Pharmacology · Antifungal Agents

Azole Antifungals — Fluconazole and Itraconazole

CYP51 inhibition, spectrum, pharmacokinetics, drug interactions, and resistance

Azoles inhibit fungal CYP51 (lanosterol 14α-demethylase), blocking the conversion of lanosterol to ergosterol — ergosterol depletion disrupts membrane fluidity and function, producing a fungistatic effect against most organisms.
Fluconazole is the first-line agent for candidemia in stable, non-neutropenic patients and for all forms of Cryptococcus consolidation and maintenance therapy — but it has no activity against Aspergillus or molds.
Itraconazole capsules require an acidic stomach and fatty food for adequate absorption — bioavailability is highly variable; the oral solution (in hydroxypropyl-β-cyclodextrin) has more predictable absorption and should be taken fasted.
All azoles are potent inhibitors of CYP3A4 and CYP2C9 — drug interactions are the most clinically significant adverse effect category; fluconazole additionally inhibits CYP2C19; check for interactions before every prescription.
Azole resistance in Candida arises primarily through ERG11 point mutations (reduced CYP51 affinity) and CDR1/CDR2 efflux pump upregulation — Candida krusei (now Pichia kudriavzevii) is intrinsically fluconazole-resistant and should never be treated with fluconazole regardless of MIC.

Abbreviations: CYP51 = lanosterol 14α-demethylase · CYP3A4 = cytochrome P450 3A4 · MIC = minimum inhibitory concentration · CSF = cerebrospinal fluid · TDM = therapeutic drug monitoring · HPβCD = hydroxypropyl-β-cyclodextrin · QTc = corrected QT interval · ERG11 = ergosterol biosynthesis gene 11 · MDR = multidrug-resistant

Mechanism of Action

CYP51 Inhibition — Ergosterol Depletion

Shared Mechanism of All Azoles

Azole nitrogen coordinates with the heme iron of fungal CYP51 (lanosterol 14α-demethylase) → blocks conversion of lanosterol → ergosterol
Ergosterol depletion → altered membrane fluidity and permeability → impaired membrane protein function and cell growth
Accumulation of toxic methylated sterol intermediates (14α-methylated sterols) contributes to toxicity
Primarily fungistatic — relies on host immune system to clear the infection; fungicidal only against Candida in some settings
Azoles also inhibit mammalian CYP enzymes (less selective than fungal CYP51) — source of hepatotoxicity and drug interactions
Activity against Aspergillus and molds: limited for fluconazole and itraconazole; better with extended-spectrum azoles (voriconazole, posaconazole) — covered in Module 3

Fluconazole vs. Itraconazole

Feature	Fluconazole	Itraconazole
Spectrum	Most Candida; Cryptococcus; Coccidioides; dermatophytes; no mold activity	Broader: Candida, Cryptococcus, endemic dimorphics (Histoplasma, Blastomyces, Sporothrix), some Aspergillus
Oral Bioavailability	~90% regardless of food or gastric pH — highly reliable	Capsule: ~55% with high-fat meal and acid; poor in achlorhydria or with PPIs. Solution: better, take fasted; serum levels unpredictable — TDM recommended
CNS Penetration	Excellent — ~60–80% of plasma; preferred for cryptococcal meningitis consolidation	Poor — not for CNS infections
Renal Adjustment	Yes — primarily renal elimination; reduce dose when CrCl <50 mL/min	No — hepatic metabolism; no renal adjustment
CYP Inhibition	CYP2C9 (major), CYP3A4 (moderate), CYP2C19 (minor)	CYP3A4 (potent, mechanism-based irreversible) — strongest CYP3A4 inhibitor of the first-generation azoles
QTc Prolongation	Yes — dose-dependent; check baseline QTc	Yes — particularly significant given strong CYP3A4 inhibition; raises levels of QTc-prolonging substrates
Key Indications	Candidemia (stable/non-neutropenic); oropharyngeal, esophageal, vaginal candidiasis; cryptococcal consolidation/maintenance; coccidioidal meningitis; prophylaxis	Histoplasmosis (preferred oral); blastomycosis; sporotrichosis; onychomycosis; dermatophytes; aspergillosis (step-down from IV)

Spectrum Gaps and Resistance

Intrinsic Resistance — Never Use Fluconazole

Organisms Where Fluconazole Fails

Candida krusei (now Pichia kudriavzevii) — intrinsically resistant; use echinocandin or amphotericin B
Aspergillus species — all first-generation azoles lack reliable activity; use voriconazole (Module 3)
Mucorales — all azoles inactive; amphotericin B required
Candida auris — variable; high rates of fluconazole resistance; echinocandin preferred
Candida glabrata (now Nakaseomyces glabrata) — intermediate susceptibility; higher fluconazole MICs; echinocandin preferred for invasive disease

Acquired Resistance Mechanisms

How Candida Escapes Azoles

ERG11 point mutations — alter CYP51 active site, reduce azole binding affinity
CDR1/CDR2 efflux pumps (ABC transporters) — active extrusion of drug from the cell
MDR1 efflux pump (MFS transporter) — fluconazole-specific efflux
ERG3 mutations — bypass ergosterol pathway; tolerate membrane with alternative sterols
Azole resistance emergence during prolonged therapy is well documented in C. glabrata and C. albicans — repeat susceptibility testing for clinical failure
Aspergillus fumigatus environmental azole resistance via CYP51A mutations — now detected globally; test susceptibility before empiric voriconazole

Clinical Rules: Itraconazole Absorption and CYP3A4 Interactions

Itraconazole capsule absorption is profoundly dependent on gastric acid and dietary fat — it requires a high-fat meal and an acidic environment to dissolve. Proton pump inhibitors, H2 blockers, antacids, and achlorhydria all significantly reduce bioavailability of the capsule formulation. When reliable levels are needed (invasive fungal infections), use the oral solution (taken fasted in HPβCD) or IV formulation and obtain trough levels — target trough above 0.5 mcg/mL for prophylaxis and above 1.0 mcg/mL for treatment.

Itraconazole is a potent mechanism-based (irreversible) CYP3A4 inhibitor — its interaction profile is broader and more severe than fluconazole's. Contraindicated combinations include: HMG-CoA reductase inhibitors metabolized by CYP3A4 (simvastatin, lovastatin — rhabdomyolysis risk); ergot alkaloids (severe vasospasm); oral midazolam and triazolam (excessive sedation); quinidine and pimozide (QTc prolongation). Check every co-administered drug for CYP3A4 substrate status before prescribing itraconazole.

Suggested References

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