Novel method for evaluating in vitro activity of anidulafungin in combination with amphotericin B or azoles

A combination of drugs possessing different targets has been used as salvage therapy, although without scientific support. In vitro studies validating such combinations are scarce, and the methodology is very laborious and time-consuming. This study proposes a flow cytometric (FC) protocol as an alternative to evaluate the effect of the combination of anidulafungin (AND) with amphotericin B (AMB) and azoles (fluconazole and voriconazole), tested upon 39 and 36 Candida strains, respectively. The concentration assayed in the combination was 0.5× MIC of each drug. The membrane potential marker DiBAC(4)(3) [Bis-(1,3-dibutylbarbituric acid) trimethine oxonol] was used for AND-AMB, and the metabolic marker FUN-1 was used for AND-azoles. Drug interaction was determined by calculating a staining index (SI): the sum of the percentage of depolarized cells (DC) after treatment with drug combinations divided by the DC of the drug alone, and the sum of the mean intensity of fluorescence (MIF) displayed by cells treated with drug combinations divided by the MIF of the drug alone for FUN-1. An SI of <1 means antagonism, an SI between 1 and 4 means no interaction, and an SI of >4 means synergism. The combination of AND and AMB by FC and checkerboard was synergistic for 46 and 43% of isolates and antagonistic for 5 and 8%, respectively. For the combination of AND and azoles, it was synergistic for 36% and antagonistic for 3% by FC and synergistic for 44% and antagonistic for 3% by checkerboard. When the FC method was compared to the gold standard checkerboard method, the agreement was 0.91 (95% confidence interval [95% CI] of 0.88 to 0.94), sensitivity was 0.88 (95% CI of 0.73 to 0.95), and specificity was 0.95 (95% CI of 0.84 to 1). Thus, FC is a rapid and reliable method (<2 h) to assess the effect of antifungal combinations.