In vitro interaction between azoles and cyclosporin A against clinical isolates of Candida albicans determined by the chequerboard method and time–kill curves

Objectives To investigate the in vitro interaction between three azoles (fluconazole, itraconazole and voriconazole) and cyclosporin A against five azole-susceptible (azole-S) and five azole-resistant (azole-R) clinical Candida albicans isolates. Methods By using a chequerboard technique and time–kill curves, synergistic, indifferent or antagonistic effects when drugs were used in combination were assessed. In the chequerboard assay, the antifungal activity of drug combinations was determined by the microdilution method based on the CLSI M27-A2 guidelines. The effects of the interactions were assessed by two non-parametric approaches (fractional inhibitory concentration index model and ΔE model). In the time–kill assay, a colony counting method was employed against one azole-S strain and one azole-R strain at 0, 6, 12, 24 and 48 h of incubation at 35°C. Results Good concordance was found between the chequerboard method and time–kill curves. Indifference or synergism was observed for azole-S isolates in interactions of azoles and cyclosporin A, while strong synergism was observed for azole-R isolates in all drug combinations. Conclusions Cyclosporin A showed potent synergism when combined with the three azoles, especially against azole-R C. albicans strains, and there was good agreement between various methods used in this study.