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Ketoconazole, an orally active antimycotic drug, is a potent inhibitor of ergosterol biosynthesis in Candida albicans when added to culture media which support yeast or mycelial growth or to cultures containing outgrown mycelium. This inhibition coincides with accumulation of sterols with a methyl group at C-14 and can thus be attributed to an interference with one of the reactions involved in the removal of the 14 alpha-methyl group of lanosterol. When administered to rats infected with C. albicans, ketocanazole also inhibits fungal synthesis of ergosterol. A six-times-higher dose is required to effect cholesterol synthesis by rat liver.

604th MEETING, CAMBRIDGE 665 known as denmert) caused total loss of its fungicidal activity towards Sphaerotheca fuliginea and Monilinia fmctigena. These workers suggested that the co-ordination of the heterocyclic meta-nitrogen atom of buthiobate to a cationic receptor site may be important for its fungicidal activity. The cationic receptor site mentioned would appear to be the Fez+ of the cytochrome P-450 catalysing step 1 of the sterol 14-demethylation sequence.

Yeast and plant membranes contain rather small amounts of cytochrome P–450 as compared with membrane fractions prepared from bovine adrenal cortex, piglet testis and rabbit liver. The agricultural fungicides azaconazole, penconazole, propiconazole and imazalil showed a much greater affinity for microsomal cytochrome P–450 isozymes of Saccharomyces cerevisiae and Candida albicans (ATCC 28516) than for cytochrome P–450 in microsomal fractions prepared from Jerusalem artichoke tubers, maize shoots, pea seedlings or tulip bulbs and for cytochrome P–450 isozymes in mitochondrial or microsomal fractions from rabbit liver, piglet testis and bovine adrenals. The medicinal azole antifungals miconazole, clotrimazole, ketoconazole, fluconazole and itraconazole also interacted at much lower concentrations with the microsomal cytochrome P–450 isozymes from S. cerevisiae and C. albicans (ATCC‐28516, ATCC 44859, B 34226/1) than with those in mammalian membranes. Itraconazole showed the highest selectivity; bifonazole was much less selective. The microsomal fraction prepared from C. albicans (isolate B 41628) contained cytochrome P–450 isozymes with a lower affinity than the microsomal fractions from other isolates for miconazole, ketoconazole, fluconazole and itraconazole. However, itraconazole showed still high affinity for these cytochrome P–450 isozymes. In animal models this C. albicans isolate was less pathogenic and was shown to be less sensitive to azole antifungals both in vitro and in vivo. Azole antifungals inhibited ergosterol synthesis at nanomolar concentrations whereas almost micromolor concentrations were needed to obtain a similar inhibition of cholesterol or phytosterol synthesis. This inhibition coincided with the accumulation of 14α‐methylsterols such as 14‐methyl‐fecosterol, 14‐methyl‐24‐methylene‐ergosterol, 14‐methyl‐ergosta‐8, 24 (28)‐dien‐3β, 6α‐diol, obtusifoliol, lanosterol and 24‐methylenedihydro‐lanosterol. In 24‐h‐old cultures of C. albicans the 3β, 6α‐diol was the major sterol. It is speculated that by making the 14α‐methylsterols less lipophilic the cells are trying to eliminate these membrane‐disturbing compounds. This suggests that the azole‐induced ergosterol depletion might represent a greater contribution to their fungicidal activity than the accumulation of 14α‐methylsterols.

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