Antifolates as antimycotics? Connection between the folic acid cycle and the ergosterol biosynthesis pathway in Candida albicans.

The increased incidence of invasive mycoses and the emerging problem of antifungal drug resistance have encouraged the search for new antifungal agents or effective combinations of existing drugs. Infections due to Candida albicans are usually treated with azole antifungals such as fluconazole, ketoconazole or itraconazole. Whilst azoles may have little or no toxicity, they generally offer rather poor fungicidal activity. Even in the absence of resistance, treatment failures or recurrent infections are not uncommon, especially in immunocompromised individuals. Here we demonstrate that the non-classical antifolate pyrimethamine shows synergy with azole antifungal compounds and interferes with the ergosterol biosynthesis pathway in C. albicans. By disturbing folate metabolism in this fungus, pyrimethamine can inhibit ergosterol production. The molecular connection between the folic acid cycle and the ergosterol biosynthesis pathway is discussed and we show that the filamentous form of this fungus is more susceptible to methotrexate than the yeast form because the drug is more effectively transported through the membrane of the filamentous form. When used to treat the hyphal form, methotrexate showed synergy with other antifungals such as azoles and terbinafine. This finding could have important clinical applications, as a combination of azoles with antifolates and/or inhibitors of folic acid synthesis could represent an attractive alternative for the treatment of C. albicans infections.