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 sinusitis There is some overlap between these groups

What are the targets for antifungal therapy?

Cell membrane Fungi use principally ergosterol instead of cholesterol DNA Synthesis Some compounds may be selectively activated by fungi, arresting DNA synthesis.

Cell Wall Unlike mammalian cells, fungi have a cell wall

Atlas of fungal Infections, Richard Diamond Ed. 1999 Introduction 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

Imidazole  Ketoconazole (Nizoril)  Itraconazole (Sporanox)  Fluconazole (Diflucan)  Voriconazole (Vfend) Triazoles “2nd generation triazole” Fluconazole Ketoconazole

Azoles - Mechanism

  In fungi, the cytochrome P450 enzyme lanosterol 14 a demethylase is responsible for the conversion of lanosterol to ergosterol Azoles bind to lanosterol 14 a 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) T 1/2 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

Advantages



Well tolerated



IV/PO formulations



Favorable pharmacokinetics Disadvantages



Fungistatic



Resistance is increasing



Narrow spectrum

 (

Drug interactions)

Fluconazole - spectrum

 Good activity against

C. albicans

and

Cryptococcus neoformans

 Non-

albicans Candida

species more likely to exhibit primary resistance

Always resistant Sometimes resistant

C. krusei > C. glabrata > C. parapsilosis C. tropicalis C. kefyr

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

Oral formulation (% bioavailibility) Intravenous formulation Clearance

C. albicans, C. tropicalis +/ C. glabrata

No

Aspergillus

Tablet (>90%) Available, no solubilizer Renal (80%) Similar

Candida

coverage as fluconazole, +

Aspergillus

Capsule (6-25%) Solution (20-60%) Available, cyclodextrin Hepatic 3A4 Broad, includes most

Candida

spp.,

Aspergillus, Fusarium sp. Not Zygomycoses

Tablet (>90%) Available, cyclodextrin Hepatic 2C19, 3A4

Serum half life (hr)

24

CSF penetration CYP 3A4 inhibition

Excellent Weak 24-30 Poor Strong 6-24 Excellent Moderate-Strong

Adverse effects

N&V, hepatic N&V, diarrhea (solution), hepatic, CHF N&V, visual disturbances, hepatic, rash R.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

Abelcet ® ABLC Amphotec ® ABCD Ambisome ® L-AMB

Ribbon-like particles Carrier lipids: DMPC, DMPG Particle size (µm) : 1.6 11 Disk-like particles Carrier lipids: Cholesteryl sulfate Particle size (µm) : 0.12 0.14 Unilaminar liposome Carrier lipids: HSPC, DSPG, cholesterol Particle size (µm)

DMPC-Dimyristoyl phospitidylcholine HSPC-Hydrogenated soy phosphatidylcholine

: 0.08

Amphotericin B

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

 Less toxic preparations: 1) Liposomal amphotericin B 2) Amphotericin B colloidal dispersion 3) 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

High Low

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

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)

Flucytosine

Fluorinated pyrimidine related to flurouracil.

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 Volume of distribution 3-6 hrs 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

Monotherapy : now limited  Candidiasis  Cryptococcosis  ?Aspergillosis }

In combination with amphotericin B or fluconazole.

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. 1999 Introduction to Medical Mycology. Merck and Co. 2001

The Fungal Cell Wall

mannoproteins b 1,3 b 1,6 glucans Cell membrane b 1,3 glucan synthase chitin ergosterol Atlas of fungal Infections, Richard Diamond Ed. 1999 Introduction to Medical Mycology. Merck and Co. 2001

Echinocandins - Pharmacology

O H 2 N OH H 3 C H HO H HO H HO O H H H O HO H NH OH H

 Cyclic lipopeptide antibiotics that interfere with fungal cell wall synthesis by inhibition of ß-(1,3) D glucan synthase

N NH H OH H O NH H O O NH H N H NH O

 Loss of cell wall glucan results in osmotic fragility

CH 3 H OH H OH

Spectrum:    Candida

species including non-

albicans

isolates resistant to fluconazole

Aspergillus

spp. but not activity against other moulds (

Fusarium

,

Zygomycosis

)

No

coverage of

Cryptococcus neoformans

Echinocandins - spectrum

Highly active

Candida albicans, Candida glabrata, Candida tropicalis, Candida krusei Candida kefyr Pneumocystis carinii

Very active

Candida parapsilosis Candida gulliermondii Aspergillus fumigatus Aspergillus flavus Aspergillus terreus Candida lusitaniae Low MIC ,with fungicidal activity and good in-vivo activity.

Low MIC, but without fungicidal activity in most instances.

Some activity

Coccidioides immitis Blastomyces dermatididis Scedosporium species Paecilomyces variotii Histoplasma 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   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?

Staining with antisera to glucan synthase subunit (Fks1p) Aniline blue staining of β (1-3) glucans – stains only at apex Beauvais et al. J. Bacteriol 2001;183:2273-79

Caspofungin - Pharmacokinetics

Absorption Distribution (Vd) Protein binding Major metabolic pathway t 1/2 ß < 2% 9.67 L 97% albumin Peptide hydrolysis, slow N-acetylation 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

Drug Polyenes Amphotericin B deoxycholate Lipo-AMB (AmBisome) ABLC (Abelcet) Amphocil Triazoles Fluconazole Itraconazole Voriconazole Echinocandins Caspofungin Dosage

1 mg/kg/day IV 3 mg/kg/day IV 5 mg/kg/day IV 3 mg/kg/day IV 400/800 mg IV 400 mg IV 4 mg/kg IV 50 mg IV x 1 day,

AWP Cost/day for 70kg Patient

£7 £554 £246 £380 £56/£112 £72 £80 £334

(Medical Letter

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