EVALUATION OF END-POINTS FOR ANTIFUNGAL SUSCEPTIBILITY DETERMINATIONSWITH LY303366

We have previously reported poor correlation between the in vitro fung icidal activity of LY303366 and MIC results in RPMI medium based upon the manufacturer's suggested susceptibility endpoint, lack of visual g rowth. Additionally, we have noted a significant trailing effect with LY303366 when MICs are determined in RPMI medium. These observations h ave led us to evaluate an alternative susceptibility endpoint for LY30 3366, an 80% reduction in growth compared with control (similar to tha t utilized for azoles). Two isolates each of Candida albicans, Candida glabrata, and Candida tropicalis were selected for testing, MICs were determined for LY303366 in RPMI 1640 medium buffered with morpholinep ropanesulfonic acid. MICs were determined with suggested (MIC100) and experimental (MIC80) endpoints, The minimal fungicidal concentration ( MFC) of LY303366 for each isolate was also determined. Time-kill cut-y es were determined in RPMI medium with each isolate at concentrations of LY303366 ranging from 0.125 to 16x MIG(80) to assess the corl elati on between MIC80 and fungicidal activity, Lastly, fungi exposed to LY3 03366 were examined via scanning electron microscope (SEM) for evidenc e of drug-induced ultrastructure change. MIC(80)s for test isolates ra nged from 0.015 to 0.12 mu g/ml and were consistently three to five we lls less than MIC(100)s. Good correlation was observed between fungici dal activity, as assessed by kill curves, and the MIG(80). SEM data re vealed significant ultrastructure changes induced by LY303366 even at sub-MIC(80)s. Based on our results demonstrating better correlation be tween MIG(80) and fungicidal activity, i.e., time-kill curves and MFCs , we suggest that 80% reduction in visible growth be utilized as the e ndpoint for susceptibility determinations with LY303366 in RPMI medium .