilman’s The Pharmacological Basis of Therapeutics 11th Edition)

7.2 Quinolones

The first quinolone, nalidixic acid, was isolated as a by-product of the synthesis of chloroquine.

It has been available for the treatment of urinary tract infections for many years.

The introduction of fluorinated 4-quinolones, such as ciprofloxacin (CIPRO), moxifloxacin (AVELOX), and gatifloxacin (TEQUIN) represents a particularly important therapeutic advance because these agents have broad antimicrobial activity and are effective after oral administration for the treatment of a wide variety of infectious diseases.

Relatively few side effects appear to accompany the use of these fluoroquinolones, and microbial resistance to their action does not develop rapidly.

Rare and potentially fatal side effects, however, have resulted in the withdrawal from the market of temafloxacin (immune hemolytic anemia), trovafloxacin (hepatotoxicity), grepafloxacin (cardiotoxicity), and clinafloxacin (phototoxicity).

In all these cases, the side effects were so infrequent as to be missed by prerelease clinical trials and detected only by postmarketing surveillance.

Chemistry

The compounds that currently are available for clinical use in the United States are quinolones containing a carboxylic acid moiety at position 3 of the primary ring structure.

Many of the newer fluoroquinolones also contain a fluorine substituent at position 6 and a piperazine moiety at position 7.

Mechanism of Action

The quinolone antibiotics target bacterial DNA gyrase and topoisomerase IV. For many gram-positive bacteria (such as S. aureus), topoisomerase IV is the

primary activity inhibited by the quinolones. In contrast, for many gram-negative bacteria (such as E. coli), DNA gyrase is

the primary quinolone target. The individual strands of double-helical DNA must be separated to permit

DNA replication or transcription. However, anything that separates the strands results in "overwinding" or

excessive positive supercoiling of the DNA in front of the point of separation. To combat this mechanical obstacle, the bacterial enzyme DNA gyrase is

responsible for the continuous introduction of negative supercoils into DNA. This is an ATP-dependent reaction requiring that both strands of the DNA be

cut to permit passage of a segment of DNA through the break; the break then is resealed.

The DNA gyrase of E. coli is composed of two 105,000-dalton A subunits and two 95,000-dalton B subunits encoded by the gyrA and gyrB genes, respectively.

The A subunits, which carry out the strand-cutting function of the gyrase, are the site of action of the quinolones.

The drugs inhibit gyrase-mediated DNA supercoiling at concentrations that correlate well with those required to inhibit bacterial growth (0.1 to 10 mg/ml). Mutations of the gene that encodes the A subunit polypeptide can confer resistance to these drugs.

Topoisomerase IV also is composed of four subunits encoded by the parC and parE genes in E. coli.

Topoisomerase IV separates interlinked (catenated) daughter DNA molecules that are the product of DNA replication. Eukaryotic cells do not contain DNA gyrase.

However, they do contain a conceptually and mechanistically similar type II DNA topoisomerase that removes positive supercoils from eukaryotic DNA to prevent its tangling during replication.

This enzyme is the target for some antineoplastic agents. Quinolones inhibit eukaryotic type II topoisomerase only at much higher

concentrations (100 to 1000 mg/ml).

Antibacterial Spectrum

The fluoroquinolones are potent bactericidal agents against E. coli and various species of Salmonella, Shigella, Enterobacter, Campylobacter, and Neisseria.

Minimal inhibitory concentrations of the fluoroquinolones for 90% of these strains (MIC90) usually are less than 0.2 mg/ml.

Ciprofloxacin is more active than norfloxacin (NOROXIN) against P. aeruginosa; values of MIC90 range from 0.5 to 6 mg/ml.

Fluoroquinolones also have good activity against staphylococci, but not against methicillin-resistant strains (MIC90 = 0.1 to 2 mg/ml).

Activity against streptococci is limited to a subset of the quinolones, including levofloxacin (LEVAQUIN), gatifloxacin (TEQUIN), and moxifloxacin (AVELOX).

Several intracellular bacteria are inhibited by fluoroquinolones at concentrations that can be achieved in plasma; these include species of Chlamydia, Mycoplasma, Legionella, Brucella, and Mycobacterium (including Mycobacterium tuberculosis).

Ciprofloxacin, ofloxacin (FLOXIN), and pefloxacin have MIC90 values from 0.5 to 3 mg/ml for M. fortuitum, M. kansasii, and M. tuberculosis; ofloxacin and pefloxacin are active in animal models of leprosy.

However, clinical experience with these pathogens remains limited. Several of the new fluoroquinolones have activity against anaerobic bacteria,

including garenoxacin and gemifloxacin. Resistance to quinolones may develop during therapy via mutations in the

bacterial chromosomal genes encoding DNA gyrase or topoisomerase IV or by active transport of the drug out of the bacteria.

No quinolone-modifying or -inactivating activities have been identified in bacteria.

Resistance has increased after the introduction of fluoroquinolones, especially in Pseudomonas and staphylococci.

Increasing fluoroquinolone resistance also is being observed in C. jejuni, Salmonella, N. gonorrhoeae, and S. pneumoniae.

The pharmacokinetic and pharmacodynamic parameters of antimicrobial agents are important in preventing the selection and spread of resistant strains and have led to description of the mutation-prevention concentration, which is the lowest concentration of antimicrobial that prevents selection of resistant bacteria from high bacterial inocula.

β-Lactams are time-dependent agents without significant postantibiotic effects, resulting in bacterial eradication when unbound serum concentrations exceed MICs of these agents against infecting pathogens for more than 40% to 50% of the dosing interval.

By contrast, fluoroquinolones are concentration-dependent agents, resulting in bacterial eradication when unbound serum area-under-the-curve-to-MIC ratios exceed 25 to 30.

These observations are now being used to assess the roles of current agents, develop new formulations, and assess potency of new antimicrobials.

Absorption, Fate, and Excretion

The quinolones are well absorbed after oral administration and are distributed widely in body tissues.

Peak serum levels of the fluoroquinolones are obtained within 1 to 3 hours of an oral dose of 400 mg, with peak levels ranging from 1.1 mg/ml for sparfloxacin to 6.4 mg/ml for levofloxacin.

Relatively low serum levels are reached with norfloxacin and limit its usefulness to the treatment of urinary tract infections.

Food does not impair oral absorption but may delay the time to peak serum concentrations.

Oral doses in adults are 200 to 400 mg every 12 hours for ofloxacin, 400 mg every 12 hours for norfloxacin and pefloxacin, and 250 to 750 mg every 12 hours for ciprofloxacin.

Bioavailability of the fluoroquinolones is greater than 50% for all agents and greater than 95% for several.

The serum half-life ranges from 3 to 5 hours for norfloxacin and ciprofloxacin to 20 hours for sparfloxacin.

The volume of distribution of quinolones is high, with concentrations of quinolones in urine, kidney, lung and prostate tissue, stool, bile, and macrophages and neutrophils higher than serum levels.

Quinolone concentrations in cerebrospinal fluid, bone, and prostatic fluid are lower than in serum.

Pefloxacin and ofloxacin levels in ascites fluid are close to serum levels, and ciprofloxacin, ofloxacin, and pefloxacin have been detected in human breast milk.

Most quinolones are cleared predominantly by the kidney, and dosages must be adjusted for renal failure.

Exceptions are pefloxacin and moxifloxacin, which are metabolized predominantly by the liver and should not be used in patients with hepatic failure. None of the agents is removed efficiently by peritoneal dialysis or hemodialysis.

Therapeutic Uses

Urinary Tract Infections

Nalidixic acid is useful only for urinary tract infections caused by susceptible microorganisms.

The fluoroquinolones are significantly more potent and have a much broader spectrum of antimicrobial activity.

Norfloxacin is approved for use in the United States only for urinary tract infections.

Comparative clinical trials indicate that the fluoroquinolones are more efficacious than trimethoprim-sulfamethoxazole for the treatment of urinary tract infections.

Prostatitis

Norfloxacin, ciprofloxacin, and ofloxacin all have been effective in uncontrolled trials for the treatment of prostatitis caused by sensitive bacteria.

Fluoroquinolones administered for 4 to 6 weeks appear to be effective in patients not responding to trimethoprim-sulfamethoxazole.

Sexually Transmitted Diseases

The quinolones are contraindicated in pregnancy. Fluoroquinolones lack activity for Treponema pallidum but have activity in

vitro against N. gonorrhoeae, C. trachomatis, and H. ducreyi. For chlamydial urethritis/cervicitis, a 7-day course of ofloxacin or sparfloxacin

is an alternative to a 7-day treatment with doxycycline or a single dose of azithromycin; other available quinolones have not been reliably effective.

A single oral dose of a fluoroquinolone such as ofloxacin or ciprofloxacin is effective treatment for sensitive strains of N. gonorrhoeae, but increasing resistance to fluoroquinolones has led to ceftriaxone being the first-line agent for this infection.

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Pelvic inflammatory disease has been treated effectively with a 14-day course of ofloxacin combined with an antibiotic with activity against anaerobes (clindamycin or metronidazole).

Chancroid (infection by H. ducreyi) can be treated with 3 days of ciprofloxacin.

Adverse Effects

Quinolones and fluoroquinolones generally are well tolerated. The most common adverse reactions involve the GI tract, with 3% to 17% of

patients reporting mostly mild nausea, vomiting, and/or abdominal discomfort. Diarrhea and antibiotic-associated colitis have been unusual. CNS side effects, predominately mild headache and dizziness, have been

seen in 0.9% to 11% of patients. Rarely, hallucinations, delirium, and seizures have occurred, predominantly in

patients who also were receiving theophylline or a nonsteroidal antiinflammatory drug.

Ciprofloxacin and pefloxacin inhibit the metabolism of theophylline, and toxicity from elevated concentrations of the methylxanthine may occur.

Nonsteroidal antiinflammatory drugs may augment displacement of g-aminobutyric acid (GABA) from its receptors by the Quinolones.

Rashes, including photosensitivity reactions, also can occur. Achilles tendon rupture or tendinitis has occurred rarely. Renal disease, hemodialysis, and steroid use may be predisposing factors. All these agents can produce arthropathy in several species of immature

animals. Traditionally, the use of quinolones in children has been contraindicated for

this reason. However, children with cystic fibrosis given ciprofloxacin, norfloxacin, and

nalidixic acid have had few, and reversible, joint symptoms. Therefore, in some cases the benefits may outweigh the risks of quinolone

therapy in children. Leukopenia, eosinophilia, and mild elevations in serum transaminases occur

rarely. QTc interval (QT interval corrected for heart rate) prolongation has been

observed with sparfloxacin and to a lesser extent with gatifloxacin and moxifloxacin.

Quinolones probably should be used only with caution in patients on class III (amiodarone) and class IA (quinidine, procainamide) antiarrhythmics.

ANTISEPTIC AND ANALGESIC AGENTS FOR URINARY TRACT INFECTIONS

The urinary tract antiseptics inhibit the growth of many species of bacteria. They cannot be used to treat systemic infections because effective

concentrations are not achieved in plasma with safe doses. However, because they are concentrated in the renal tubules, they can be

administered orally to treat infections of the urinary tract. Furthermore, effective antibacterial concentrations reach the renal pelves and

the bladder. Treatment with such drugs can be thought of as local therapy: only in the

kidney and bladder, with the rare exceptions mentioned below, are adequate therapeutic levels achieved.

Methenamine

Methenamine is a urinary tract antiseptic and prodrug that owes its activity to its capacity to generate formaldehyde.

Chemistry

Methenamine is hexamethylenetetramine (hexamethylenamine). It has the following structure:

The compound decomposes in water to generate formaldehyde, according to the following reaction:

At pH 7.4, almost no decomposition occurs; however, the yield of formaldehyde is 6% of the theoretical amount at pH 6 and 20% at pH 5.

Thus, acidification of the urine promotes the formaldehyde-dependent antibacterial action.

The reaction is fairly slow, and 3 hours are required to reach 90% completion.

Antimicrobial Activity

Nearly all bacteria are sensitive to free formaldehyde at concentrations of about 20 mg/ml.

Urea-splitting microorganisms (e.g., Proteus spp.) tend to raise the pH of the urine and thus inhibit the release of formaldehyde.

Microorganisms do not develop resistance to formaldehyde.

Pharmacology and Toxicology

Methenamine is absorbed orally, but 10% to 30% decomposes in the gastric juice unless the drug is protected by an enteric coating.

Because of the ammonia produced, methenamine is contraindicated in hepatic insufficiency.

Excretion in the urine is nearly quantitative. When the urine pH is 6 and the daily urine volume is 1000 to 1500 ml, a daily

dose of 2 g will yield a concentration of 18 to 60 mg/ml of formaldehyde; this is more than the MIC for most urinary tract pathogens.

Various poorly metabolized acids can be used to acidify the urine. Low pH alone is bacteriostatic, so acidification serves a double function.

The acids commonly used are mandelic acid and hippuric acid (UREX, HIPREX).

Gastrointestinal distress frequently is caused by doses greater than 500 mg four times a day, even with enteric-coated tablets.

Painful and frequent micturition, albuminuria, hematuria, and rashes may result from doses of 4 to 8 g/day given for longer than 3 to 4 weeks.

Once the urine is sterile, a high dose should be reduced. Because systemic methenamine has low toxicity at the typically used doses,

renal insufficiency does not constitute a contraindication to the use of methenamine alone, but the acids given concurrently may be detrimental.

Methenamine mandelate is contraindicated in renal insufficiency. Crystalluria from the mandelate moiety can occur. Methenamine combines with sulfamethizole and perhaps other sulfonamides

in the urine, which results in mutual antagonism.

Therapeutic Uses and Status

Methenamine is not a primary drug for the treatment of acute urinary tract infections, but it is of value for chronic suppressive treatment.

The agent is most useful when the causative organism is E. coli, but it usually can suppress the common gram-negative offenders and often S. aureus and S. epidermidis as well.

Enterobacter aerogenes and Proteus vulgaris are usually resistant. Urea-splitting bacteria (mostly Proteus) make it difficult to control the urine

pH. The physician should strive to keep the pH below 5.5.

(Goodman and Gilman’s The Pharmacological Basis of Therapeutics 11th Edition)

7.3 Nitrofurantoin

Nitrofurantoin (FURADANTIN, MACROBID, others) is a synthetic nitrofuran that is used for the prevention and treatment of infections of the urinary tract.

Antimicrobial Activity

Enzymes capable of reducing nitrofurantoin appear to be crucial for its activation.

Highly reactive intermediates are formed, and these seem to be responsible for the observed capacity of the drug to damage DNA.

Bacteria reduce nitrofurantoin more rapidly than do mammalian cells, and this is thought to account for the selective antimicrobial activity of the compound.

Bacteria that are susceptible to the drug rarely become resistant during therapy.

Nitrofurantoin is active against many strains of E. coli and enterococci. However, most species of Proteus and Pseudomonas and many species of Enterobacter and Klebsiella are resistant.

Nitrofurantoin is bacteriostatic for most susceptible microorganisms at concentrations of 32 mg/ml or less and is bactericidal at concentrations of 100 mg/ml and more.

The antibacterial activity is higher in an acidic urine.

Pharmacology and Toxicity

Nitrofurantoin is absorbed rapidly and completely from the GI tract. The macrocrystalline form of the drug is absorbed and excreted more slowly. Antibacterial concentrations are not achieved in plasma following ingestion of

recommended doses because the drug is eliminated rapidly. The plasma half-life is 0.3 to 1 hour; about 40% is excreted unchanged into

the urine. The average dose of nitrofurantoin yields a concentration in urine of

approximately 200 mg/ml. This concentration is soluble at pH >5, but the urine should not be alkalinized

because this reduces antimicrobial activity. The rate of excretion is linearly related to the creatinine clearance, so in

patients with impaired glomerular function, the efficacy of the drug may be decreased and the systemic toxicity increased.

Nitrofurantoin colors the urine brown. The most common untoward effects are nausea, vomiting, and diarrhea; the

macrocrystalline preparation is better tolerated. Various hypersensitivity reactions occur occasionally. These include chills, fever, leukopenia, granulocytopenia, hemolytic anemia

[associated with glucose-6-phosphate dehydrogenase deficiency, cholestatic jaundice, and hepatocellular damage.

Chronic active hepatitis is an uncommon but serious side effect. Acute pneumonitis with fever, chills, cough, dyspnea, chest pain, pulmonary

infiltration, and eosinophilia may occur within hours to days of the initiation of therapy; these symptoms usually resolve quickly after discontinuation of the drug.

More insidious subacute reactions also may be noted, and interstitial pulmonary fibrosis can occur in patients taking the drug chronically.

This appears to be due to generation of oxygen radicals as a result of redox cycling of the drug in the lung.

Elderly patients are especially susceptible to the pulmonary toxicity of Nitrofurantoin.

Megaloblastic anemia is rare. Various neurological disorders are observed occasionally. Headache, vertigo, drowsiness, muscular aches, and nystagmus are readily

reversible, but severe polyneuropathies with demyelination and degeneration of both sensory and motor nerves have been reported; signs of denervation and muscle atrophy result.

Neuropathies are most likely to occur in patients with impaired renal function and in persons on long-continued treatment.

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Toxic reactive metabolites may contribute to some adverse reactions. The oral dosage of nitrofurantoin for adults is 50 to 100 mg four times a day

with meals and at bedtime. Alternatively, the daily dosage is better expressed as 5 to 7 mg/kg in four

divided doses (not to exceed 400 mg). A single 50- to 100-mg dose at bedtime may be sufficient to prevent

recurrences. The daily dose for children is 5 to 7 mg/kg but may be as low as 1 mg/kg for

long-term therapy. A course of therapy should not exceed 14 days, and repeated courses should

be separated by rest periods. Pregnant women, individuals with impaired renal function (creatinine

clearance < 40 ml/min), and children younger than 1 month of age should not receive nitrofurantoin.

Nitrofurantoin is approved only for the treatment of urinary tract infections caused by microorganisms known to be susceptible to the drug.

Currently, bacterial resistance to nitrofurantoin is more frequent than resistance to fluoroquinolones or trimethoprim-sulfamethoxazole, making nitrofurantoin a second-line agent for treatment of urinary tract infections.

Nitrofurantoin also is not recommended for treatment of pyelonephritis or prostatitis.

However, nitrofurantoin is effective for prophylaxis of recurrent urinary tract infections.

Phenazopyridine

Phenazopyridine hydrochloride (PYRIDIUM, others) is not a urinary antiseptic.

However, it does have an analgesic action on the urinary tract and alleviates symptoms of dysuria, frequency, burning, and urgency.

The usual dose is 200 mg three times daily. The compound is an azo dye, which colors urine orange or red; the patient

should be so informed. Gastrointestinal upset is seen in up to 10% of patients and can be reduced by

administering the drug with food; overdosage may result in methemoglobinemia.

Phenazopyridine has been marketed since 1925 and has had dual prescription/over-the-counter (OTC) marketing status since 1951.

As part of their ongoing review of OTC drug products, the FDA is currently in the process of evaluating products containing less than 200 mg phenazopyridine to determine whether these products generally are recognized as safe and effective as urinary analgesics.

The outcome of this evaluation will determine the continued availability of OTC phenazopyridine products in the United States.

Products containing 200 mg phenazopyridine are sold by prescription, but their long-term availability in the marketplace also may be affected by the FDA's final OTC ruling.

(Goodman and Gilman’s The Pharmacological Basis of Therapeutics 11th Edition)

7.4 Aminoglycosides

The aminoglycoside group includes gentamicin, tobramycin, amikacin, netilmicin, kanamycin, streptomycin, and neomycin.

These drugs are used primarily to treat infections caused by aerobic gram-negative bacteria; streptomycin is an important agent for the treatment of tuberculosis.

In contrast to most inhibitors of microbial protein synthesis, which are bacteriostatic, the aminoglycosides are bactericidal inhibitors of protein synthesis. Mutations affecting proteins in the bacterial ribosome, the target for these drugs, can confer marked resistance to their action.

However, most commonly resistance is due to acquisition of plasmids or transposon-encoding genes for aminoglycoside-metabolizing enzymes or from impaired transport of drug into the cell.

Thus there can be cross-resistance between members of the class. These agents contain amino sugars linked to an aminocyclitol ring by

glycosidic bonds. They are polycations, and their polarity is responsible in part for

pharmacokinetic properties shared by all members of the group. For example, none is absorbed adequately after oral administration,

inadequate concentrations are found in cerebrospinal fluid (CSF), and all are excreted relatively rapidly by the normal kidney.

Although aminoglycosides are widely used and important agents, serious toxicity limits their usefulness.

All members of the group share the same spectrum of toxicity, most notably nephrotoxicity and ototoxicity, which can involve the auditory and vestibular functions of the eighth cranial nerve.

History and Source

Aminoglycosides are natural products or semisynthetic derivatives of compounds produced by a variety of soil actinomycetes.

Streptomycin was first isolated from a strain of Streptomyces griseus. Gentamicin and netilmicin are broad-spectrum antibiotics derived from

species of the actinomycete Micromonospora. The difference in spelling (-micin) compared with the other aminoglycoside

antibiotics (-mycin) reflects this difference in origin. Tobramycin is one of several components of an aminoglycoside complex

(nebramycin) that is produced by S. tenebrarius. It is most similar in antimicrobial activity and toxicity to gentamicin. In contrast to the other aminoglycosides, amikacin, a derivative of kanamycin,

and netilmicin, a derivative of sisomicin, are semisynthetic products.

Other aminoglycoside antibiotics have been developed (e.g., arbekacin, isepamicin, and sisomicin), but they have not been introduced into clinical practice in the United States because numerous potent, less toxic alternatives (e.g., broad-spectrum b-lactam antibiotics and quinolones) are available.

Chemistry

The aminoglycosides consist of two or more amino sugars joined in glycosidic linkage to a hexose nucleus, which usually is in a central position.

This hexose, or aminocyclitol, is either streptidine (found in streptomycin) or 2-deoxystreptamine (found in all other available aminoglycosides).

These compounds thus are aminoglycosidic aminocyclitols, although the simpler term aminoglycoside is used commonly to describe them.

A related compound, spectinomycin, is an aminocyclitol that does not contain amino sugars.

The aminoglycoside families are distinguished by the amino sugars attached to the aminocyclitol.

In the neomycin family, which includes neomycin B and paromomycin, an aminoglycoside used orally for the treatment of intestinal parasitic infections, there are three amino sugars attached to the central 2-deoxystreptamine.

The kanamycin and gentamicin families have only two such amino sugars. Neomycin B has the following structural formula: In the kanamycin family, which includes kanamycins A and B, amikacin, and

tobramycin, two amino sugars are linked to a centrally located 2-deoxystreptamine moiety; one of these is a 3-aminohexose.

Amikacin is a semisynthetic derivative prepared from kanamycin A by acylation of the 1-amino group of the 2-deoxystreptamine moiety with 2-hydroxy-4-aminobutyric acid.

The gentamicin family, which includes gentamicins C1 , C1a , and C2 , sisomicin, and netilmicin (the 1-N-ethyl derivative of sisomicin), contains a different 3-amino sugar (garosamine). Variations in methylation of the other amino sugar result in the different components of gentamicin.

These modifications appear to have little effect on biological activity. Streptomycin differs from the other aminoglycoside antibiotics in that it

contains streptidine rather than 2-deoxystreptamine, and the aminocyclitol is not in a central position.

Mechanism of Action

The aminoglycoside antibiotics are rapidly bactericidal. Bacterial killing is concentration-dependent:

The higher the concentration, the greater is the rate at which bacteria are killed.

A post-antibiotic effect, i.e., residual bactericidal activity persisting after the serum concentration has fallen below the minimum inhibitory concentration (MIC), also is characteristic of aminoglycoside antibiotics; the duration of this effect also is concentration dependent.

These properties probably account for the efficacy of once-daily dosing regimens of aminoglycosides. Although much is known about their capacity to inhibit protein synthesis and decrease the fidelity of translation of mRNA at the ribosome, the precise mechanism responsible for the rapidly lethal effect of aminoglycosides on bacteria is unknown.

Aminoglycosides diffuse through aqueous channels formed by porin proteins in the outer membrane of gram-negative bacteria to enter the periplasmic space.

Transport of aminoglycosides across the cytoplasmic (inner) membrane depends on electron transport in part because of a requirement for a membrane electrical potential (interior negative) to drive permeation of these antibiotics.

This phase of transport has been termed energy-dependent phase I (EDP1). It is rate-limiting and can be blocked or inhibited by divalent cations (e.g.,

Ca2+ and Mg2+), hyperosmolarity, a reduction in pH, and anaerobic conditions. The last two conditions impair the ability of the bacteria to maintain the membrane potential, which is the driving force necessary for transport.

Thus the antimicrobial activity of aminoglycosides is reduced markedly in the anaerobic environment of an abscess, in hyperosmolar acidic urine, and in other conditions that limit EDP1.

Once inside the cell, aminoglycosides bind to polysomes and interfere with protein synthesis by causing misreading and premature termination of mRNA translation (Figure 45-2). The resulting aberrant proteins may be inserted into the cell membrane, leading to altered permeability and further stimulation of aminoglycoside transport.

This phase of aminoglycoside transport, termed energy-dependent phase II (EDP2), is poorly understood; however, EDP2 may link to disruption of the structure of the cytoplasmic membrane, perhaps by the aberrant proteins.

This concept is consistent with the observed progression of the leakage of small ions, followed by larger molecules and, eventually, by proteins from the bacterial cell prior to aminoglycoside-induced death.

This progressive disruption of the cell envelope, as well as other vital cell processes, may help to explain the lethal action of aminoglycosides.

The primary intracellular site of action of the aminoglycosides is the 30S ribosomal subunit, which consists of 21 proteins and a single 16S molecule of RNA.

At least three of these ribosomal proteins, and perhaps the 16S ribosomal RNA as well, contribute to the streptomycin-binding site, and alterations of these molecules markedly affect the binding and subsequent action of streptomycin.

For example, a single amino acid substitution of asparagine for lysine at position 42 of one ribosomal protein (S12) prevents binding of the drug; the resulting mutant is totally resistant to streptomycin.

Substitution of glutamine for lysine creates a mutant that actually requires streptomycin for survival.

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The other aminoglycosides also bind to the 30S ribosomal subunit; however, they also appear to bind to several sites on the 50S ribosomal subunit.

Aminoglycosides also cause misreading of the mRNA template and incorporation of incorrect amino acids into the growing polypeptide chains. Aminoglycosides vary in their capacity to cause misreading presumably owing to differences in their affinities for specific ribosomal proteins.

Although there appears to be a strong correlation between bactericidal activity and the ability to induce misreading, it remains to be established that this is the primary mechanism of aminoglycoside-induced cell death.

Microbial Resistance to the Aminoglycosides

Bacteria may be resistant to aminoglycosides because of failure of the antibiotic to penetrate intracellularly, low affinity of the drug for the bacterial ribosome, or inactivation of the drug by microbial enzymes.

Clinically, drug inactivation is the most common mechanism for acquired microbial resistance to aminoglycosides.

The genes encoding aminoglycoside-modifying enzymes are acquired primarily by conjugation and transfer of resistance plasmids.

These enzymes phosphorylate, adenylate, or acetylate specific hydroxyl or amino groups.

Amikacin is a suitable substrate for only a few of these inactivating enzymes, thus strains that are resistant to multiple other drugs tend to be susceptible to amikacin.

The metabolites of the aminoglycosides may compete with the unaltered drug for transport across the inner membrane, but they are incapable of binding effectively to ribosomes and interfering with protein synthesis.

A significant percentage of clinical isolates of Enterococcus faecalis and E. faecium are highly resistant to all aminoglycosides.

Infections caused by aminoglycoside-resistant strains of enterococci can be especially difficult to treat because of the loss of the synergistic bactericidal activity between a penicillin or vancomycin and an aminoglycosid and because these strains often also are cross-resistant to vancomycin and penicillin.

Resistance to gentamicin indicates cross-resistance to tobramycin, amikacin, kanamycin, and netilmicin because the inactivating enzyme is bifunctional and can modify all these aminoglycosides.

Owing to differences in the chemical structures of streptomycin and other aminoglycosides, this enzyme does not modify streptomycin, which is inactivated by another enzyme; consequently, gentamicin-resistant strains of enterococci may be susceptible to streptomycin.

Natural resistance to aminoglycosides may be caused by failure of the drug to penetrate the cytoplasmic (inner) membrane.

Penetration of drug across the outer membrane of gram-negative microorganisms into the periplasmic space can be slow, but resistance on this basis is unimportant clinically.

Transport of aminoglycosides across the cytoplasmic membrane is an oxygen-dependent active process.

Strictly anaerobic bacteria thus are resistant to these drugs because they lack the necessary transport system.

Similarly, facultative bacteria are resistant when they are grown under anaerobic conditions.

Resistance owing to mutations that alter ribosomal structure is relatively uncommon.

Missense mutations in Escherichia coli that substitute a single amino acid in a crucial ribosomal protein may prevent binding of streptomycin.

Although highly resistant to streptomycin, these strains are not widespread in nature.

Similarly, only 5% of strains of Pseudomonas aeruginosa exhibit such ribosomal resistance to streptomycin.

It has been estimated that approximately half the streptomycin-resistant strains of enterococci are ribosomally resistant.

Because ribosomal resistance usually is specific for streptomycin, these strains of enterococci remain sensitive to a combination of penicillin and gentamicin in vitro.

Antibacterial Spectrum of the Aminoglycosides

The antibacterial activity of gentamicin, tobramycin, kanamycin, netilmicin, and amikacin is directed primarily against aerobic gram-negative bacilli. Kanamycin, like streptomycin, has a more limited spectrum compared with other aminoglycosides; in particular, it should not be used to treat infections caused by Serratia or P. aeruginosa.

Aminoglycosides have little activity against anaerobic microorganisms or facultative bacteria under anaerobic conditions.

Their action against most gram-positive bacteria is limited, and they should not be used as single agents to treat infections caused by gram-positive bacteria.

In combination with a cell wall-active agent, such as a penicillin or vancomycin, an aminoglycoside (streptomycin and gentamicin have been tested most extensively) produces a synergistic bactericidal effect in vitro against enterococci, streptococci, and staphylococci.

Clinically, the superiority of aminoglycoside combination regimens over b-lactams alone is not proven except in relatively few infections (discussed below).

The aerobic gram-negative bacilli vary in their susceptibility to the aminoglycosides.

Tobramycin and gentamicin exhibit similar activity against most gram-negative bacilli, although tobramycin usually is more active against P. aeruginosa and some Proteus spp.

Many gram-negative bacilli that are resistant to gentamicin because of plasmid-mediated inactivating enzymes also will be resistant to tobramycin. Amikacin and, in some instances, netilmicin retain their activity against

gentamicin-resistant strains because they are a poor substrate for many of the aminoglycoside-inactivating enzymes.

ABSORPTION, DISTRIBUTION, DOSING, AND ELIMINATION OF THE AMINOGLYCOSIDES

Absorption

The aminoglycosides are highly polar cations and therefore are very poorly absorbed from the gastrointestinal tract.

Less than 1% of a dose is absorbed after either oral or rectal administration. The drugs are not inactivated in the intestine and are eliminated quantitatively

in the feces. Long-term oral or rectal administration of aminoglycosides may result in

accumulation to toxic concentrations in patients with renal impairment. Absorption of gentamicin from the gastrointestinal tract may be increased by

gastrointestinal disease (e.g., ulcers or inflammatory bowel disease). Instillation of these drugs into body cavities with serosal surfaces also may result in rapid absorption and unexpected toxicity, i.e., neuromuscular blockade.

Similarly, intoxication may occur when aminoglycosides are applied topically for long periods to large wounds, burns, or cutaneous ulcers, particularly if there is renal insufficiency.

All the aminoglycosides are absorbed rapidly from intramuscular sites of injection.

Peak concentrations in plasma occur after 30 to 90 minutes and are similar to those observed 30 minutes after completion of an intravenous infusion of an equal dose over a 30-minute period.

These concentrations typically range from 4 to 12 mg/ml following a 1.5 to 2 mg/kg dose of gentamicin, tobramycin, or netilmicin and from 20 to 35 mg/ml following a 7.5 mg/kg dose of amikacin or kanamycin.

In critically ill patients, especially those in shock, absorption of drug may be reduced from intramuscular sites because of poor perfusion.

Distribution

Because of their polar nature, the aminoglycosides do not penetrate into most cells, the central nervous system (CNS), and the eye.

Except for streptomycin, there is negligible binding of aminoglycosides to plasma albumin.

The apparent volume of distribution of these drugs is 25% of lean body weight and approximates the volume of extracellular fluid.

Concentrations of aminoglycosides in secretions and tissues are low. High concentrations are found only in the renal cortex and the endolymph and

perilymph of the inner ear; the high concentration in these sites likely contribute to the nephrotoxicity and ototoxicity caused by these drugs.

As a result of active hepatic secretion, concentrations in bile approach 30% of those found in plasma, but this represents a very minor excretory route for the aminoglycosides.

Penetration into respiratory secretions is poor. Diffusion into pleural and synovial fluid is relatively slow, but concentrations

that approximate those in the plasma may be achieved after repeated administration.

Inflammation increases the penetration of aminoglycosides into peritoneal and pericardial cavities.

Concentrations of aminoglycosides achieved in CSF with parenteral administration usually are subtherapeutic.

In experimental animals and human beings, concentrations in CSF in the absence of inflammation are less than 10% of those in plasma; this value may approach 25% when there is meningitis.

Intrathecal or intraventricular administration of aminoglycosides has been used to achieve therapeutic levels, but the availability of third- and fourth-generation cephalosporins has made this unnecessary in most cases.

Penetration of aminoglycosides into ocular fluids is so poor that effective therapy of bacterial endophthalmitis requires periocular and intraocular injections of the drugs.

Administration of aminoglycosides to women late in pregnancy may result in accumulation of drug in fetal plasma and amniotic fluid.

Streptomycin and tobramycin can cause hearing loss in children born to women who receive the drug during pregnancy.

Insufficient data are available regarding the other aminoglycosides; it is therefore recommended that they be used with caution during pregnancy and only for strong clinical indications in the absence of suitable alternatives.

Dosing

Recommended doses of individual aminoglycosides in the treatment of specific infections are given in later sections of this chapter.

Current practice is to give the total daily dose as a single injection, although historically it was administered as two or three equally divided doses. Numerous studies have shown that administration of the total dose once daily is associated with less toxicity and is just as effective as multiple-dose regimens.

This diminished toxicity is probably due to a threshold effect from accumulation of drug in the inner ear or in the kidney.

More drug accumulates with higher plasma concentrations, particularly at trough, and with prolonged periods of exposure.

Elimination of aminoglycoside from these organs occurs more slowly when plasma concentrations are relatively high.

A once-daily dosing regimen, despite the higher peak concentration, provides a longer period when concentrations fall below the threshold for toxicity than does a multiple-dose regimen (12 hours versus less than 3 hours total in the example shown), accounting for its lower toxicity.

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Aminoglycoside bactericidal activity, on the other hand, is related directly to the peak concentration achieved because aminoglycosides cause concentration-dependent killing and a concentration-dependent postantibiotic effect.

This enhanced activity at higher concentrations probably accounts for the equivalent efficacy of a once-daily regimen compared with a multiple-dose regimen despite the relatively prolonged periods of time that plasma concentrations are "subtherapeutic,"i.e., below the MIC.

Numerous studies in a variety of clinical settings employing virtually every commonly used aminoglycoside have demonstrated that once-daily regimens are as safe or safer than multiple-dose regimens with equal efficacy.

Once-daily dosing also costs less and is administered more easily. For these reasons, administration of aminoglycosides as a single daily dose

generally is preferred; exceptions include use in pregnancy, neonatal and pediatric infections and low-dose combination therapy of bacterial endocarditis because data documenting equivalent safety and efficacy are inadequate.

Once-daily dosing also should be avoided in patients with creatinine clearances of less than 20 to 25 ml/min because accumulation is likely to occur.

Less frequent dosing (e.g., every 48 hours) is more appropriate for these patients.

Whether once-daily or multiple-daily dosing is chosen, the dose must be adjusted for patients with creatinine clearances of below 80 to 100 ml/min and plasma concentrations must be monitored.

Use of nomograms may be helpful in selecting initial doses, but variability in aminoglycoside clearance among patients is too large for these to be relied on for more than a few days.

If it is anticipated that the patient will be treated with an aminoglycoside for more than 3 to 4 days, then plasma concentrations should be monitored to avoid drug accumulation.

In addition, aminoglycosides generally should not be used as single agents except for urinary tract infections because of relatively poor tissue penetration and poorer outcomes associated with aminoglycoside monotherapy.

For twice- or thrice-daily dosing regimens, both peak and trough plasma concentrations are determined.

The trough sample is obtained just before a dose, and the peak sample is obtained 60 minutes after intramuscular injection or 30 minutes after an intravenous infusion given over 30 minutes.

The peak concentration documents that the dose produces therapeutic concentrations, generally accepted to be 4 to 10 mg/ml for gentamicin, netilmicin, and tobramycin and 15 to 30 mg/ml for amikacin and streptomycin.

The trough concentration is used to avoid toxicity by monitoring for accumulation of drug.

Trough concentrations should be less than 1 to 2 mg/ml for gentamicin, netilmicin, and tobramycin and 5 to 10 mg/ml for amikacin and streptomycin.

Monitoring of aminoglycoside plasma concentrations also is important when using a once-daily dosing regimen, although peak concentrations are not determined routinely (these will be three to four times higher than the peak achieved with a multiple-daily-dosing regimen).

Several approaches may be used to determine that drug is being cleared and not accumulating.

The simplest method is to obtain a trough sample 24 hours after dosing and adjust the dose to achieve the recommended plasma concentration, e.g., below 1 to 2 mg/ml in the case of gentamicin or tobramycin.

This approach probably is the least desirable. An undetectable trough concentration could reflect grossly inadequate dosing in patients who clear the drug rapidly with prolonged periods (perhaps well over half the dosing interval) during which concentrations are subtherapeutic.

In contrast, a 24-hour trough concentration target of 1 to 2 mg/ml actually would increase aminoglycoside exposure compared with a multiple-daily-dosing regimen which defeats the goal of providing a washout with concentrations of 0 to 1 mg/ml between 18 to 24 hours after a dose.

A second approach relies on nomograms to target a range of concentrations in a sample obtained earlier in the dosing interval.

For example, if the plasma concentration from a sample obtained 8 hours after a dose of gentamicin is between 1.5 and 6 mg/ml, then the concentration at 18 hours will be less than 1 mg/ml.

Target ranges of 1 to 1.5 mg/ml for gentamicin at 18 hours for patients with creatinine clearances above 50 ml/min and 1 to 2.5 mg/ml for those with clearances below 50 ml/min also have been used.

This method also tends to be inaccurate, particularly when conditions that alter aminoglycoside clearance are present.

The most accurate method for monitoring plasma levels for dose adjustment is to measure the concentration in two plasma samples drawn several hours apart (e.g., at 2 and 12 hours after a dose).

The clearance then can be calculated and the dose adjusted to achieve the desired target range.

Elimination

The aminoglycosides are excreted almost entirely by glomerular filtration, and urine concentrations of 50 to 200 mg/ml are achieved.

A large fraction of a parenterally administered dose is excreted unchanged during the first 24 hours, with most of this appearing in the first 12 hours.

The half-lives of the aminoglycosides in plasma are similar and vary between 2 and 3 hours in patients with normal renal function.

Renal clearance of aminoglycosides is approximately two-thirds of the simultaneous creatinine clearance; this observation suggests some tubular reabsorption of these drugs.

After a single dose of an aminoglycoside, disappearance from the plasma exceeds renal excretion by 10% to 20%; however, after 1 to 2 days of

therapy, nearly 100% of subsequent doses eventually is recovered in the urine.

This lag period probably represents saturation of binding sites in tissues. The rate of elimination of drug from these sites is considerably longer than

from plasma; the half-life for tissue-bound aminoglycoside has been estimated to range from 30 to 700 hours.

For this reason, small amounts of aminoglycosides can be detected in the urine for 10 to 20 days after drug administration is discontinued. Aminoglycoside bound to renal tissue exhibits antibacterial activity and protects experimental animals against bacterial infections of the kidney even when the drug no longer can be detected in serum.

The concentration of aminoglycoside in plasma produced by the initial dose depends only on the volume of distribution of the drug.

Since the elimination of aminoglycosides depends almost entirely on the kidney, a linear relationship exists between the concentration of creatinine in plasma and the half-life of all aminoglycosides in patients with moderately compromised renal function.

In anephric patients, the half-life varies from 20 to 40 times that determined in normal individuals.

Because the incidence of nephrotoxicity and ototoxicity is related to the concentration to which an aminoglycoside accumulates, it is critical to reduce the maintenance dosage of these drugs in patients with impaired renal function.

The size of the individual dose, the interval between doses, or both can be altered.

There is no conclusive information on the best approach, and even the currently accepted therapeutic range has been questioned.

The most consistent plasma concentrations are achieved when the loading dose is given in milligrams per kilogram of body weight; and since aminoglycosides are distributed minimally in fatty tissue, the lean or expected body weight should be used.

There are obvious difficulties in using any of these approaches for ill patients with rapidly changing renal function.

In addition, even when known factors are taken into consideration, concentrations of aminoglycosides achieved in plasma after a given dose vary widely among patients. If the extracellular volume is expanded, the volume of distribution is increased, and concentrations will be reduced.

For unknown reasons, aminoglycoside clearances are increased and half-lives are reduced in patients with cystic fibrosis; the volume of distribution is increased in patients with leukemia.

Patients with anemia (hematocrit <25%) have a concentration in plasma that is higher than expected probably because of reduction in the number of binding sites on red blood cells.

Determination of the concentration of drug in plasma is an essential guide to the proper administration of aminoglycosides.

In patients with life-threatening systemic infections, aminoglycoside concentrations should be determined several times per week (more frequently if renal function is changing) and should be determined within 24 to 48 hours of a change in dosage.

Aminoglycosides can be removed from the body by either hemodialysis or peritoneal dialysis.

Approximately 50% of the administered dose is removed in 12 hours by hemodialysis, which has been used for the treatment of overdosage.

As a general rule, a dose equal to half the loading dose administered after each hemodialysis should maintain the plasma concentration in the desired range; however, a number of variables make this a rough approximation at best.

Continuous arteriovenous hemofiltration (CAVH) and continuous venovenous hemofiltration (CVVH) will result in aminoglycoside clearances approximately equivalent to 15 and 15 to 30 ml/min of creatinine clearance, respectively, depending on the flow rate.

The amount of aminoglycoside removed can be replaced by administering approximately 15% to 30% of the maximum daily dose (Table 45-2) each day. Frequent monitoring of plasma drug concentrations is again crucial.

Peritoneal dialysis is less effective than hemodialysis in removing aminoglycosides.

Clearance rates are approximately 5 to 10 ml/min for the various drugs but are highly variable.

If a patient who requires dialysis has bacterial peritonitis, a therapeutic concentration of the aminoglycoside probably will not be achieved in the peritoneal fluid because the ratio of the concentration in plasma to that in peritoneal fluid may be 10:1.

Thus it is recommended that antibiotic be added to the dialysate to achieve concentrations equal to those desired in plasma. For intermittent dosing via peritoneal dialysate, 2 mg/kg of amikacin is added to the bag once a day.

The corresponding dose for gentamicin, netilmicin, or tobramycin is 0.6 mg/kg.

For continuous dosing, the dose for amikacin is 12 mg/L (25 mg/L loading dose in the first bag), and the dose for gentamicin, netilmicin, or tobramycin is 4 mg/L in each bag (8 mg/L loading dose).

This should be preceded by administration of a loading dose, either parenterally or in dialysis fluid.

Although excretion of aminoglycosides is similar in adults and children older than 6 months of age, half-lives of the drugs may be prolonged significantly in the newborn: 8 to 11 hours in the first week of life in newborns weighing less than 2 kg and approximately 5 hours in those weighing more than 2 kg (Yow, 1977).

Thus it is critically important to monitor concentrations of aminoglycosides during treatment of neonates.

Aminoglycosides can be inactivated by various penicillins in vitro and in patients with end-stage renal failure thus making dosage recommendations even more difficult.

Amikacin appears to be the least affected by this interaction.

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UNTOWARD EFFECTS OF THE AMINOGLYCOSIDES

All aminoglycosides have the potential to produce reversible and irreversible vestibular, cochlear, and renal toxicity.

These side effects complicate the use of these compounds and make their proper administration difficult.

Ototoxicity

Vestibular and auditory dysfunction can follow the administration of any of the aminoglycosides.

Studies of animals and human beings have documented progressive accumulation of these drugs in the perilymph and endolymph of the inner ear. Accumulation occurs predominantly when concentrations in plasma are high.

Diffusion back into the bloodstream is slow; the half-lives of the aminoglycosides are five to six times longer in the otic fluids than in plasma.

Back-diffusion is concentration dependent and is facilitated at the trough concentration of drug in plasma.

Ototoxicity is more likely to occur in patients with persistently elevated concentrations of drug in plasma.

However, even a single dose of tobramycin has been reported to produce slight temporary cochlear dysfunction during periods when the concentration in plasma is at its peak.

Ototoxicity has been linked to mutations in a mitochondrial ribosomal RNA gene, indicating that a genetic predisposition exists for this side effect.

Oxidant stress probably plays a role, and ras activation has been implicated. Ototoxicity is largely irreversible and results from progressive destruction of

vestibular or cochlear sensory cells, which are highly sensitive to damage by Aminoglycosides.

Studies in guinea pigs exposed to large doses of gentamicin reveal degeneration of the type I sensory hair cells in the central part of the crista ampullaris (vestibular organ) and fusion of individual sensory hairs into giant hairs.

Similar studies with gentamicin and tobramycin also demonstrate loss of hair cells in the cochlea of the organ of Corti.

With increasing dosage and prolonged exposure, damage progresses from the base of the cochlea, where high-frequency sounds are processed, to the apex, which is necessary for the perception of low frequencies.

While these histological changes correlate with the ability of the cochlea to generate an action potential in response to sound, the biochemical mechanism for ototoxicity is poorly understood.

Early changes induced by aminoglycosides have been shown in experimental ototoxicity to be reversible by Ca2+.

Once sensory cells are lost, however, regeneration does not occur; retrograde degeneration of the auditory nerve follows, resulting in irreversible hearing loss. It has been suggested that aminoglycosides interfere with the active transport system essential for the maintenance of the ionic balance of the endolymph.

This would lead to alteration in the normal concentrations of ions in the labyrinthine fluids, with impairment of electrical activity and nerve conduction.

Eventually, the electrolyte changes, or perhaps the drugs themselves, damage the hair cells irreversibly.

The degree of permanent dysfunction correlates with the number of destroyed or altered sensory hair cells and is thought to be related to sustained exposure to the drug.

Interestingly, total dose and duration of aminoglycoside exposure and other risk factors, such as advanced age, bacteremia, liver disease, and renal disease, that reasonably might predispose one to ototoxicity have not been proven to do so.

Repeated courses of aminoglycosides, each probably resulting in the loss of more cells, seem to lead to deafness.

Drugs such as ethacrynic acid and furosemide potentiate the ototoxic effects of the aminoglycosides in animals; data implicating furosemide are less convincing in humans.

Hearing loss following exposure to these agents also is more likely to develop in patients with preexisting auditory impairment.

Although all aminoglycosides are capable of affecting cochlear and vestibular function, some preferential toxicity is evident.

Streptomycin and gentamicin produce predominantly vestibular effects, whereas amikacin, kanamycin, and neomycin primarily affect auditory function; tobramycin affects both equally.

The incidence of ototoxicity is extremely difficult to determine. Data from audiometry suggest that the incidence may be as high as 25%. The relative incidence appears to be equal for tobramycin, gentamicin, and

amikacin. Initial studies in laboratory animals and human beings suggested that

netilmicin is less ototoxic than other aminoglycosides; however, the incidence of ototoxicity from netilmicin is not negligible such complications developed in 10% of patients in one clinical Trial of Netilmicin.

The incidence of vestibular toxicity is particularly high in patients receiving streptomycin; nearly 20% of individuals who received 500 mg twice daily for 4 weeks for enterococcal endocarditis developed clinically detectable irreversible vestibular damage.

In addition, up to 75% of patients who received 2 g streptomycin for more than 60 days showed evidence of nystagmus or postural imbalance.

Since the initial symptoms may be reversible, it is recommended that patients receiving high doses and/or prolonged courses of aminoglycosides be monitored carefully for ototoxicity; however, deafness may occur several weeks after therapy is discontinued.

Clinical Symptoms of Cochlear Toxicity

A high-pitched tinnitus often is the first symptom of toxicity. If the drug is not discontinued, auditory impairment may develop after a few

days. The tinnitus may persist for several days to 2 weeks after therapy is stopped. Since perception of sound in the high-frequency range (outside the

conversational range) is lost first, the affected individual is not always aware of the difficulty, and it will not be detected unless careful audiometric examination is carried out.

If the hearing loss progresses, the lower sound ranges are affected, and conversation becomes difficult.

Clinical Symptoms of Vestibular Toxicity

Moderately intense headache lasting 1 or 2 days may precede the onset of labyrinthine dysfunction.

This is followed immediately by an acute stage in which nausea, vomiting, and difficulty with equilibrium develop and persist for 1 to 2 weeks.

Prominent symptoms include vertigo in the upright position, inability to perceive termination of movement ("mental past-pointing"), and difficulty in sitting or standing without visual cues.

Drifting of the eyes at the end of a movement so that both focusing and reading are difficult, a positive Romberg test, and rarely, pendular trunk movement and spontaneous nystagmus are outstanding signs.

The acute stage ends suddenly and is followed by the appearance of manifestations consistent with chronic labyrinthitis, in which, although symptomless while in bed, the patient has difficulty when attempting to walk or make sudden movements; ataxia is the most prominent feature.

The chronic phase persists for approximately 2 months; it is gradually superseded by a compensatory stage in which symptoms are latent and appear only when the eyes are closed.

Adaptation to the impairment of labyrinthine function is accomplished by the use of visual cues and deep proprioceptive sensation for determining movement and position.

It is more adequate in the young than in the old but may not be sufficient to permit the high degree of coordination required in many special trades. Recovery from this phase may require 12 to 18 months, and most patients have some permanent residual damage.

Although there is no specific treatment for the vestibular deficiency, early discontinuation of the drug may permit recovery before irreversible damage of the hair cells.

Nephrotoxicity

Approximately 8% to 26% of patients who receive an aminoglycoside for more than several days will develop mild renal impairment that is almost always reversible.

The toxicity results from accumulation and retention of aminoglycoside in the proximal tubular cells.

The initial manifestation of damage at this site is excretion of enzymes of the renal tubular brush border.

After several days, there is a defect in renal concentrating ability, mild proteinuria, and the appearance of hyaline and granular casts.

The glomerular filtration rate is reduced after several additional days. The nonoliguric phase of renal insufficiency is thought to be due to the effects

of aminoglycosides on the distal portion of the nephron with a reduced sensitivity of the collecting-duct epithelium to endogenous antidiuretic hormone.

While severe acute tubular necrosis may occur rarely, the most common significant finding is a mild rise in plasma creatinine (5 to 20 mg/ml; 40 to 175 mM).

Hypokalemia, hypocalcemia, and hypophosphatemia are seen very infrequently.

The impairment in renal function is almost always reversible because the proximal tubular cells have the capacity to regenerate.

Several variables appear to influence nephrotoxicity from aminoglycosides. Toxicity correlates with the total amount of drug administered. Consequently, toxicity is more likely to be encountered with longer courses of

therapy. Continuous infusion is more nephrotoxic in animals than is intermittent

dosing; constantly elevated concentrations of drug in plasma above a critical level, which is manifest by elevated trough serum concentrations, correlate with toxicity in human beings (Keating et al., 1979).

The nephrotoxic potential varies among individual aminoglycosides. The relative toxicity correlates with the concentration of drug found in the

renal cortex in experimental animals. Neomycin, which concentrates to the greatest degree, is highly nephrotoxic in

human beings and should not be administered systemically. Streptomycin does not concentrate in the renal cortex and is the least nephrotoxic.

Most of the controversy has concerned the relative toxicities of gentamicin and tobramycin.

Gentamicin is concentrated in the kidney to a greater degree than is tobramycin, but several controlled clinical trials have given different estimates of their relative nephrotoxicities.

If differences between the renal toxicity of these two aminoglycosides do exist in human beings, they appear to be slight.

Comparative studies with amikacin, sisomicin, and netilmicin are not conclusive.

Other drugs, such as amphotericin B, vancomycin, angiotensin-converting enzyme inhibitors, cisplatin, and cyclosporine, may potentiate aminoglycoside-induced nephrotoxicity.

Furosemide enhances the nephrotoxicity of aminoglycosides in rats if there is concurrent fluid depletion.

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Clinical studies have not proven conclusively that furosemide itself potentiates nephrotoxicity but volume depletion and wasting of K+ that accompany its use have been incriminated.

Advanced age, liver disease, diabetes mellitus, and septic shock have been suggested as risk factors for the development of nephrotoxicity from aminoglycosides, but data are not convincing.

Note, however, that renal function in the elderly patient is overestimated by measurement of creatinine concentration in plasma, and overdosing will occur if this value is used as the only guide in this patient population.

Even though aminoglycosides consistently alter the structure and function of renal proximal tubular cells, these effects usually are reversible.

The most important result of this toxicity may be reduced excretion of the drug, which, in turn, predisposes to ototoxicity.

Monitoring drug concentrations in plasma is useful, particularly during prolonged and/or high-dose therapy.

However, it never has been proven that toxicity can be prevented by avoiding excessive peak or trough concentrations of aminoglycosides.

In fact, experience with once-daily dosing regimens strongly suggests that high peaks (e.g., 25 mg/ml or higher) do not increase toxicity.

The biochemical events leading to tubular cell damage and glomerular dysfunction are poorly understood but may involve perturbations of the structure of cellular membranes.

Aminoglycosides inhibit various phospholipases, sphingomyelinases, and ATPases, and they alter the function of mitochondria and ribosomes.

Because of the ability of cationic aminoglycosides to interact with anionic phospholipids, these drugs may impair the synthesis of membrane-derived autacoids and intracellular second messengers such as prostaglandins, inositol phosphates, and diacylglycerol.

Derangements of prostaglandin metabolism may explain the relationship between tubular damage and reduction in glomerular filtration rate.

Others have observed morphological changes in glomerular endothelial cells (decreased number of endothelial fenestrations) and reduction in the glomerular capillary ultrafiltration coefficient in animals receiving aminoglycosides.

Ca2+ has been shown to inhibit the uptake and binding of aminoglycosides to the renal brush-border luminal membrane in vitro, and supplementary dietary Ca2+ attenuates experimental nephrotoxicity.

Aminoglycosides eventually are internalized by pinocytosis. Morphologically, there is clear evidence of accumulation of drug in liposomes,

a means by which aminoglycosides are trapped, concentrated (up to 50 times the plasma concentration) and prepared for extrusion into the urine as multilamellar phospholipid structures called myeloid bodies.

Neuromuscular Blockade

An unusual toxic reaction of acute neuromuscular blockade and apnea has been attributed to the aminoglycosides.

The order of decreasing potency for blockade is neomycin, kanamycin, amikacin, gentamicin, and tobramycin.

In humans, neuromuscular blockade generally has occurred after intrapleural or intraperitoneal instillation of large doses of an aminoglycoside; however, the reaction can follow intravenous, intramuscular, and even oral administration of these agents.

Most episodes have occurred in association with anesthesia or the administration of other neuromuscular blocking agents. Patients with myasthenia gravis are particularly susceptible to neuromuscular blockade by aminoglycosides.

Aminoglycosides may inhibit prejunctional release of acetylcholine while also reducing postsynaptic sensitivity to the transmitter, but Ca2+ can overcome this effect, and the intravenous administration of a calcium salt is the preferred treatment for this toxicity.

Inhibitors of acetylcholinesterase (e.g., edrophonium and neostigmine) also have been used with varying degrees of success.

Other Effects on the Nervous System

The administration of streptomycin may produce dysfunction of the optic nerve, including scotomas, presenting as enlargement of the blind spot.

Among the less common toxic reactions to streptomycin is peripheral neuritis. This may be due either to accidental injection of a nerve during the course of

parenteral therapy or to toxicity involving nerves remote from the site of antibiotic administration.

Paresthesia, most commonly perioral but also present in other areas of the face or in the hands, occasionally follows the use of the antibiotic and usually appears within 30 to 60 minutes after injection of the drug. It can persist for several hours.

Other Untoward Effects

In general, the aminoglycosides have little allergenic potential; anaphylaxis and rash are unusual.

Rare hypersensitivity reactions¾including skin rashes, eosinophilia, fever, blood dyscrasias, angioedema, exfoliative dermatitis, stomatitis, and anaphylactic shock¾have been reported.

Parenterally administered aminoglycosides are not associated with pseudomembranous colitis, probably because they do not disrupt the normal anaerobic flora.

(Goodman and Gilman’s The Pharmacological Basis of Therapeutics 11th Edition)