ABSTRACT Cryptococcosis is a life-threatening disease often associated with HIV infection. Three Cryptococcus species CYP51 enzymes were purified and catalyzed the 14α-demethylation of lanosterol, eburicol, and obtusifoliol. The investigational agent VT-1129 bound tightly to all three CYP51 proteins (dissociation constant [ K d ] range, 14 to 25 nM) with affinities similar to those of fluconazole, voriconazole, itraconazole, clotrimazole, and ketoconazole ( K d range, 4 to 52 nM), whereas VT-1129 bound weakly to human CYP51 ( K d , 4.53 μM). VT-1129 was as effective as conventional triazole antifungal drugs at inhibiting cryptococcal CYP51 activity (50% inhibitory concentration [IC 50 ] range, 0.14 to 0.20 μM), while it only weakly inhibited human CYP51 activity (IC 50 , ∼600 μM). Furthermore, VT-1129 weakly inhibited human CYP2C9, CYP2C19, and CYP3A4, suggesting a low drug-drug interaction potential. Finally, the cellular mode of action for VT-1129 was confirmed to be CYP51 inhibition, resulting in the depletion of ergosterol and ergosta-7-enol and the accumulation of eburicol, obtusifolione, and lanosterol/obtusifoliol in the cell membranes.