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)