ANTI-BACTERIAL CHEMOTHERAPY..(1+2)

By Dr Israa Omar

A. ANTIMICROBIAL AGENTS THAT INTERFERE WITH THE SYNTHESIS OR ACTION OF FOLATE

sulfonamides• Examples include sulfadiazine ,

sulfadimidine , sulfamethoxazole (intermediate acting)• Sulfametopyrazine (long acting)• Sulfasalazine (poorly absorbed in the

gastrointestinal tract) • Sulfamethoxazole (in combination with

trimethoprim as co-trimoxazole).

Mechanism of action• Sulfonamide is a structural analogue of p-aminobenzoic

acid (PABA), which is an essential precursor in the synthesis of folic acid in bacteria.

• folate is required for the synthesis of the precursors of DNA and RNA both in bacteria and in mammals, but whereas bacteria need to synthesis folic acid, mammals can obtain it from dietary sources.

• Sulfonamides compete with PABA for the enzyme dihydropteroate synthetase, and the effect of the sulfonamide may be overcome by adding excess PABA.

Mechanism of action• The action of a sulfonamide is to inhibit growth of the

bacteria, not to kill them; that is to say, it is bacteriostatic rather than bactericidal.

• The action is impaired in the presence of pus or products of tissue breakdown, because these contain thymidine and purines, which bacteria utilize directly, bypassing the requirement for folic acid.

• Resistance to the drugs, which is common, is plasmid-mediated and results from the synthesis of a bacterial enzyme insensitive to the drug.

Pharmacokinetics• Most sulfonamides are readily absorbed in the

gastrointestinal tract and reach maximum concentrations in the plasma in 4-6 hours.

• They are usually not given topically because of the risk of sensitization or allergic reactions.

• The drugs pass into inflammatory exudates and cross both placental and blood-brain barriers.

• They are metabolized mainly in the liver, the major product being an acetylated derivative that lacks antibacterial action.

Side effects• Mild to moderate side effects include nausea and

vomiting, headache and mental depression.• Cyanosis caused by methaemoglobinaemia (Fe3+HB)

may occur but is a lot less alarming than it looks.• Serious adverse effects include hepatitis,

hypersensitivity reactions (rashes, fever, anaphylactic reactions), bone marrow depression

• Crystalluria; this effect results from the precipitation of acetylated metabolites in the urine.

Clinical uses of sulfonamides

• Combined with trimethoprim (Trimethoprim/sulfamethoxazole ‘co-trimoxazole’) for Pneumocystis carinii.

• Combined with pyrimethamine (Sulfadoxine/pyrimethamine) for drug-resistant malaria , and for toxoplasmosis.

• In inflammatory bowel disease: sulfasalazine (sulfapyridine-aminosalicylate combination.

• For infected burns (silver sulfadiazine given topically).• For some sexually transmitted infections (e.g. trachoma,

chlamydia, chancroid).• For respiratory infections: use now confined to a few special

problems (e.g. infection with Nocardia).• For acute urinary tract infection (now seldom used).

TRIMETHOPRIM • Trimethoprim is chemically related to the

antimalarial drug pyrimethamine , both being folate antagonists.

• Structurally, it resembles the pteridine moiety of folate and the similarity is close enough to fool the bacterial dihydrofolate reductase, which is many times more sensitive to trimethoprim than the equivalent enzyme in humans .

Pharmacokinetic aspects • Trimethoprim is given orally. • It is fully absorbed from the gastrointestinal tract and

widely distributed throughout the tissues and body fluids.

• It reaches high concentrations in the lungs and kidneys, and fairly high concentrations in the cerebrospinal fluid (CSF).

• When given with sulfamethoxazole, about half the dose of each is excreted within 24 hours.

• Because trimethoprim is a weak base, its elimination by the kidney increases with decreasing urinary pH.

Antimicrobial agents that interfere with the synthesis or action of folate

• Sulfonamides are bacteriostatic; they act by interfering with folate synthesis and thus with nucleotide synthesis.

• Unwanted effects include crystalluria and hypersensitivities.

• Trimethoprim is bacteriostatic. It acts by antagonizing folate.

• Co-trimoxazole is a mixture of trimethoprim with sulfamethoxazole, which affects bacterial nucleotide synthesis at two points in the pathway

Clinical uses of trimethoprim/co-trimoxazole

• For urinary tract and respiratory infections: trimethoprim, used on its own, is usually preferred.

• For infection with Pneumocystis carinii, which causes pneumonia in patients with AIDS: co-trimoxazole is used in high dose.

Cell Wall Inhibitors

Cell Wall Inhibitors

• b-Lactams– Penicillins– Cephalosporins– Monobactams (Aztreonam)– Carbapenems (Imipenem)

• Non b-Lactams– Vancomycin (glycopeptide)– Cycloserine– Fosfomycin – Bacitracin (cyclic peptide)

Penicillins

S

N

CH3

CH3

COOHO

R1

Cephalosporins

R2

COOHO

N

SR1

Beta Lactam Ring

β

β

Carbapenems

N

O

R1

R2

COOHMonobactams

O

N

R2

R1

Beta Lactam Ring

β

β

Cell coverings of bacteria • Inner: Cell / plasma membrane• Outer: Cell wall• Peptidoglycan layer consists of glycan chains• Glycan chains: linear strands of two alternating

amino sugars N-acetylglucosamine (Nag) N-acetylmuramic acid (Nam)

• Cross linking of these strands by peptide chains

Gram-neg & gram-pos Cell Walls

β-LACTAM ANTIBIOTICS 1. Penicillins and cephalosporins

• Penicillins and cephalosporins – Part of group of drugs called β –lactams• Have shared chemical structure called β-lactam ring

– Competitively inhibits function of penicillin-binding proteins• Inhibits peptide bridge formation between glycan

molecules• This causes the cell wall to develop weak points at

the growth sites and become fragile.

Dye Binds to Exposed Cell Wall Peptidoglycan, Gram positive (Rt.)

Mechanism of action of PENICILLIN

• All β-lactam antibiotics interfere with the synthesis of the bacterial cell wall peptidoglycan.

• After attachment to penicillin-binding proteins on bacteria (there may be seven or more types in different organisms), they inhibit the transpeptidation enzyme that cross-links the peptide chains attached to the backbone of the peptidoglycan.

Mechanism of action

Mechanism of action

• The final bactericidal event is the inactivation of an inhibitor of autolytic enzymes{in other word promotion OR stimulation of autolytic enzyme} in the cell wall, leading to lysis of the bacterium.

• Some organisms, referred to as 'tolerant', have defective autolytic enzymes and are inhibited but not lysed in the presence of the drug.

Types of penicillin and their antimicrobial activity

• The first penicillins were the naturally occurring benzylpenicillin and its congeners, including phenoxymethylpenicillin.

• Benzylpenicillin is active against a wide range of organisms and is the drug of first choice for many infections.

• Its main drawbacks are poor absorption in the gastrointestinal tract (which means it must be given by injection) and its susceptibility to bacterial β-lactamases

• Various semisynthetic penicillins have been prepared by adding different side-chains to the penicillin nucleus.

• β-lactamase-resistant penicillins (e.g. flucloxacillin)• Broad-spectrum penicillins (e.g. ampicillin,

pivampicillin and amoxicillin) • Ticarcilin with antipseudomonal activity against P.

aeruginosa.• Amoxicillin is sometimes combined with β-

lactamase inhibitor clavulanic acid as co-amoxiclav

Pharmacokinetic aspects • When given orally, different penicillins are absorbed

to differing degrees depending on their stability in acid and their adsorption to foodstuffs in the gut.

• Penicillins can be given IM or IV , but intrathecal administration is inadvisable, particularly in the case of benzylpenicillin, as it can cause convulsions.

• The penicillins are widely distributed in body fluids, passing into joints; into pleural and pericardial cavities; into bile, saliva and milk; and across the placenta.

•

Pharmacokinetic aspects • Being lipid-insoluble, they do not enter mammalian{check

it} cells and do not, therefore, cross the blood-brain barrier unless the meninges are inflamed, in which case they readily reach therapeutically effective concentrations in the CSF as well.

• Elimination of most penicillins occurs rapidly and is mainly renal, 90% being through tubular secretion.

• The relatively short plasma half-life is a potential problem in the clinical use of benzylpenicillin

•

Unwanted effects • Penicillins are relatively free from direct toxic effects

(other than their pro-convulsant effect when given intrathecally).

• The main unwanted effects are –Hypersensitivity reactions caused by the

degradation products of penicillin, which combine with host protein and become antigenic. – Skin rashes and fever are common;–A delayed type of serum sickness occurs

infrequently.

Unwanted effects • Much more serious is acute anaphylactic shock,

which although fortunately very rare, may in some cases be fatal.

• When given orally, penicillins, particularly the broad-spectrum type, alter the bacterial flora in the gut. This can be associated with gastrointestinal disturbances and in some cases with superinfection by other, penicillin-insensitive, micro-organisms.

Clinical uses of the penicillins• Bacterial meningitis (e.g. By N. Meningitidis,

S.Pneumoniae): benzylpenicillin, high doses IV• Bone and joint infections (e.g. With staph. Aureus):

flucloxacillin• Skin and soft tissue infections (e.g. With strep.

Pyogenes or staph. Aureus): benzylpenicillin, flucloxacillin; animal bites: co-amoxiclav

• Pharyngitis (from strep. Pyogenes): phenoxylmethylpenicillin

• Otitis media (organisms commonly include strep. Pyogenes, haemophilus influenzae): amoxicillin

• Bronchitis (mixed infections common): amoxicillin• Pneumonia: amoxicillin• Urinary tract infections (e.g. with Escherichia coli):

amoxicillin• Gonorrhea: amoxicillin (plus probenecid)• Syphilis: procaine benzylpenicillin• Endocarditis (e.g. with Strep. viridans or Enterococcus

faecalis)• Serious infections with Pseudomonas aeruginosa:

ticarcillin, piperacillin.• EOL

Lecture No:02CEPHALOSPORINS ANDCEPHAMYCINS

• They are usually classified in arbitrary in terms of chronological order in which they were produced:– First generation : cephalexin, cefadroxil – Second generation: cefuroxime, cephamandole and

cefoxitin– Third generation: cefotaxime, ceftazidine, cefixime

and ceftrixone, more active against g-ve– Fourth generation: cefepime highly resistant to beta

lactamase enzyme

Mechanism of action

• The mechanism of action of these agents is similar to that of the penicillins: interference with bacterial peptidoglycan synthesis after binding to the β-lactam-binding proteins.

Resistance• Resistance to this group of drugs has increased

because of plasmid-encoded or chromosomal β-lactamase.

• Nearly all Gram-negative bacteria have a chromosomal gene coding for a β-lactamase that is more active in hydrolyzing cephalosporins than penicillins, and in several organisms a single mutation can result in high-level constitutive production of this enzyme.

Pharmacokinetic aspects

• Some cephalosporins may be given orally , but most are given parenterally, IM or IV.

• After absorption, they are widely distributed in the body and some, such as cefotaxime, cefuroxime and ceftriaxone, cross the blood-brain barrier.

• Excretion is mostly via the kidney, largely by tubular secretion, but 40% of ceftriaxone is eliminated in the bile

Adverse effects

• Hypersensitivity reactions, very similar to those seen with penicillin, may occur, and there may be some cross-sensitivity; about 10% of penicillin-sensitive individuals will have allergic reactions to cephalosporins.

• Nephrotoxicity has been reported (especially with cefradine), as has drug-induced alcohol intolerance.

• Diarrhea can occur with oral cephalosporins

Clinical uses of the cephalosporins

• Septicaemia (e.g. cefuroxime, cefotaxime)• Pneumonia caused by susceptible organisms• Meningitis (e.g. ceftriaxone, cefotaxime)• Biliary tract infection• Urinary tract infection (especially in pregnancy

or in patients unresponsive to other drugs)• Sinusitis (e.g. cefadroxil).

OTHER β-LACTAM ANTIBIOTICS

CARBAPENEMS AND MONOBACTAMS

• Carbapenems and monobactams were developed to deal with β-lactamase-producing Gram-negative organisms resistant to penicillins

2. Carbapenem• Imipenem, an example of a carbapenem, acts in the

same way as the other β-lactams . • It has a very broad spectrum of antimicrobial activity,

being active against many aerobic and anaerobic Gram-positive and Gram-negative organisms.

• Imipenem was originally resistant to all β-lactamases, but some organisms now have chromosomal genes that code for imipenem-hydrolysing β-lactamases.

• It is sometimes given together with cilastatin, which inhibits its inactivation by renal enzymes..

• Meropenem is similar but is not metabolized by the kidney.

• Ertapenem has a broad spectrum of antibacterial actions but is licensed only for a limited range of indications

Unwanted effects

• Unwanted effects are generally similar to those seen with other β-lactams, nausea and vomiting being the most frequently seen.

• Neurotoxicity can occur with high plasma concentrations

3 .MONOBACTAMS • The main monobactam is aztreonam, a simple

monocyclic β-lactam with a complex substituent which is resistant to most β-lactamases.

• It is given parenterally and has a plasma half-life of 2 hours.

• Aztreonam has an unusual spectrum of activity and is effective only against Gram-negative aerobic rods such as pseudomonads, N.meningitidis and H.influenzae.

• It has no action against Gram-positive organisms or anaerobes

Unwanted effects

• Unwanted effects are, in general, similar to those of other β-lactam antibiotics, but this agent does not necessarily cross-react immunologically with penicillin and its products, and so does not usually cause allergic reactions in penicillin-sensitive individuals.

Non b-Lactams

1 .Vancomycin• Non Beta-Lactam • Teicoplanin is similar but longer lasting• Inhibits transglycosylase enzyme from binding to D-ala-D-

ala; stops elongation of peptidoglycan chain; Bactericidal• IV only; Renal elimination• Used to treat Serious infections (sepsis) due to Staph,

Methicillin-resistant strains, Alt. to β-lactams in endocarditis caused by Staphylococci and Streptococci

• C.difficile enterocolitis is treated orally with vancomycin, metronidazole is preferred

Adverse effect of vancomycin• Red neck syndrome“{red man syndrome"

(RMS), which has also been called "red neck syndrome." RMS is an idiopathic infusion reaction, which is not thought to involve drug-specific antibodies and, in contrast to allergic reactions, may develop with the first administration of vancomycin.}

• Phlebitis at injection site• Ototoxicity / nephrotoxicity with

aminoglycoside

Bacitracin

• Cyclic peptide mixture – not β-Lactam• Nephrotoxic; Topical use only • Blocks dephosphorylation of lipid deliver to

peptidoglycan subunits of the growing cell wall• Often combine with other antibiotics

Fosfomycin• Inhibits synthesis of N-acetylmuramic acid by

inhibiting enolpyruvyl transferase• One single oral dose• Uncomplicated urinary tract infection esp. in

women• Unchanged drug present in high conc. in urine

up to 46 hrs

CYCLOSERINE

• Structural analog of D-alanine• Exclusively use in strains of Mycobecteria

resistant to first line anti tuberculous drugs• Serious dose related nervous toxicity i.e.

psychosis, tremors, convulsions

Interference with cell membrane integrity

Drugs that interfere with cell membrane integrity

• Few damage cell membrane• Polymixn B (most common) & Polymixn E (colistin)

Common ingredient in first-aid skin ointments• Binds membrane of Gram –ve cells• Alters permeability• Leads to leakage of cell and cell death• Also bind eukaryotic cells but to lesser extent• Limits use to topical application• Can cause severe neurotoxicity and nephrotoxic

Daptomycin• Cyclic lipopeptide• Treat infections by resistant gram positive organism

MRSA and vancomycin-resistant enterococci• Work by rapidly depolarizing bacterial membrane,

thus disrupting multiple aspects of membrane function and inhibit synthesis of DNA and RNA

• It is bactericidal antibiotics and can be inactivated by lung surfactant; thus is not indicated in treatment of pneumonia

• Can cause potential muscle toxicity and elevation of liver transaminases

Antibiotic - Penicillin

Antibiotics:Antibiotics are specific chemical substances

derived from or produced by living organisms that are capable of inhibiting the life processes

of other organisms. The first antibiotics were isolated from microorganisms but some are now

obtained from higher plants and animals. Over 3,000 antibiotics have been identified but only a few dozen are used in medicine. Antibiotics are

the most widely prescribed class of drugs comprising 12% of the prescriptions in the

United States.The penicillins were the first antibiotics

discovered as natural products from the mold Penicillium. In 1928, Sir Alexander Fleming,

professor of bacteriology at St. Mary's Hospital in London, was culturing Staphylococcus aureus.

He noticed zones of inhibition where mold spores were growing. He named the mold

Penicillium rubrum. It was determined that a secretion of the mold was effective against

Gram-positive bacteria.Penicillins as well as cephalosporins are called

beta-lactam antibiotics and are characterized by three fundamental structural requirements: the

fused beta-lactam structure (shown in the blue and red rings, a free carboxyl acid group (shown

in red bottom right), and one or more substituted amino acid side chains (shown in

black). The lactam structure can also be viewed as the covalent bonding of pieces of two amino

acids - cysteine (blue) and valine (red).Penicillin-G where R = an ethyl pheny group, is

the most potent of all penicillin derivatives. It has several shortcomings and is effective only

against gram-positive bacteria. It may be broken down in the stomach by gastric acids

and is poorly and irregularly absorbed into the blood stream. In addition many disease

producing staphylococci are able to produce an enzyme capable of inactivating penicillin-G.

Various semisynthetic derivatives have been produced which overcome these shortcomings.

Powerful electron-attracting groups attached to the amino acid side chain such as in

phenethicillin prevent acid attack. A bulky group attached to the amino acid side chain

provides steric hindrance which interfers with the enzyme attachment which would deactivate the pencillins i.e. methicillin. Refer to Table 2 for

the structures.

Finally if the polar character is increased as in ampicillin or carbenicillin, there is a greater

activiity against Gram-negative bacteria .

Click for larger image

Penicillin Mode of Action - Enzyme Inhibition:All penicillin derivatives produce their

bacteriocidal effects by inhibition of bacterial cell wall synthesis. Specifically, the cross linking

of peptides on the mucosaccharide chains is prevented. If cell walls are improperly made

cell walls allow water to flow into the cell causing it to burst.

Resemblances between a segment of penicillin structure and the backbone of a peptide chain

have been used to explain the mechanism of action of beta-lactam antibiotics. The structures

of a beta-lactam antibiotic and a peptide are shown on the left for comparison. Follow the

trace of the red oxygens and blue nitrogen atoms.

Penicillin and ala-ala peptide - Chime innew window

Click for larger image Image courtesy of the University of Texas-

Houston Medical School.

Mechanisms of Drug Actions by Enzyme Inhibition:

Gram-positive bacteria possess a thick cell wall composed of a cellulose-like structural sugar

polymer covalently bound to short peptide units in layers.The polysaccharide portion of the

peptidoglycan structure is made of repeating units of N-acetylglucosamine linked b-1,4 to N-

acetylmuramic acid (NAG-NAM).The peptide varies, but begins with L-Ala and

ends with D-Ala. In the middle is a dibasic amino acid, diaminopimelate (DAP). DAP

(orange) provides a linkage to the D-Ala (gray) residue on an adjacent peptide.

The bacterial cell wall synthesis is completed when a cross link between two peptide chains

attached to polysaccharide backbones is formed.The cross linking is catalyzed by the enzyme

transpeptidase. First the terminal alanine from each peptide is hydrolyzed and secondly one alanine is joined to lysine through an amide

bond.Penicillin binds at the active site of the

transpeptidase enzyme that cross-links the peptidoglycan strands. It does this by mimicking

the D-alanyl-D-alanine residues that would normally bind to this site. Penicillin irreversibly inhibits the enzyme transpeptidase by reacting

with a serine residue in the transpeptidase. This reaction is irreversible and so the growth of the

bacterial cell wall is inhibited.Since mammal cells do not have the same type of cell walls, penicillin specifically inhibits only

bacterial cell wall synthesis.Bacterial Resistance to Penicillin:

As early as the 1940s, bacteria began to combat the effectiveness of penicillin.

Penicillinases (or beta-lactamases) are enzymes produced by structurally susceptable bacteria

which renders penicillin useless by hydrolysing the peptide bond in the beta-lactam ring of the nucleus. Penicillinase is a response of bacterial

adaptation to its adverse environment, namely the presence of a substance which inhibits its

growth. Many other anitbiotics are also rendered ineffective because of this same type

of resistance.Severe Allergic Shock Reactions to Penicillin:

It is estimated that between 300-500 people die each year from penicillin-induced anaphylaxis, a

severe allergic shock reaction to penicillin. In afflicted individuals, the beta-lactam ring binds

to serum proteins, initiating an IgE-mediated inflammatory response .

Cephalosporins:Cephalosporins are the second major group of

beta-lactam antibiotics. They differ from penicillins by having the beta-lactam ring as a 6

member ring. The other difference, which is more significant from a medicinal chemistry

stand point, is the existence of a functional group (R) at position 3 of the fused ring system.

This now allows for molecular variations to effect changes in properties by diversifying the

groups at position 3.The first member of the newer series of beta-lactams was isolated in 1956 from extracts of Cephalosporium acremonium, a sewer fungus.

Like penicillin, cephalosporins are valuable because of their low toxicity and their broad

spectrum of action against various diseases. In this way, cephalosporin is very similar to

penicillin. Cephalosporins are one of the most widely used antibiotics, and economically speaking, has about 29% of the antibiotic

market. The cephalosporins are possibly the single most important group of antibiotics today

and are equal in importance to penicillin.The structure and mode of action of the

cephalosporins are similar to that of penicillin. They affect bacterial growth by inhibiting cell wall synthesis, in Gram-positive and negative

bacteria.Some brand names include: cefachlor, cefadroxil,

cefoxitin, ceftriaxone.Cephalexin - Chime innew window

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