Background: Efflux by membrane-based pumps is a mechanism of azole resistance for Candida spp. These pumps are constitutive components of the fungal genome & we hypothesized that their expression might contribute to the trailing growth (incomplete growth inhibition even at very high drug concentrations) seen during antifungal susceptibility testing (AFST). MC-510011 & MC-510027 are milbemycins (MLBs) that inhibit the ATP binding cassette (ABC) efflux pumps CDR1 and CDR2. We sought to determine whether these compounds were synergistic with FLU and if they eliminated trailing growth. Methods: Interactions between fluconazole (FLU) and MLBs were assessed using methods based on NCCLS M-27A microdilution AFST. Results: By checkerboard testing of 11 strains (4 C. albicans, 3 C. tropicalis, 1 C. parapsilosis, 2 C. lusitaniae, 1 C. krusei), the median (range) Fractional Inhibitory Concentration Index (FICI) based on an MIC read at 48h as an optically clear well (MIC-0) was 0.06 (0.01- 0.5) for MC-510011 and 0.25 (0.01-0.51) for MC-510027. MLBs showed weak antifungal effects on their own. The optimal concentrations for synergy were 8 and 4 µg/mL for the two MLBs, respectively. We assessed the effect of these fixed concentrations of the MLBs combined with FLU in 45 strains (7 C. albicans, 1 C. dubliniensis, 10 C. glabrata, 1 C. krusei, 4 C. lusitaniae, 12 C. parapsilosis, 10 C. tropicalis). Of these, 19 strains exhibited trailing growth. Addition of either MLB eliminated trailing in all strains and reduced the FLU MIC up to 256-fold. For the 26 strains without trailing growth, addition of MLBs reduced the FLU MIC up to 32-fold for both compounds. Conclusion: These MLBs are strongly synergistic with FLU in vitro. Constitutive expression of CDR1 and CDR2 may contribute to the trailing growth phenomenon seen during AFST.
Full conference title:
42nd Interscience Conference on Antimicrobial Agents and Chemotherapy
- ICAAC 42nd