A NEW TRIAZOLE, VORICONAZOLE (UK-109,496), BLOCKS STEROL BIOSYNTHESISIN CANDIDA-ALBICANS AND CANDIDA-KRUSEI

Voriconazole (UK-109,496) is a novel triazole derivative with potent b road-spectrum activity against various fungi, including some that are inherently resistant to fluconazole, such as Candida krusei. In this s tudy we compared the effect of subinhibitory concentrations of voricon azole and fluconazole on sterol biosynthesis of fluconazole-resistant and -susceptible Candida albicans strains, as well as C.krusei, in an effort to delineate the precise mode of action of voriconazole. Vorico nazole MICs ranged from 0.003 to 4 mu g/ml, while fluconazole MICs ran ged from 0.25 to >64 mu g/ml. To investigate the effects of voriconazo le and fluconazole on candidal sterols, yeast cells were grown in the absence and presence of antifungals. In untreated C. albicans controls , ergosterol was the major sterol (accounting for 53.6% +/- 2.2% to 71 .7% +/- 7.8% of the total) in C. albicans and C. krusei strains. There was no significant difference between the sterol compositions of the fluconazole-susceptible and -resistant C. albicans isolates. Voriconaz ole treatment led to a decrease in the total sterol content of both C. albicans strains tested. In contrast, exposure to fluconazole did not result in a significant reduction in the total sterol content of the three candidal strains tested (P > 0.5). Sas-liquid chromatographic an alysis revealed profound changes in the sterol profiles of both C. alb icans strains and of C. krusei in response to voriconazole. This antif ungal agent exerted a similar effect on the sterol compositions of bot h fluconazole-susceptible ana -resistant C. albicans strains. Interest ingly, a complete inhibition of ergosterol synthesis and accumulation of its biosynthetic precursors were observed in both strains treated w ith voriconazole. In contrast, fluconazole partially inhibited ergoste rol synthesis. Analysis of sterols obtained from a fluconazole-resista nt C. albicans strain grown in the presence of different concentration s of voriconazole showed that this agent inhibits ergosterol synthesis in a dose-dependent manner. In C. krusei, voriconazole significantly inhibited ergosterol synthesis (over 75% inhibition). C. krusei cells treated with voriconazole accumulated the following biosynthetic inter mediates: squalene, 4,14-dimethylzymosterol, and 24-methylenedihydrola nosterol. Accumulation of these methylated sterols is consistent with the premise that this agent functions by inhibiting fungal P-450-depen dent 14 alpha-demethylase. As expected, treating C. krusei with flucon azole minimally inhibited ergosterol synthesis. Importantly, our data indicate that voriconazole is more effective than fuconazole in blocki ng candidal sterol biosynthesis, consistent with the different antifun gal potencies of these compounds.