Pharmacology · Systemic Antifungals

Antifungal Drug Therapy

Mechanisms, clinical applications, adverse effects, and key comparisons for major systemic and topical antifungals.

📘 Overview of Antifungal Therapy

  • Fungal infections range from superficial dermatophytosis to life-threatening systemic mycoses.
  • Antifungal drugs exploit biochemical differences between fungal and human cells.
  • Primary therapeutic targets:
    • Ergosterol synthesis pathway (polyenes, azoles, terbinafine).
    • Cell wall integrity (echinocandins).
    • Nucleic acid synthesis (flucytosine).
    • Microtubule function (griseofulvin).
  • Key challenges:
    • Emerging resistance.
    • Drug–drug interactions (particularly azoles).
    • Nephrotoxicity, hepatotoxicity, and infusion-related reactions.
  • Selection depends on:
    • Pathogen identification and susceptibility.
    • Infection site (CNS penetration required for meningitis).
    • Host immune status and organ function.
  • High-yield: Amphotericin B remains the drug of choice for many severe systemic mycoses, but its nephrotoxicity limits use.

🧬 Polyenes

  • Drugs: Amphotericin B (systemic), Nystatin (topical only).
  • Mechanism of action:
    • Amphiphilic structure — contains both polar and nonpolar regions.
    • Binds to ergosterol in the fungal cell membrane.
    • Forms transmembrane channels ("pores") that disrupt ion gradients.
    • Leakage of intracellular K⁺, Mg²⁺, and other metabolites → cell death.
    • Fungicidal effect.
  • Resistance:
    • Altered ergosterol content (reduced concentration in membrane).
    • Changes in membrane lipid composition reduce drug binding.
  • Amphotericin B — clinical use:
    • Broadest antifungal spectrum among all agents.
    • Drug of choice (or co-DOC) for:
      • Cryptococcal meningitis (often combined with flucytosine).
      • Mucormycosis (Zygomycosis).
      • Severe histoplasmosis, blastomycosis, coccidioidomycosis.
      • Visceral leishmaniasis (off-label).
  • Nystatin:
    • Too toxic for systemic administration.
    • Used topically for mucosal and cutaneous candidiasis.
    • Available as creams, ointments, and oral suspensions.
  • Pharmacokinetics (Amphotericin B):
    • Administered by slow intravenous infusion.
    • Poor CNS penetration — intrathecal administration may be required for select CNS infections.
    • Extremely long elimination half-life (> 2 weeks).
    • Cleared via hepatic metabolism and renal excretion.
  • Adverse effects — infusion-related:
    • Fever, chills, rigors, and hypotension (histamine-mediated).
    • Typically occur during or shortly after IV infusion.
    • Test dose recommended before full therapeutic dose.
    • Pre-treatment with NSAIDs, antihistamines, meperidine, or corticosteroids can attenuate symptoms.
  • Adverse effects — dose-dependent toxicity:
    • Nephrotoxicity — the most clinically significant.
      • Reduced glomerular filtration rate.
      • Renal tubular acidosis (type 1 or 2).
      • Hypokalemia and hypomagnesemia.
      • Anemia via decreased erythropoietin production.
    • Mitigation strategies:
      • Volume expansion with sodium loading (saline).
      • Liposomal amphotericin B formulations (reduced nephrotoxicity).
      • Combination therapy (e.g., with flucytosine) allows dose reduction.
  • Exam trap: Amphotericin B is not effective against Candida glabrata or Candida krusei (resistance patterns).

⚗️ Azoles

  • Drugs: Ketoconazole, Fluconazole, Itraconazole, Voriconazole, Clotrimazole, Miconazole.
  • Mechanism of action:
    • Inhibit 14α-demethylase (lanosterol 14α-demethylase) — a fungal cytochrome P450 enzyme.
    • Blocks conversion of lanosterol to ergosterol.
    • Ergosterol depletion → membrane instability and impaired fungal growth.
    • Fungistatic in most settings (concentration-dependent fungicidal activity for some agents).
  • Resistance:
    • Reduced intracellular drug accumulation (efflux pump overexpression).
    • Target site mutations (altered 14α-demethylase).
    • Upregulation of alternative sterol synthesis pathways.
  • Ketoconazole:
    • First-generation oral azole.
    • Co-drug of choice for paracoccidioidomycosis.
    • Backup agent for blastomycosis and histoplasmosis.
    • Also used orally for mucocutaneous candidiasis and dermatophytes.
    • Absorption reduced by antacids and gastric acid suppressors.
  • Fluconazole:
    • Drug of choice for:
      • Esophageal candidiasis.
      • Invasive candidiasis (non-critically ill).
      • Coccidioidomycosis.
    • Used for prophylaxis and suppression in cryptococcal meningitis.
    • Excellent CSF penetration — the only azole that reliably enters the CNS.
    • Eliminated renally (largely unchanged in urine).
  • Itraconazole and Voriconazole:
    • Drugs of choice for:
      • Blastomycosis.
      • Sporotrichosis.
      • Aspergillosis (voriconazole is preferred).
    • Backup agents for many other mycoses and candidiasis.
    • Itraconazole absorption increased by food and acidic environment.
    • Voriconazole has activity against fluconazole-resistant Candida species.
  • Topical azoles (Clotrimazole, Miconazole):
    • Used for superficial candidal and dermatophytic infections.
    • Minimal systemic absorption.
    • Available as creams, powders, troches, and vaginal formulations.
  • Pharmacokinetic considerations:
    • All azoles inhibit hepatic CYP450 enzymes (especially CYP3A4).
    • Significant drug–drug interactions — warfarin, statins, benzodiazepines, calcium channel blockers, cyclosporine, etc.
    • Fluconazole is primarily renally excreted; dose adjustment required in renal impairment.
    • Ketoconazole and itraconazole are hepatically metabolized.
  • Adverse effects:
    • Endocrine effects — reduced cortisol and testosterone synthesis.
      • Decreased libido.
      • Gynecomastia.
      • Menstrual irregularities.
    • Hepatotoxicity — elevated transaminases; rare but severe.
    • GI distress, headache, rash.
    • QT prolongation (particularly with voriconazole).
    • Visual disturbances (voriconazole — photopsia, color vision changes).
Agent Key Use CNS Penetration Elimination
FluconazoleCandidiasis, coccidioidomycosis, cryptococcal suppression✅ HighRenal (unchanged)
ItraconazoleBlastomycosis, sporotrichosis, aspergillosis❌ PoorHepatic
VoriconazoleAspergillosis (DOC), fluconazole-resistant Candida⚠️ ModerateHepatic
KetoconazoleParacoccidioidomycosis, backup for histo/blasto❌ PoorHepatic

🔬 Other Antifungals

  • Flucytosine (5-FC)
    • Mechanism:
      • Converted by fungal cytosine deaminase to 5-fluorouracil (5-FU).
      • 5-FU is triphosphorylated → incorporated into fungal RNA → disrupts protein synthesis.
      • 5-FU also converted to 5-fluorodeoxyuridine monophosphate (5-FdUMP) → inhibits thymidylate synthase → depletes thymine → impairs DNA synthesis.
    • Clinical use:
      • Never used as monotherapy — rapid resistance emergence.
      • Combination therapy with amphotericin B for severe candidiasis and cryptococcal infections.
      • Excellent CSF penetration.
    • Adverse effects:
      • Bone marrow suppression (leukopenia, thrombocytopenia) — mechanism shared with anticancer 5-FU.
      • GI intolerance, hepatotoxicity.
      • Renal impairment (requires dose adjustment).
  • Griseofulvin
    • Mechanism:
      • Deposits in newly forming keratin.
      • Disrupts fungal microtubule assembly → inhibits mitosis.
    • Clinical use:
      • Active only against dermatophytes (Trichophyton, Microsporum, Epidermophyton).
      • Oral administration (not topically effective).
      • Used for tinea capitis, tinea unguium, and extensive dermatophytosis.
    • Adverse effects:
      • Disulfiram-like reaction — avoid alcohol.
      • Headache, GI upset, rash.
      • Hepatotoxicity (rare).
  • Terbinafine
    • Mechanism:
      • Inhibits squalene epoxidase → blocks ergosterol synthesis upstream.
      • Accumulation of squalene is toxic to fungal cells.
    • Clinical use:
      • Active only against dermatophytes.
      • Considered superior to griseofulvin for onychomycosis (nail infections).
      • Good oral bioavailability and tissue penetration (keratinophilic).
    • Adverse effects:
      • GI distress, headache, rash.
      • Hepatotoxicity — monitor liver function tests.
      • Loss of taste (dysgeusia) — reversible.
  • Echinocandins (Caspofungin, Micafungin, Anidulafungin)
    • Mechanism:
      • Inhibit β-1,3-D-glucan synthase.
      • Disrupt fungal cell wall synthesis → osmotic instability and cell lysis.
    • Clinical use:
      • IV administration only.
      • Backup agents for disseminated and mucocutaneous Candida infections.
      • Second-line therapy for invasive aspergillosis.
      • Effective against Candida species, including fluconazole-resistant strains.
      • No activity against Cryptococcus, Fusarium, or Mucorales.
    • Adverse effects:
      • Infusion-related reactions (flushing, rash).
      • Hepatotoxicity — monitor LFTs.
      • Generally better tolerated than amphotericin B.
  • High-yield: Echinocandins are the preferred agents for candidemia in critically ill patients due to favorable safety profile and activity against resistant species.
Drug Target Route Key Spectrum
FlucytosineDNA/RNA synthesisPO/IVCandida, Cryptococcus (combo)
GriseofulvinMicrotubulesPODermatophytes only
TerbinafineSqualene epoxidasePO/topicalDermatophytes only
Echinocandinsβ-glucan synthaseIVCandida, Aspergillus

📊 Comparative Overview

  • By mechanism:
    • Membrane disruption: polyenes (Amphotericin B, Nystatin).
    • Ergosterol synthesis inhibition: azoles, terbinafine.
    • Cell wall synthesis inhibition: echinocandins.
    • Nucleic acid disruption: flucytosine.
    • Microtubule disruption: griseofulvin.
  • By spectrum:
    • Broadest: Amphotericin B.
    • Dermatophyte-specific: Griseofulvin, Terbinafine.
    • Candida/Aspergillus: azoles, echinocandins.
    • Cryptococcus: Amphotericin B + flucytosine; fluconazole for suppression.
  • By CNS penetration:
    • Good: Fluconazole, Flucytosine.
    • Poor: Amphotericin B (intrathecal needed), itraconazole, ketoconazole.
    • Intermediate: Voriconazole.
  • By toxicity profile:
    • Nephrotoxic: Amphotericin B.
    • Hepatotoxic: Azoles, terbinafine, echinocandins.
    • Myelosuppressive: Flucytosine.
    • Infusion reactions: Amphotericin B, echinocandins.
Polyene → Membrane pores → Ion leakage → Fungicidal
Azole → 14α-demethylase inhibition → Ergosterol depletion → Fungistatic
Echinocandin → β-glucan synthase inhibition → Cell wall disruption → Fungicidal (Candida)

🩺 Clinical Pearls & High-Yield Facts

  • For cryptococcal meningitis:
    • Induction: Amphotericin B + flucytosine.
    • Consolidation/suppression: Fluconazole.
    • Fluconazole alone is insufficient for acute induction.
  • For invasive aspergillosis:
    • Voriconazole is the drug of choice.
    • Echinocandins are alternative or salvage therapy.
    • Amphotericin B is third-line (or combination).
  • For mucormycosis:
    • Amphotericin B is the cornerstone.
    • Azoles (except posaconazole) and echinocandins are not effective.
    • Surgical debridement is often required.
  • For dermatophytosis:
    • Terbinafine is first-line for onychomycosis.
    • Griseofulvin is still used for tinea capitis in children.
    • Topical azoles are effective for limited skin involvement.
  • Drug–drug interactions to remember:
    • Azoles + warfarin → increased INR (bleeding risk).
    • Azoles + statins → rhabdomyolysis risk.
    • Azoles + cyclosporine/tacrolimus → nephrotoxicity.
    • Azoles + rifampin → reduced azole efficacy (CYP3A4 induction).
  • Pregnancy considerations:
    • Amphotericin B is preferred for systemic infections.
    • Azoles are generally avoided in the first trimester.
    • Fluconazole high dose is teratogenic.
  • Common exam trap: Fluconazole is not effective against Aspergillus — voriconazole or itraconazole must be used.
  • Board pearl: Liposomal amphotericin B has equivalent efficacy and significantly less nephrotoxicity compared to conventional deoxycholate formulation.