Interactions between triazoles and amphotericin B against Cryptococcus neoformans

The interaction of amphotericin B (AmB) and azole antifungal agents in the treatment of fungal infections is still a controversial Issue. A checkerboa rd titration broth microdilution based method that adhered to the recommend ations of the National Committee for Clinical Laboratory Standards was appl ied to study the in vitro interactions of AmB with fluconazole (PLC), itrac onazole (ITC), and the new investigational triazole SCH 56592 (SCH) against 15 clinical isolates of Cryptococcus neoformans, Synergy, defined as a fra ctional inhibitory concentration (FIC) index of less than or equal to 0.50, was observed for 7% of the isolates in studies of the interactions of both FLC-AmB and ITC-AmB and for 33% of the isolates in studies of the SCH-AmB interactions; additivism (FICs, >0.50 to 1.0) was observed for 67, 73, and 53% of the isolates in studies of the FLC-AmB, ITC-AmB, and SCH-AmB interac tions, respectively; indifference (FICs, > 1.0 to less than or equal to 2.0 ) was observed for 26, 20, and 14% of the isolates in studies of the FLC-Am B, ITC-AmB, and SCH-AmB interactions, respectively. Antagonism (FIC >2.0) w as not observed. When synergy was not achieved, there was still a decrease, although not as dramatic, in the MIC of one or both drugs when they were u sed in combination. To investigate the effects of FLC-AmB combination thera py in vivo, we established an experimental model of systemic cryptococcosis in BALB/c mice by intravenous injection of cells of C. neoformans 2337, a clinical isolate belonging to serotype D against which the combination of F LC and AmB yielded an additive interaction in vitro. Both survival and tiss ue burden studies showed that combination therapy was more effective than F LC alone and that combination therapy was at least as effective as AmB give n as a single drug. On the other hand, when cells of C. neoformans 2337 wer e grown in FLC-containing medium, a pronounced increase in resistance to su bsequent exposures to AmB was observed. In particular, killing experiments conducted with nonreplicating cells showed that preexposure to FLC abolishe d the fungicidal activity of the polyene. However, this apparent antagonism was not observed in vivo. Rather, when the two drugs were used sequentiall y for the treatment of systemic murine cryptococcosis, a reciprocal potenti ation was often observed. Our study shows that (i) the combination of triaz oles and AmB is significantly more active than either drug alone against C. neoformans in vitro and (ii) the concomitant or sequential use of FLC and AmB for the treatment of systemic murine cryptococcosis results in a positi ve interaction.