Isavuconazole has pharmacokinetic advantages over voriconazole for invasive mold infections. Which specific voriconazole pharmacokinetic limitation does isavuconazole overcome?

• A Voriconazole exhibits nonlinear (saturable) pharmacokinetics and extreme inter-individual variability due to CYP2C19 polymorphisms (poor metabolizers vs. ultrarapid metabolizers); isavuconazole has linear PK and less CYP2C19 dependence ✓
• B Voriconazole is poorly absorbed orally while isavuconazole has 98% bioavailability
• C Voriconazole causes QTc prolongation while isavuconazole shortens the QTc interval
• D Voriconazole cannot be used in renal failure due to accumulation of cyclodextrin vehicle; isavuconazole has no cyclodextrin and a better renal safety profile but with the same PK variability

Explanation

Voriconazole undergoes saturable hepatic metabolism (nonlinear pharmacokinetics), causing disproportionate plasma level increases with dose escalation. It is primarily metabolized by CYP2C19, CYP3A4, and CYP2C9. CYP2C19 polymorphisms create enormous variability: poor metabolizers (PM, 15-20% Asians, 3-5% Caucasians) have 4-fold higher exposure than extensive metabolizers, while ultrarapid metabolizers may have subtherapeutic levels. This mandates therapeutic drug monitoring (TDM) of voriconazole. Isavuconazole has linear pharmacokinetics, broader (though incomplete) CYP3A4/CYP3A5 metabolism, and less dramatic CYP2C19 dependence, resulting in more predictable plasma concentrations. Both options C and D are partially true (voriconazole prolongs QTc while isavuconazole shortens it; IV voriconazole cyclodextrin accumulates in renal failure) but the PK variability question specifically addresses CYP2C19.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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