5-Fluorocytosine antagonizes the action of sterol biosynthesis inhibitors in Candida glabrata

The concentration-dependent antagonistic interaction between 5-fluorocytosine and a sterol biosynthesis inhibitor (SBI) was studied using intact cells and cell-free extracts of Candida glabrata. 5-Fluorocytosine promoted incorporation of radioactivity into 4-desmethylsterols (P < 0.01), and enhanced the relative and absolute increases of ergosterol (P < 0.05) in C. glabrata incubated aerobically with an SBI (miconazole or amorolfine). Further aerobic incubation of C. glabrata with combinations of a nucleic acid or protein synthesis inhibitor (rifampicin or chlortetracycline) and an SBI (miconazole) promoted a similar increase in ergosterol biosynthesis. In contrast, 5-fluorocytosine reduced the incorporation of radioactivity into 4,4-dimethylsterols (P < 0.01), but had no obvious effect on the absolute ergosterol level in C. glabrata incubated statically with miconazole. In cell-free extracts of cultures previously incubated with 5-fluorocytosine, ergosterol synthesis was less sensitive to the action of miconazole. Antagonism between 5-fluorocytosine and the SBI is thus mediated by a reversal of inhibition of intracellular ergosterol synthesis. The possible mechanisms underlying antagonism between 5-fluorocytosine and SBIs that inhibit different sites of the sterol biosynthesis pathway, as well as its clinical relevance to combination therapy, are discussed.