An Introduction to Anti-fungal Pharmacology

The following slides were generously supplied by

Professor Russell E. Lewis, Pharm.D., BCPS

University of Houston College of Pharmacy, University of Texas M.D. Anderson Cancer Center.

With lecture notes written by Hannah Woodcock and Jenny Bartholomew, University of Manchester, UK.

Types of fungal infections - Mycoses

Superficial mycoses Affect the skin, hair and nails

Subcutaneous mycoses (tropical) Affect the muscle and connective tissue

immediately below the skin Systemic (invasive) mycoses

Involve the internal organs Primary vs. opportunistic

Allergic mycoses Affect lungs or sinuses Patients may have chronic asthma, cystic fibrosis

or sinusitisThere is some overlap between these

groups

What are the targets for antifungal therapy?

Cell membraneFungi use principally ergosterol instead of cholesterol

Cell WallUnlike mammalian cells, fungi have a cell wall

DNA SynthesisSome compounds may be selectively activated by fungi, arresting DNA synthesis.

Atlas of fungal Infections, Richard Diamond Ed. 1999Introduction to Medical Mycology. Merck and Co. 2001

Cell Membrane Active Antifungals

Cell membrane • Polyene antibiotics - Amphotericin B, lipid formulations - Nystatin (topical)

• Azole antifungals - Ketoconazole - Itraconazole - Fluconazole - Voriconazole - Miconazole, clotrimazole (and other topicals)

Azole Antifungals for Systemic Infections

Ketoconazole (Nizoril) Itraconazole (Sporanox) Fluconazole (Diflucan) Voriconazole (Vfend)

Imidazole

Triazoles

“2nd generationtriazole”

Fluconazole Ketoconazole

Azoles - Mechanism

In fungi, the cytochrome P450-enzyme lanosterol 14-demethylase is responsible for the conversion of lanosterol to ergosterol

Azoles bind to lanosterol 14-demethylase inhibiting the production of ergosterol

Some cross-reactivity is seen with mammalian cytochrome p450 enzymes

Drug Interactions Impairment of steroidneogenesis

(ketoconazole, itraconazole)

Effect of azoles on C. albicans

Before exposure After exposure

Azoles - Pharmacodynamics

Concentration-independent fungistatic agents Dosage escalation may be necessary

when faced with more resistant fungal species (e.g. Candida glabrata)

Goal of dosing is to maintain AUC:MIC >50 i.e. maintain concentrations 1-2 x MIC

for the entire dosing interval

Ketoconazole

Spectrum: yeasts and moulds - poor absorption limits its role for severe infections, generally used in mucosal infections only

Pharmacokinetics Variable oral absorption, dependent on pH

(often given with cola or fruit juice) T1/2 7-10 hours Protein binding > 99% Hepatic, bile and kidney elimination

Ketoconazole - Adverse effects

Adverse effects N&V, worse with higher doses (800 mg/day) Hepatoxicity (2-8%), increase in

transaminases, hepatitis Dose related inhibition of CYP P450

responsible for testosterone synthesis Gynecomastia, oligosperma, decreased libido

Dose-related inhibition of CYP P450 responsible for adrenal cortisol synthesis

Ketoconazole - Drug Interactions

Potent inhibitor of cytochrome P450 3A4 Rifampin and phenytoin decrease ketoconazole

levels Ketoconazole increases cyclosporin, warfarin,

astemizole, corticosteroid, and theophylline levels Many of these drug interactions are

severe Drugs that increase gastric pH will decrease

blood levels of ketoconazole Antacids, omeprazole, H2 blockers

Ketoconazole - Dose

Serious infections 800 mg/day PO Other: 200-400 mg/day PO

Cost $2.50 per 200 mg tablet

Fluconazole

Well tolerated IV/PO

formulations Favorable

pharmacokinetics

Fungistatic Resistance is

increasing Narrow

spectrum (Drug

interactions)

Advantages Disadvantages

Fluconazole - spectrum

Good activity against C. albicans and Cryptococcus neoformans

Non-albicans Candida species more likely to exhibit primary resistance

C. krusei > C. glabrata > C. parapsilosis

C. tropicalis

C. kefyr

Always resistant Sometimes resistant

Fluconazole - resistance Primary resistance (seen in severely ill or

immunocompromised patients) Selection of resistant species or

subpopulations Replacement with more resistant strain

Secondary resistance (seen in patients with AIDS who experienced recurrent orophayrngeal candidiasis and received long-term fluconazole therapy) Genetic mutation Upregulation of efflux pumps

Mechanisms of antifungal resistance

Target enzyme modification

Ergosterol biosynthetic pathway

Efflux pumps Drug import

White TC, Marr KA, Bowden RA. Clin Microbiol Review 1998;11:382-402

Fluconazole - What is not covered

Candida krusei +/- Candida glabrata Aspergillus species and other

moulds

Fluconazole - Pharmacokinetics

Available as both IV and PO Bioavailibility > 90%

Linear pharmacokinetics t 1/2 = ~24 hours Cmax (400 mg IV) = 20 µg/ml (steady state) Protein binding < 12% Vd 0.85 L/kg (widely distributed) >90% excreted unchanged through the

kidney

Fluconazole - adverse effects/monitoring

N&V, rash: More likely with high doses and in AIDS

patients Asymptomatic increase in LFTs (7%)

Drug interactions: May increase phenytoin, cyclosporin,

rifabutin, warfarin, and zidovudine concentrations

Rifampin reduced fluconazole levels to half (even though FLU is not a major substrate)

Fluconazole - Dosing

Mucosal candidiasis 100-200 mg/day (150 mg tablet vulvovaginal

candidiasis) Systemic fungal infections

400-800 mg q24h > 800 mg q24h in unstable patient, S-DD

isolate, or if non-albicans spp. (except C. krusei)

Maintenance for cryptococcal meningitis 400 mg q24h

Key Biopharmaceutical Characteristics of the Triazole Antifungals

Fluconazole Itraconazole Voriconazole

Spectrum vs. Candida and Aspergillus

C. albicans, C. tropicalis +/- C. glabrataNo Aspergillus

Similar Candida coverage as fluconazole, + Aspergillus

Broad, includes most Candida spp., Aspergillus, Fusarium sp. Not Zygomycoses

Oral formulation(% bioavailibility)

Tablet (>90%) Capsule (6-25%)Solution (20-60%)

Tablet (>90%)

Intravenous formulation

Available, no solubilizer

Available, cyclodextrin

Available, cyclodextrin

Clearance Renal (80%) Hepatic 3A4 Hepatic 2C19, 3A4

Serum half life (hr)

24 24-30 6-24

CSF penetration Excellent Poor Excellent

CYP 3A4 inhibition

Weak Strong Moderate-Strong

Adverse effects N&V, hepatic N&V, diarrhea (solution), hepatic, CHF

N&V, visual disturbances, hepatic, rashR.E. Lewis 2002. Exp Opin Pharmacother 3:1039-57.

Itraconazole Solution - Side Effects

Taste disturbances Nausea and vomiting Osmotic diarrhea (especially at

doses > 400 mg/day) Long-term compliance often difficult

Voriconazole - Side Effects

Visual disturbances (~ 30%) Decreased vision, photophobia, altered color

perception and ocular discomfort IV > oral No evidence of structural damage to retina

Reversible alterations in function of retinal rods and cones

Testing 2 weeks after the end of treatment demonstrates a return to normal function

Long term effects?..caution against night-time driving Effects may be intensified by hallucinations (2-5%)

Amphotericin B Polyene antibiotic Fermentation product of

Streptomyces nodusus Binds sterols in fungal cell membrane Creates transmembrane channel and

electrolyte leakage. Active against most fungi except

Aspergillus terreus, Scedosporium spp.

Lipid Amphotericin B Formulations

Ribbon-like particlesRibbon-like particlesCarrier lipids: DMPC, Carrier lipids: DMPC, DMPGDMPGParticle size Particle size (µm): 1.6-: 1.6-11 11

Abelcet Abelcet ®® ABLC ABLC Amphotec Amphotec ®® ABCD ABCD Ambisome Ambisome ®® L-AMB L-AMB

Disk-like particlesDisk-like particlesCarrier lipids: Cholesteryl Carrier lipids: Cholesteryl sulfate sulfateParticle size Particle size (µm): 0.12-: 0.12-0.14 0.14

UnilaminarUnilaminar liposomeliposomeCarrier lipids: HSPC, Carrier lipids: HSPC, DSPG, cholesterolDSPG, cholesterolParticle size Particle size (µm) : 0.08 : 0.08

DMPC-Dimyristoyl phospitidylcholineDMPG- Dimyristoyl phospitidylcglycerol

HSPC-Hydrogenated soy phosphatidylcholineDSPG-Distearoyl phosphitidylcholine

Amphotericin B

Classic amphotericin B deoxycholate (Fungizone™) formulation: serious toxic side effects.

Less toxic preparations:1) Liposomal amphotericin B2) Amphotericin B colloidal dispersion3) Amphotericin B lipid complex

Amphotericin B - Pharmacokinetics

Absorption from the GI tract is negligible Oral solution sometimes used to

decontaminate gut; few side effects Only reliable method of administration is

IV Selective distribution into deep tissue

sites, with slow release of drug

kidney > liver > spleen > lung > heart > skeletal muscle > brain > bone > CSF > eye

LowHigh

Amphotericin B - Metabolic elimination

Metabolic fate is unknown, drug accumulates in tissues and then is slowly released Drug levels can be measured in the kidney, liver,

and spleen up to 1 year after receiving drug Dosages of amphotericin B are generally not

altered due to decreased elimination of the drug in kidney dysfunction

Hemodialysis does not alter serum drug concentrations except in hyperlipidemic patients

Amphotericin B - Elimination

Inverse correlation between patient age and elimination of AmB, Age, elimination, side effects

Paediatric patients often tolerate amphotericin B better than adults

Amphotericin B - Nephrotoxicity

Most significant delayed toxicity Renovascular and tubular mechanisms

Vascular-decrease in renal blood flow leading to drop in GFR, azotemia

Tubular-distal tubular ischemia, wasting of potassium, sodium, and magnesium

Enhanced in patients who are volume depleted or who are on concomitant nephrotoxic agents

Amphotericin B - Manoeuvers employed to blunt nephrotoxicity…

Sodium loading-> blunt the vasoconstriction and tubular-glomerular feedback Administration of 500 ml -1000 ml of

NaCl before and after amphotericin B infusion

Amphotericin B - Drug Interactions

Enhanced nephrotoxicity Nephrotoxic drugs

Cyclosporine, aminoglycosides, foscarnet, pentamidine

Antineoplastic agents Cisplatin, nitrogen mustards

Amphotericin B - Clinical Uses

The drug of choice for: Cryptococcal meningitis Mucormycosis (zygomycosis) Invasive fungal infection, not

responding to other therapy

Amphotericin B - Dosing and Administration

“Test dose” 1.0 mg in 25-100ml 5% dextrose infused over 10 minutes used to evaluate possibility of anaphylactic reaction No longer recommended, current product has

fewer impurities Current recommendation- Start with ~30% of

target dose, infuse for 15 minutes, stop infusion, and monitor patient for adverse effects before resuming infusion

Rapidly escalate to full dosages within 48-72 hours

Delay in giving full dose = worse clinical outcome

Cell Wall Active Antifungals

Cell membrane • Polyene antibiotics • Azole antifungals

DNA/RNA synthesis • Pyrimidine analogues - Flucytosine

Cell wall • Echinocandins -Caspofungin acetate (Cancidas)

Fluorinated pyrimidinerelated to flurouracil.

Flucytosine

Flucytosine Restricted spectrum of activity.

Acquired Resistance.> result of monotherapy> rapid onset

Due to:1) Decreased uptake (permease activity)2) Altered 5-FC metabolism (cytosine

deaminase or UMP pyrophosphorylase activity)

Flucytosine - pharmacokinetics

Oral absorption complete

Plasma half-life 3-6 hrs

Volume of distribution

0.7-1l/kg (low)

Plasma protein binding

~12%

Flucytosine - side effects

Infrequent – include D&V, alterations in liver function tests and blood disorders.

Blood concs need monitoring when used in conjunction with Amphotericin B.

Flucytosine – Clinical uses

Candidiasis Cryptococcosis ?Aspergillosis

} In combination with amphotericin B or fluconazole.

Monotherapy : now limited

Cell Wall Active Antifungals

Cell membrane • Polyene antibiotics • Azole antifungals

DNA/RNA synthesis • Pyrimidine analogues - Flucytosine

Cell wall • Echinocandins -Caspofungin acetate (Cancidas)

Atlas of fungal Infections, Richard Diamond Ed. 1999Introduction to Medical Mycology. Merck and Co. 2001

The Fungal Cell Wall

mannoproteins

1,6glucans

1,3

chitin

ergosterol

1,3 glucansynthase

Cellmembrane

Atlas of fungal Infections, Richard Diamond Ed. 1999Introduction to Medical Mycology. Merck and Co. 2001

Echinocandins - Pharmacology Cyclic lipopeptide antibiotics that

interfere with fungal cell wall synthesis by inhibition of ß-(1,3) D-glucan synthase

Loss of cell wall glucan results in osmotic fragility

Spectrum: Candida species including non-

albicans isolates resistant to fluconazole

Aspergillus spp. but not activity against other moulds (Fusarium, Zygomycosis)

No coverage of Cryptococcus neoformansOH

H H

HO

OH

H NH

O

NHH

HO

H2N

HO O

NH

H3C

HO H

H

O

NH

H

HO OH

H

H NH

NH

O

CH3

OH

H

HO

N

OH

OH

H

Echinocandins - spectrumHighly activeCandida albicans, Candida glabrata,Candida tropicalis, Candida kruseiCandida kefyrPneumocystis carinii

Low MIC ,with fungicidal activity and good in-vivo activity.

Very activeCandida parapsilosisCandida gulliermondiiAspergillus fumigatusAspergillus flavusAspergillus terreusCandida lusitaniae

Low MIC, but without fungicidal activity in most instances.

Some activityCoccidioides immitisBlastomyces dermatididisScedosporium speciesPaecilomyces variotiiHistoplasma capsulatum

Detectable activity, which might have therapeutic potential for man (in some cases in combination with other drugs).

Echinocandins act at the apical tips of Aspergillus hyphae

DiBAC

Bowman et al. Antimicrob Agent Chemother 2002;46:3001-12

Echinocandins-Spectrum vs. Moulds

Staining with antisera to glucan synthase subunit

(Fks1p)

Active against Aspergillus species Glucan synthase

localized in apical tips Activity against other

yeast and moulds is less well described or variable Mycelial forms of

endemic mycoses?

Beauvais et al. J. Bacteriol 2001;183:2273-79Beauvais et al. J. Bacteriol 2001;183:2273-79

Aniline blue staining of β (1-3) glucans –stains only at apex

Caspofungin - Pharmacokinetics

Absorption < 2%

Distribution (Vd) 9.67 L

Protein binding 97% albumin

Major metabolic pathway Peptide hydrolysis, slow

N-acetylation

t 1/2 ß 9-11 hours

CNS penetration

Dosage adjustment

Probably poor

Moderate-severe hepatic dysfunction

Drug-Drug interactions Significant interactions CSA? FK-506, mycophenolate? Inducers of 3A4?

Caspofungin acetate

IV only

Indication: Invasive candidiasis Invasive aspergillosis refractory to other therapies

Dosage and administration 70 mg day 1, followed by 50 mg daily

Increase to 70 mg per day in non-responders Decrease to 35 mg per day in moderate-severe hepatic

dysfunction (Child-Pugh 7-9)

Antiviral Drug Products Advisory Committee, January 10, 2001- www.FDA.gov

Caspofungin - Adverse effects

Most common AEs are infusion related: Intravenous site irritation (15-20%) Mild to moderate infusion-related AE including

fever, headache, flushing, erythema, rash (5-20%) Symptoms consistent with histamine release (2%)

Most AEs were mild and did not require treatment discontinuation

Most common laboratory AE Asymptomatic elevation of serum transaminases

(10-15%) Clinical experience to date suggests that these drugs

are extremely well-tolerated

Antiviral Drug Products Advisory Committee, January 10, 2001- www.FDA.gov

PolyenesAmphotericin B deoxycholateLipo-AMB (AmBisome)ABLC (Abelcet)Amphocil

1 mg/kg/day IV3 mg/kg/day IV5 mg/kg/day IV3 mg/kg/day IV

£7£554£246£380

TriazolesFluconazoleItraconazoleVoriconazole

400/800 mg IV400 mg IV 4 mg/kg IV

£56/£112£72£80

EchinocandinsCaspofungin 50 mg IV x 1 day, £334

Drug Dosage AWP Cost/day for 70kg Patient

(Medical Letter 2002;44:63-65; Lancet 2003;362:1142-1151)