The Evaluation of the Combination of Caspofungin with Voriconazole and Amphotericin B against Clinical Candida krusei Isolates by Etest and Disk Diffusion Methods

Background: C. krusei is an opportunistic fungal pathogen that known of its intrinsic fluconazole resistance and its frequency is increasing especially among hematology patients. The increase in the frequency of high mortality fungal infections have accelerated the efforts of new drug development with broad spectrum, low toxicity and the studies including their combination. However, there is no standardized method to evaluate the activity of drug combinations.

Aims: To evaluate the activity of caspofungin (CAS) with voriconazole (VOR) and amphotericin B (AMB) alone and in combination and the utility of Etest and disk diffusion methods for antifungal combinations.

Methodology: The minimum inhibitory concentrations of VOR, CAS and AMB against 30 clinical C. krusei isolates were determined by using Etest, disk diffusion and reference broth microdilution methods. Combinations of CAS with VOR and CAS with AMB were evaluated using disk diffusion (three different ways) and Etest (two different ways) methods.

Results: All isolates tested were susceptible to VOR and CAS in vitro by all three methods. Categorical agreements of Etest and disk diffusion methods with reference microdilutiontest were 100% for CAS and VOR (for each method), 86.7% and 50% for AMB, respectively. In the all ways of both combination methods, we did not observe distinctly antagonistic or synergic interaction.

Conclusion: Etest and disk diffusion could be easy, convenient, and nontime-consuming alternative methods to evaluate the antifungal combinations. The combinations of CAS with VOR and AMB exhibited promising results because of an apparent antagonistic interaction was not detected in this study