Fungal infections range from superficial dermatophyte infections to life-threatening invasive mycoses in immunocompromised patients. The development of antifungal drugs is challenging due to the eukaryotic nature of fungal cells, which share many cellular targets with human cells, limiting therapeutic options and increasing toxicity risks.
What Are Antifungal Drugs?
Antifungals are classified by their mechanism of action and spectrum of activity. Systemic antifungals are used for invasive infections, while topical agents treat mucocutaneous disease. The major classes include azoles, polyenes, echinocandins, allylamines, flucytosine, and griseofulvin.
Drug Classes and Mechanisms
Azoles inhibit lanosterol 14-alpha-demethylase, a cytochrome P450 enzyme required for ergosterol synthesis, disrupting fungal cell membrane integrity. Fluconazole has excellent bioavailability and good activity against Candida species but limited activity against molds. Itraconazole has broader spectrum including Aspergillus. Voriconazole is first-line for invasive aspergillosis. Posaconazole and isavuconazole provide extended spectrum including Mucorales. Azoles are substrate inhibitors of CYP enzymes, causing significant drug-drug interactions.
Polyenes include amphotericin B and nystatin. Amphotericin B binds ergosterol in fungal cell membranes, forming pores that cause ion leakage and cell death. It has the broadest antifungal spectrum but significant infusion-related reactions and nephrotoxicity. Nystatin is too toxic for systemic use and is limited to topical and oral suspension formulations.
Echinocandins such as caspofungin, micafungin, and anidulafungin inhibit beta-(1,3)-D-glucan synthase, disrupting fungal cell wall synthesis. They are fungicidal against Candida species and fungistatic against Aspergillus. Their excellent safety profile makes them first-line for invasive candidiasis.
Allylamines including terbinafine inhibit squalene epoxidase, blocking ergosterol synthesis at an earlier step than azoles. Terbinafine is highly effective for dermatophyte infections including onychomycosis due to its fungicidal activity and keratin affinity.
Flucytosine is a pyrimidine analog converted to fluorouracil within fungal cells, inhibiting DNA and RNA synthesis. It is always used in combination (typically with amphotericin B) due to rapid resistance development as monotherapy.
Griseofulvin binds tubulin and disrupts fungal mitosis. It accumulates in keratin, making it useful for dermatophyte infections of skin, hair, and nails.
Therapeutic Uses
Superficial candidiasis responds to topical azoles or nystatin. Invasive candidiasis is treated with echinocandins as first-line. Invasive aspergillosis requires voriconazole. Cryptococcal meningitis in HIV is managed with amphotericin B plus flucytosine induction followed by fluconazole consolidation. Dermatophyte infections are treated with topical or systemic terbinafine.
Adverse Effects
Azoles cause hepatotoxicity, QT prolongation, and endocrine effects including gynecomastia and adrenal suppression with prolonged use. Amphotericin B causes infusion reactions (fever, rigors, hypotension) and dose-dependent nephrotoxicity requiring monitoring. Echinocandins are well tolerated with mild gastrointestinal effects. Terbinafine may cause taste disturbance and rare hepatotoxicity.
Key Clinical Considerations
Azole drug interactions are extensive and require careful medication review. Therapeutic drug monitoring is recommended for voriconazole and posaconazole to ensure efficacy and avoid toxicity. Antifungal resistance is an emerging concern, particularly in Candida auris and azole-resistant Aspergillus fumigatus. Prophylaxis with fluconazole or echinocandins is used in high-risk hematology patients.
Conclusion
Antifungal pharmacotherapy requires balancing efficacy with toxicity, particularly in critically ill immunocompromised patients. The echinocandin class has improved the safety of treating invasive candidiasis, while azoles remain central for most other fungal infections despite their interaction potential.
