In Vitro Synergistic Activity of Caspofungin Plus Plymyxin B Against Fluconazole-Resistant

Background: Candida species account for most invasive fungal infections, and the emergence of fluconazole and caspofungin resistance is problematic. Overcoming resistance with synergism between 2 drugs may be useful. In a 2013 in vitro study, caspofungin plus colistin (polymyxin E) was found to act synergistically against fluconazole-resistant and susceptible Candida albicans isolates. The purpose of our study was to extend this finding by evaluating caspofungin plus polymyxin B for in vitro synergy against fluconazole-resistant Candida glabrata isolates. Materials and Methods: A total of 7 fluconazole- resistant C. glabrata bloodstream infection isolates were obtained from 2010-2011. Ofthese, 2isolates were also resistant to caspofungin. Minimum inhibitory concentrations( MICs) for caspofungin and polymyxinB were determined by Etest and broth microdilution. Clinical and Laboratory Standards Institute breakpoints were used for fluconazole and caspofungin MIC interpretations. No interpretive guidelines exist for testing polymyxin B against C. glabrata. Synergy testing with caspofungin( 1 x MIC) and polymyxinB( (MIC)-M-1/2) was performed using a modified bacterial Etest synergy method and time-kill assay. Results: With the Etest synergy method, 4 out of 7 isolates showed in vitro synergy and 1 out of 7 showed additivity. The remaining isolates (both caspofungin resistant) showed indifference. Using the time-kill assay, 1 out of 7 isolates showed synergy, 1 showed additivity and the remaining 5 (including both caspofungin-resistant isolates) showed indifference. Conclusions: Caspofungin susceptibility may be required for synergism between caspofungin and polymyxin B. Further synergy testing with caspofungin plus polymyxin B and additional fluconazole-resistant C. glabrata isolates should be performed. In vitro synergy/additivity may or may not correlate with in vivo benefit.