The fungal pathogen Aspergillus fumigatus causes serious illness and often death when it invades tissues, especially in immunocompromised individuals. The
azole class of drugs is the most commonly prescribed treatment for many
fungal infections and acts on the
ergosterol biosynthesis pathway. One common mechanism of acquired
azole drug resistance in fungi is the prevention of
drug accumulation to toxic levels in the cell. While
drug efflux is a well-known resistance strategy, reduced
azole import would be another strategy to maintain low intracellular
azole levels. Recently,
azole uptake in Candida albicans and other yeasts was analyzed using [(3)H]
fluconazole. Defective
drug import was suggested to be a potential mechanism of drug resistance in several pathogenic fungi, including Cryptococcus neoformans, Candida krusei, and Saccharomyces cerevisiae. We have adapted and developed an assay to measure
azole accumulation in A. fumigatus using radioactively labeled
azole drugs, based on previous work done with C. albicans. We used this assay to study the differences in
azole uptake in A. fumigatus isolates under a variety of
drug treatment conditions, with different morphologies and with a select mutant strain with deficiencies in the
sterol uptake and biosynthesis pathway. We conclude that
azole drugs are specifically selected and imported into the fungal cell by a pH- and
ATP-independent facilitated diffusion mechanism, not by passive diffusion. This method of
drug transport is likely to be conserved across most fungal species.