Impact of the order of initiation of fluconazole and amphotericin B in sequential or combination therapy on killing of Candida albicans in vitro and in a rabbit model of endocarditis and pyelonephritis

In vitro time-kill studies and a rabbit model of endocarditis and pyeloneph ritis were used to define the impact that the order of exposure of Candida albicans to fluconazole (FLC) and amphotericin B (AMB), as sequential and c ombination therapies, had on the susceptibility of C. albicans to AMB and o n the outcome. The contribution of FLC-induced resistance to AMB for C. alb icans also was assessed. In vitro, AMB monotherapy rapidly killed each of f our C. albicans strains; FLC alone was fungistatic. Preincubation of these fungi with FLC for 18 h prior to exposure to AMB decreased their susceptibi lities to AMB for 8 to >40 h. Induced resistance to AMB was transient, but the duration of resistance increased with the length of FLC preincubation, Yeast sequentially incubated with FLC followed by AMB plus FLC (FLC-->AMB+F LC) showed fungistatic growth kinetics similar to that of fungi that were e xposed to FLC alone. This antagonistic effect persisted for at least 24 h. Simultaneous exposure of C. albicans to AMB and FLC [AMB+FLC(simult)] demon strated activity similar to that with AMB alone for AMB concentrations of g reater than or equal to1 mug/ml; antagonism was seen using an AMB concentra tion of 0.5 mug/ml. The in vitro findings accurately predicted outcomes in our rabbit infection model. In vivo, AMB monotherapy and treatment with AMB for 24 h followed by AMB plus FLC (AMB-->AMB+FLC) rapidly sterilized kidne ys and cardiac vegetations. AMB+FLC(simult) and FLC-->AMB treatments were s lower in clearing fungi from infected tissues. FLC monotherapy and FLC-->AM B+FLC were both fungistatic and were the least active regimens. No adverse interaction was observed between AMB and FLC for the AMB-->FLC regimen. How ever, FLC-->AMB treatment was slower than AMB alone in clearing fungi from tissues. Thus, our in vitro and in vivo studies both demonstrate that preex posure of C. albicans to FLC reduces fungal susceptibility to AMB. The leng th of FLC preexposure and whether AMB is subsequently used alone or in comb ination with FLC determine the duration of induced resistance to AMB.