Anti-Candida drugs--the biochemical basis for their activity

The past years have seen a continuous effort toward the synthesis of new antifungal agents. Most of them belong to the N-substituted imidazoles and triazoles. Another interesting series of antifungals are the allylamines. Biochemically, both the azole derivatives and the allylamines belong to the class of ergosterol biosynthesis inhibitors and thus differ from the polyene macrolide antibiotics. Indeed, it is now believed that the antifungal action of the polyenes, nystatin and amphotericin B, is due to a direct interaction with ergosterol itself. A more detailed analysis of the ergosterol biosynthesis inhibitors revealed that ergosterol depletion is the consequence of the interaction of the azole derivatives, e.g., miconazole, ketoconazole, and itraconazole, with the cytochrome P-450 involved in the 14 alpha-demethylation of lanosterol. Both the accumulation of 14 alpha-methylsterols and the concomitant decreased ergosterol content affect the membranes and membrane-bound enzymes of yeast and fungi. The allylamines seem to act by inhibition of the squalene epoxidase resulting in ergosterol depletion and accumulation of squalene. The target for the fluorinated pyrimidine, flucytosine, is completely different. Its antifungal properties may result from its conversion to 5-fluorouracil. The latter is then phosphorylated and incorporated into RNA, thus disrupting the protein synthesis in the yeast cell. These different biochemical targets for the antifungals of use in candidosis are discussed in this paper.