Azole antifungals exert their fungistatic effect by inhibiting which cytochrome P450 enzyme in fungi?

Explanation

## Azole Mechanism of Action **Key Point:** Azoles (imidazoles: ketoconazole, miconazole; triazoles: fluconazole, itraconazole, voriconazole, posaconazole) inhibit **CYP51 (lanosterol 14α-demethylase)**, a fungal-specific cytochrome P450 enzyme. ### Biochemical Basis 1. **Target enzyme:** Lanosterol 14α-demethylase (CYP51) catalyzes a critical step in ergosterol synthesis 2. **Mechanism:** Azoles bind the heme iron of CYP51, blocking the demethylation of lanosterol 3. **Consequence:** Depletion of ergosterol + accumulation of toxic sterol precursors → cell membrane dysfunction 4. **Result:** Fungistatic effect (growth inhibition; not fungicidal in most cases) ### Why CYP51 is Selective ```mermaid flowchart TD A[Azole antifungal]:::action --> B[Binds heme iron of CYP51]:::action B --> C[Blocks lanosterol demethylation]:::action C --> D[Ergosterol depletion]:::outcome D --> E[Membrane integrity lost]:::urgent E --> F[Fungistatic effect]:::outcome classDef action fill:#10b981,stroke:#059669,color:#fff classDef outcome fill:#3b82f6,stroke:#1e40af,color:#fff classDef urgent fill:#ef4444,stroke:#dc2626,color:#fff ``` **High-Yield:** Fungal CYP51 is structurally distinct from human CYP enzymes, allowing selective inhibition. However, azoles **do inhibit human CYP3A4, CYP2C9, and CYP2D6** at higher concentrations, causing significant drug interactions. **Mnemonic:** **AZOLE → CYP51** (fungal-specific demethylase). Remember: azoles are **fungistatic**, not fungicidal (except at high doses). **Clinical Pearl:** Triazoles (fluconazole, itraconazole) are preferred over imidazoles because they have greater selectivity for fungal CYP51 vs. human cytochromes, reducing systemic toxicity. [cite:Harrison 21e Ch 207]