Background: Posaconazole (SCH 56592) is a novel triazole, in phase III clinical trials, with potent activity against many fungal pathogens including Aspergillus, Candida, Coccidioides and Cryptococcus. To better understand the molecular basis of POS resistance in A. fumigatus, resistant laboratory isolates were selected. Results: Spontaneous POS resistant mutants of A. fumigatus strain ND158 (MIC 0.03 mg/L) arose at a frequency of 10-8 and fell into 2 susceptibility groups, moderately resistant and highly resistant. The mutants were analyzed for changes in transcript levels of efflux pump genes (mdr1 and mdr2) and the genes encoding the POS target site, cytochrome P450 14a-demethylase (cyp51A and cyp51B). No changes in gene expression were detected. Furthermore, the mutants did not exhibit changes in [14C] POS accumulation. DNA sequencing revealed that each resistant isolate had a point mutation in cyp51A resulting in the substitution for glycine 54 of either arginine (MIC 1 mg/L) or tryptophan (MIC 8 mg/L). The same mutations were identified in 3 clinical A. fumigatus isolates exhibiting similar POS MICs, but not in susceptible clinical isolates. To verify that the mutations were responsible for the resistance phenotype we introduced the mutant alleles into the chromosome of a POS susceptible A. fumigatus strain. The transformants exhibited reductions in POS susceptibility comparable to the original mutants. Conclusion: Amino acid substitutions that confer POS resistance in A. fumigatus appear to be restricted to a single amino acid close to the N-terminus of the cytochrome P450 14a-demethylase. The unusual location suggests that POS interacts with the 14a-demethylase in a novel manner. This may account for the finding that voriconazole resistant A. fumigatus isolates remain POS susceptible (Manavathu et. al. J. Antimicrob. Chemother. 2000 46:229-234).
Full conference title:
42nd Interscience Conference on Antimicrobial Agents and Chemotherapy
- ICAAC 42nd