Background: Posaconazole (SCH56592) is a broad spectrum novel triazole in phase III trials. To better understand the molecular basis of POS resistance we analyzed a series of Candida albicans isolates from a patient undergoing azole treatment for refractory oropharyngeal candidiasis. Results: 7 sequential isolates were verified as being related by DNA fingerprinting. Based on POS MICs the strains fell into 2 groups; the first 4 isolates exhibited a similar susceptibility (MIC 0.25 mg/L) and the 3 later isolates exhibited an additional 8-fold increase (MIC 2 mg/L). All 7 isolates exhibited significant decreases in susceptibility to fluconazole (MIC 32 mg/L) and itraconazole (isolates 1-4 MIC 1 mg/L, isolates 5-7 MIC 16 mg/L). Since azole resistance has been associated with overexpression of ERG11 or efflux pump genes we monitored gene expression using real time PCR and Northern blot analysis. None of the isolates exhibited significant changes in expression of CDR1, CDR2, MDR1 or ERG11. The ERG11 genes were sequenced to determine if the susceptibility changes were due to an amino acid substitution that interfered with azole binding. The first 4 isolates each had 5 separate missense mutations, the resulting amino acid changes have all been previously implicated in fluconazole resistance. The 5th isolate had an additional, novel mutation (L230L), in one copy of ERG11 and the last 2 isolates were homozygous for this change. The ERG11 alleles from the 1st and 7th isolate, along with the wildtype gene, were expressed in Saccharomyces cerevisiae and the resulting strains tested for azole resistance. All ERG11 alleles conferred decreased susceptibility to fluconazole but only the ERG11 allele with all 6 mutations (including L230L) conferred decreased susceptibility to POS. Conclusion: Multiple mutations in ERG11 are required to confer POS resistance in C. albicans.
Full conference title:
42nd Interscience Conference on Antimicrobial Agents and Chemotherapy
- ICAAC 42nd