Bifonazole and clotrimazole. Their mode of action and the possible reason for the fungicidal behaviour of bifonazole

Bifonazole (Bay h 4502, Mycospor) and clotrimazole (Bay b 5097, Canesten) are potent inhibitors of ergosterol synthesis in yeasts and dermatophytes. Inhibition of demethylation of 4,4',14-trimethylsterols is accepted as primary mode of action responsible for their fungistatic efficacy. In Candida albicans, Microsporum canis, Trichophyton mentagrophytes as well as in Epidermophyton floccosum the ergosterol precursor 24-methylendihydrolanosterol accumulates, whereas in Torulopsis glabrata lanosterol accumulation occurs, due to the fact that in this organism side chain alkylation proceeds after demethylation reactions. Bifonazole additionally leads to a generally decreased rate of sterol biosynthesis as compared to clotrimazole, due to a direct inhibition of microsomal HMG-CoA-reductase. The additional fungicidal effects of bifonazole are considered to originate from a sequential action by inhibition of HMG-CoA-reductase and of Cytochrome P450.