Antifungal Drugs

• Ergosterol Pathway Disruption (Azoles and Polyenes): The primary target of most antifungal drugs is ergosterol, the predominant sterol in fungal cell membranes that is functionally analogous to cholesterol in mammalian cells. Azole antifungals (imidazoles and triazoles) inhibit the cytochrome P450-dependent enzyme lanosterol 14α-demethylase (also called CYP51), which catalyzes a critical step in ergosterol synthesis. This enzyme converts lanosterol to 4,4-dimethyl-cholesta-8,14,24-trienol; inhibition causes accumulation of toxic sterol precursors and depletion of ergosterol, destabilizing the fungal cell membrane and disrupting membrane-bound enzyme function. Polyene antifungals (amphotericin B, nystatin) achieve selectivity through their preferential binding to ergosterol over cholesterol, though some mammalian toxicity occurs due to incomplete selectivity. The mechanism is concentration-dependent: at low concentrations, azoles produce fungistatic effects through membrane disruption; at higher concentrations, they may exert fungicidal activity through additional effects on membrane permeability and oxidative stress.

• Fungal Cell Wall Synthesis Inhibition (Echinocandins): Echinocandins represent a structurally distinct class targeting β-(1,3)-D-glucan synthase, a transmembrane enzyme complex that catalyzes polymerization of glucose into β-(1,3)-D-glucan, a major structural component of the fungal cell wall unique to fungi and absent in mammals. The echinocandin molecule, a large cyclic lipopeptide, binds to the Fks1 protein (a component of the glucan synthase complex) and inhibits glucan synthesis, causing cell wall weakening, membrane rupture, and ultimately cell death. This mechanism is inherently more selective than ergosterol targeting because mammalian cells lack cell walls entirely. Echinocandins are fungicidal against most Candida species and Aspergillus fumigatus but show limited efficacy against Cryptococcus neoformans (which has minimal β-glucan) and are inactive against most Zygomycetes.

• Nucleic Acid Synthesis Disruption (Flucytosine): Flucytosine (5-FC) is a pyrimidine analog that undergoes fungal-specific metabolism; it is converted intracellularly by cytidine deaminase to fluorouracil (5-FU), which is then converted to fluorodeoxyuridylate (FdUMP), an inhibitor of thymidylate synthase, and subsequently to fluorouridine triphosphate (FUTP), which is incorporated into fungal RNA. This dual mechanism—inhibiting DNA synthesis through thymidylate synthase blockade and disrupting RNA function through FUTP incorporation—leads to fungistatic effects. Mammalian cells typically lack high cytidine deaminase activity, providing some selectivity, though toxicity can occur if mammalian enzymes metabolize 5-FC to 5-FU. Resistance develops relatively rapidly through loss of cytidine deaminase or altered uracil phosphoribosyltransferase activity, limiting 5-FC use as monotherapy.

• Membrane Transport Disruption (Allylamines): Terbinafine, an allylamine antifungal, inhibits squalene epoxidase, an earlier step in ergosterol synthesis than azoles. This causes accumulation of toxic squalene and depletion of downstream sterols, but the mechanism differs from azoles in that it primarily affects the fungal rather than human sterol pathway. Terbinafine shows particularly high activity against dermatophytes due to its lipophilicity and concentration in skin, hair, and nails.

• Resistance Mechanisms and Clinical Implications: Fungal resistance emerges through multiple pathways: (1) mutations in the target enzyme reducing antifungal binding (e.g., FKS mutations in echinocandin-resistant Candida, CYP51 mutations in azole-resistant Aspergillus or Candida); (2) overexpression of the target enzyme; (3) upregulation of efflux pumps (particularly ABC transporters and MFS transporters) that actively remove azoles from fungal cells, a major mechanism of azole resistance in Candida auris and other emerging resistant species; (4) altered cell wall composition or reduced glucan content conferring echinocandin resistance; and (5) development of alternative biochemical pathways bypassing the drug target. Understanding these resistance mechanisms is increasingly critical as multidrug-resistant fungi (notably Candida auris) emerge globally.

• Immunosuppression as Primary Driver: Advanced immunosuppression (CD4 count <50 cells/μL in untreated HIV/AIDS, prolonged neutropenia in hematologic malignancies, organ transplantation with intensive immunosuppressive regimens) fundamentally increases risk for both superficial and invasive fungal infections. In HIV/AIDS, opportunistic fungal infections including Candida esophagitis, cryptococcal meningitis, and coccidioidal infections increase dramatically when CD4 counts fall below 100 cells/μL. Hematologic malignancy patients undergoing intensive chemotherapy experience prolonged periods of profound neutropenia (absolute neutrophil count <500/μL) creating ecological niches for fungal overgrowth, particularly Candida and Aspergillus species.

• Indwelling Catheters and Medical Devices: Central venous catheters (both short-term and tunneled), urinary catheters, and other indwelling foreign bodies serve as substrates for biofilm formation by Candida species. Biofilms represent organized communities of fungi embedded in extracellular matrix, conferring enhanced resistance to both immune mechanisms and antifungal drugs; this is a major driver of catheter-associated candidemia. The longer the catheter remains in place and the more compromised the immune system, the higher the risk.

• Antecedent Broad-Spectrum Antibiotic Exposure: Extended use of broad-spectrum antibiotics (particularly fluoroquinolones, cephalosporins, and carbapenems) disrupts normal bacterial flora that normally compete with Candida for ecological niches in the gastrointestinal tract and other mucosal surfaces. This bacterial suppression allows Candida overgrowth and translocation, particularly in critically ill patients with disrupted epithelial barriers (ICU patients, those post-abdominal surgery).

• Fungal Species-Specific Epidemiology: Different antifungals are indicated for different species. Candida albicans remains the most common cause of invasive candidiasis but is generally susceptible to all antifungal classes; however, non-albicans Candida species (particularly C. glabrata, C. auris, C. tropicalis) show variable susceptibility patterns, with C. glabrata showing elevated echinocandin MICs in some settings, and C. auris demonstrating multidrug resistance to azoles and sometimes echinocandins. Aspergillus fumigatus is the most common cause of invasive aspergillosis in immunocompromised patients; triazoles (voriconazole, posaconazole) are standard therapy, though azole-resistant A. fumigatus is emerging. Cryptococcus neoformans causes meningitis particularly in AIDS patients; amphotericin B plus flucytosine remains the gold standard for CNS disease. Coccidioides immitis and Histoplasma capsulatum predominate in specific geographic regions (southwestern United States and Ohio/Mississippi River valleys, respectively).

• Geographic and Epidemiologic Factors: Certain fungi are endemic to specific regions; recognition of patient travel history or residence is critical. Coccidioidal infection risk is elevated in residents of or travelers to the American Southwest. Histoplasma exposure occurs in areas with Ohio and Mississippi River valley soil contamination. Blastomyces dermatitidis similarly has geographic restriction. Recognition of these epidemiologic patterns guides appropriate antifungal selection.

• Candidal Infections: Oral candidiasis (thrush) presents as white plaques on the tongue, palate, and buccal mucosa that can be wiped away, revealing erythematous or bleeding mucosa beneath; patients often report dysphagia and altered taste. Esophageal candidiasis causes severe odynophagia (pain with swallowing) and substernal chest pain, particularly when the infection extends into the mid-to-distal esophagus. Vulvovaginal candidiasis manifests as vaginal pruritus, cottage cheese–like discharge, vulvar erythema, and dysuria. In invasive candidiasis (candidemia), patients present with fever that may not respond to broad-spectrum antibiotics, often with hemoculture positivity; dissemination can occur to virtually any organ (CNS causing meningitis, eyes causing endophthalmitis, heart causing endocarditis, kidneys causing pyelonephritis). The clinical presentation of candidemia may be indistinguishable from bacterial sepsis.

• Aspergillosis Spectrum: Allergic bronchopulmonary aspergillosis (ABPA) presents with chronic productive cough, bronchial obstruction, and wheezing in patients with asthma or cystic fibrosis due to hypersensitivity to Aspergillus antigens; imaging shows pulmonary infiltrates. Chronic pulmonary aspergillosis (CPA) manifests as chronic cavitary lung disease in patients with preexisting structural lung disease (prior tuberculosis, emphysema), presenting with progressive dyspnea, cough productive of hemoptysis, and constitutional symptoms. Invasive pulmonary aspergillosis (IPA) in severely immunocompromised patients presents with fever, cough, dyspnea, and sometimes hemoptysis, with imaging showing nodules, halos (ground-glass opacity surrounding nodules indicating hemorrhage—highly suggestive in neutropenic patients), or cavitation. Disseminated aspergillosis can involve skin, sinuses, and CNS.

• Cryptococcal Meningitis: The classic presentation in AIDS patients with CD4 <100 cells/μL includes fever, headache, meningismus (though less pronounced than bacterial meningitis), and cognitive dysfunction or altered mental status. Surprisingly, meningeal signs are often absent despite significant CNS involvement. Patients may present insidiously with subacute course (weeks), distinguishing it from acute bacterial meningitis. Immunocompetent hosts rarely develop disease unless exposed to massive inocula.

• Dermatophytosis: Tinea pedis (athlete's foot) presents with maceration, erythema, and scaling between toes or on plantar surface; the fourth and fifth interdigital spaces are classically involved. Tinea corporis shows expanding annular plaques with central clearing and erythematous raised borders. Onychomycosis manifests as nail thickening, discoloration (yellow, brown, or white), and crumbling; distal subungual onychomycosis is most common, with infection beginning at the hyponychium and progressing proximally. Tinea capitis in children presents with scaly patches of alopecia; some species (particularly Trichophyton tonsurans in North America) cause black dot ringworm with minimal inflammation.

• Endemic Fungal Infections: Coccidioidomycosis presents acutely as a pneumonia-like illness (coccidioidal pneumonia) with fever, cough, chest pain, and respiratory symptoms often 1-3 weeks after inhaling Coccidioides spores; some patients progress to chronic cavitary disease or disseminated disease with meningitis, bone/joint infections, or skin lesions. Histoplasmosis presents similarly as acute pulmonary disease or can disseminate to involve reticuloendothelial system (hepatosplenomegaly), CNS, or endocardium.

• Physical Examination Findings: Oral thrush shows white plaques on mucous membranes; vulvovaginal candidiasis shows vulvar erythema and thick discharge; tinea corporis shows annular lesions with active erythematous borders; nails with onychomycosis are thickened and discolored; skin lesions of disseminated candidiasis may appear as small nodules; fundoscopic examination in candidemia may reveal cotton-wool spots or hemorrhages consistent with endophthalmitis.

• Histopathology and Direct Visualization: The gold standard for diagnosing many fungal infections involves direct visualization through microscopy and/or histology. KOH (potassium hydroxide) preparation of skin, nail, or hair specimens dissolves cellular material, allowing visualization of fungal elements (hyphae, spores) under light microscopy; sensitivity is 40-70% for tinea corporis and may be lower for onychomycosis. Gram stain shows budding yeast as gram-positive cocci; Candida appears as gram-positive yeast and pseudohyphae. PAS (periodic acid-Schiff) stain and GMS (Gomori methenamine silver) stain are gold standards for histologic identification, with GMS particularly useful for demonstrating fungal elements in tissue sections; these are especially valuable for diagnosing invasive fungal infections through tissue biopsy. India ink preparation of CSF reveals Cryptococcus neoformans as a yeast surrounded by a clear halo (the polysaccharide capsule), though sensitivity is only 50-80% in cryptococcal meningitis.

• Fungal Cultures: Culture remains the gold standard for organism identification and antifungal susceptibility testing. Candida grows readily on standard media (blood cultures, Sabouraud dextrose agar); culture of blood, CSF, urine, or tissue establishes invasive infection. Culture results typically take 2-7 days but provide species identification and susceptibility testing critical for directing therapy. For Aspergillus, respiratory culture positivity alone does not diagnose invasive disease (can represent colonization); positive culture combined with compatible clinical and radiologic findings suggests invasive aspergillosis.

• Molecular and Antigen Detection: β-D-Glucan is a fungal cell wall component detectable in serum; levels >60 pg/mL suggest invasive fungal infection (sensitivity 60-80% for invasive candidiasis, higher for Aspergillus); it is not species-specific and can be falsely elevated with glucan-containing medications or dialysis membranes. Galactomannan antigen detection in serum or BAL fluid is relatively specific for Aspergillus (sensitivity 60-90% for invasive aspergillosis) and can help guide earlier diagnosis before culture results return. Cryptococcal antigen (CrAg) testing in serum and CSF is highly sensitive (>95%) and specific for Cryptococcus; a positive serum CrAg in an AIDS patient with CD4 <100 cells/μL warrants prophylaxis or treatment. PCR-based assays for fungal species are increasingly available in specialized centers and can provide rapid species identification.

Amphotericin B (polyene)

• Nephrotoxicity: the dose-limiting toxicity. Afferent arteriolar vasoconstriction lowers GFR, while cholesterol-binding pores in tubular membranes cause potassium and magnesium wasting and a distal (type 1) renal tubular acidosis. Monitor renal function and electrolytes frequently (commonly every 2-3 days, and more often during dose escalation or with concurrent nephrotoxins); guidelines do not specify a uniform interval. Pre-infusion normal saline loading and use of a lipid/liposomal formulation are the standard mitigations endorsed in IDSA fungal infection guidelines. There is no antidote — repletion and formulation switching are the reversal strategy.
• Infusion reactions: fever, rigors, and hypotension ("shake and bake"), cytokine-mediated; premedication with antipyretics/antihistamines is used.
• Normocytic anemia: from suppressed erythropoietin production.

Azoles

• CYP450 inhibition: the class effect that drives most harm — elevated levels of warfarin, statins (rhabdomyolysis), calcineurin inhibitors, and QT-prolonging drugs. All azoles except isavuconazole prolong the QT interval.
• Hepatotoxicity: transaminase elevation to overt hepatitis; check LFTs before and during prolonged therapy.
• Ketoconazole: blocks human 17,20-desmolase and 11β-hydroxylase, producing gynecomastia, decreased libido, and adrenal insufficiency — systemic use is now largely abandoned.
• Itraconazole: negative inotropy; avoid in heart failure.
• Voriconazole: transient visual disturbances/photopsia, hallucinations, phototoxicity with squamous cell carcinoma risk, and fluoride-associated periostitis on long courses; therapeutic drug monitoring is recommended given CYP2C19 polymorphism.
• Pregnancy: azoles are teratogenic; CDC STI guidelines favor topical azoles for vulvovaginal candidiasis in pregnancy.

Other classes

• Flucytosine: converted to 5-FU by gut bacterial and fungal cytosine deaminase. Human cells lack cytosine deaminase — this is the basis of the drug's selective toxicity, and marrow injury arises largely from 5-FU generated by gut flora. Result: dose-dependent bone marrow suppression, mucositis, and hepatotoxicity. Requires renal dose adjustment, serial CBCs, and where available serum level monitoring.
• Echinocandins: best tolerated; histamine-mediated infusion flushing and mild transaminitis.
• Terbinafine: taste and smell disturbance, headache, and rare hepatotoxicity — baseline LFTs.
• Griseofulvin: CYP450 inducer (oral contraceptive and warfarin failure), disulfiram-like reaction with alcohol, teratogenic.

• Amphotericin B + electrolytes: a patient on amphotericin with rising creatinine, hypokalemia, hypomagnesemia, and a non-anion-gap acidosis is the classic stem. Single best next step: saline hydration, aggressive K⁺/Mg²⁺ repletion, and switch to a liposomal formulation. Distractor to avoid — stopping all antifungal therapy in a patient with proven invasive disease.
• Cryptococcal meningitis induction: liposomal amphotericin B plus flucytosine, then fluconazole consolidation and maintenance, per IDSA cryptococcosis guidance. Flucytosine is never used alone (rapid resistance). Serial lumbar punctures for elevated opening pressure — not steroids — manage the raised ICP.
• Echinocandins for candidemia: IDSA candidiasis guidance makes an echinocandin the initial agent for candidemia, including Candida glabrata and C. auris. The examiners' trap: echinocandins penetrate CSF and urine poorly, so Candida meningitis, endophthalmitis, and cystitis call for another agent. For urinary tract candidiasis specifically, use fluconazole (preferred) or amphotericin B deoxycholate — lipid/liposomal formulations do not achieve useful urinary concentrations.
• Voriconazole is first-line for invasive aspergillosis (IDSA), with isavuconazole an accepted alternative primary agent (better tolerated, does not prolong QT). Voriconazole's giveaway adverse effect is transient visual disturbance/photopsia; long-term use adds phototoxicity and periostitis.
• Ketoconazole = antiandrogen: gynecomastia, decreased libido, and adrenal suppression from blockade of human steroidogenic CYP enzymes. The same property explains its historical use in Cushing syndrome.
• Azole = CYP inhibitor; griseofulvin = CYP inducer: an azole plus warfarin raises INR; griseofulvin causes oral contraceptive failure. Terbinafine inhibits CYP2D6.
• Dermatophyte drug choice: terbinafine (squalene epoxidase inhibitor, accumulates in keratin) for onychomycosis; griseofulvin (microtubule/mitotic spindle disruption) remains a standard oral option for tinea capitis in children. Topical agents fail in nail and scalp disease.
• Nystatin is topical only — swish-and-swallow works for thrush, but odynophagia in an AIDS patient means esophageal candidiasis and systemic fluconazole, not nystatin. Itraconazole in a heart failure patient is another planted wrong answer.