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Antimicrobial Agents and Chemotherapy, June 1998, p. 1387-1391, Vol. 42, No. 6
0066-4804/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Evaluation of Endpoints for Antifungal Susceptibility Determinations with LY303366

Michael E. Klepser,1,* Erika J. Ernst,1 Michael E. Ernst,1 Shawn A. Messer,2 and Michael A. Pfaller2

College of Pharmacy, The University of Iowa,1 and Department of Pathology, The University of Iowa Hospitals and Clinics,2 Iowa City, Iowa 52242

Received 24 November 1997/Returned for modification 9 March 1998/Accepted 31 March 1998

We have previously reported poor correlation between the in vitro fungicidal activity of LY303366 and MIC results in RPMI medium based upon the manufacturer's suggested susceptibility endpoint, lack of visual growth. Additionally, we have noted a significant trailing effect with LY303366 when MICs are determined in RPMI medium. These observations have led us to evaluate an alternative susceptibility endpoint for LY303366, an 80% reduction in growth compared with control (similar to that 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 morpholinepropanesulfonic 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 curves were determined in RPMI medium with each isolate at concentrations of LY303366 ranging from 0.125 to 16× MIC80 to assess the correlation between MIC80 and fungicidal activity. Lastly, fungi exposed to LY303366 were examined via scanning electron microscope (SEM) for evidence of drug-induced ultrastructure change. MIC80s for test isolates ranged from 0.015 to 0.12 µg/ml and were consistently three to five wells less than MIC100s. Good correlation was observed between fungicidal activity, as assessed by kill curves, and the MIC80. SEM data revealed significant ultrastructure changes induced by LY303366 even at sub-MIC80s. Based on our results demonstrating better correlation between MIC80 and fungicidal activity, i.e., time-kill curves and MFCs, we suggest that 80% reduction in visible growth be utilized as the endpoint for susceptibility determinations with LY303366 in RPMI medium.

* Corresponding author. Mailing address: S412 Pharmacy Building, The University of Iowa College of Pharmacy, Iowa City, IA 52242-1112. Phone: (319) 335-8861. Fax: (319) 353-5646. E-mail: michael-klepser{at}uiowa.edu.

Antimicrobial Agents and Chemotherapy, June 1998, p. 1387-1391, Vol. 42, No. 6
0066-4804/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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