Antifungal agents target the unique components of fungal cell membranes and cell walls, exploiting structural differences between fungal and mammalian cells to achieve selective toxicity. The incidence of invasive fungal infections has increased substantially with the growing population of immunocompromised patients, including those receiving chemotherapy, immunosuppressants, and prolonged intensive care.
What Are Antifungal Agents?
Fungi are eukaryotic organisms that share many cellular features with mammalian cells, making the development of selective antifungal therapy challenging. The fungal cell membrane contains ergosterol rather than cholesterol, and the fungal cell wall is composed of glucans, mannans, and chitin, structures absent in mammalian cells. These differences provide the basis for most antifungal drug targets. Antifungals are classified by their mechanism of action and spectrum of activity, which determines their clinical use.
Mechanism of Action
Azoles such as fluconazole, itraconazole, voriconazole, and posaconazole inhibit lanosterol 14-alpha-demethylase, a CYP450-dependent enzyme that converts lanosterol to ergosterol in the fungal cell membrane. Depletion of ergosterol and accumulation of toxic sterol precursors disrupts membrane integrity and function. Azoles are fungistatic against most fungi, meaning they inhibit growth rather than killing the organism. Voriconazole is the preferred agent for invasive aspergillosis, while fluconazole is used for candida infections and cryptococcal meningitis.
Polyenes such as amphotericin B bind directly to ergosterol in the fungal cell membrane, forming pores that allow leakage of intracellular ions and macromolecules, causing rapid fungicidal activity. Amphotericin B has the broadest spectrum of any antifungal agent and is used for severe invasive fungal infections. However, its clinical utility is limited by significant toxicity, particularly infusion-related reactions and nephrotoxicity. Lipid formulations of amphotericin B reduce toxicity while maintaining efficacy.
Echinocandins such as caspofungin, micafungin, and anidulafungin inhibit beta-1,3-glucan synthase, an enzyme essential for synthesis of the fungal cell wall component beta-glucan. Inhibition of this enzyme leads to osmotic instability and cell death. Echinocandins are fungicidal against Candida species and are first-line therapy for invasive candidiasis. They have limited activity against Cryptococcus and are ineffective against Aspergillus, where they are used as salvage therapy.
Terbinafine inhibits squalene epoxidase, an early enzyme in the ergosterol biosynthesis pathway. Accumulation of squalene and depletion of ergosterol disrupts membrane function. Terbinafine is highly effective for dermatophyte infections of skin and nails and is the first-line oral agent for onychomycosis.
Therapeutic Uses
Antifungals are used for superficial infections involving skin, hair, and nails, and for invasive infections affecting deep tissues and bloodstream. Superficial infections are often treated with topical azoles or terbinafine. Invasive candidiasis is treated with echinocandins or fluconazole depending on species and susceptibility. Aspergillosis requires voriconazole. Cryptococcal meningitis in HIV patients is treated with amphotericin B and flucytosine induction followed by fluconazole maintenance.
Adverse Effects
Azoles cause hepatotoxicity, gastrointestinal intolerance, and QT prolongation. They are potent inhibitors of CYP450 enzymes, causing numerous drug interactions. Amphotericin B causes infusion-related fever, chills, and rigors, and dose-dependent nephrotoxicity. Echinocandins are well tolerated, with minimal drug interactions and rare hepatotoxicity. Terbinafine can cause taste disturbance and rare hepatotoxicity.
Contraindications
Azoles are contraindicated in patients with significant hepatic impairment and during coadministration with drugs that prolong the QT interval. Amphotericin B should be used cautiously in patients with pre-existing renal impairment. Dose adjustments for echinocandins are needed in moderate to severe hepatic impairment.
Conclusion
Antifungal therapy requires careful consideration of the infecting organism, site of infection, immune status of the host, and drug toxicity profiles. The limited number of antifungal classes and increasing resistance, particularly among Candida and Aspergillus species, underscore the need for antifungal stewardship and continued drug development.
