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Antimicrobial Agents and Chemotherapy, November 2006, p. 3680-3688, Vol. 50, No. 11
0066-4804/06/$08.00+0     doi:10.1128/AAC.00369-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Derivation of an In Vivo Drug Exposure Breakpoint for Flucytosine against Candida albicans and Impact of the MIC, Growth Rate, and Resistance Genotype on the Antifungal Effect

School of Medicine, University of Manchester, Stopford Building, Oxford Road, Manchester M13 9PT, United Kingdom,¹ Pediatric Oncology Branch, National Cancer Institute, National Institutes of Health, 10 Center Dr., Bethesda, Maryland 20892,² Ordway Research Institute, 150 New Scotland Avenue, Albany, New York 12208,³ Wythenshawe Hospital, Southmoor Rd., Manchester, M23 9LT, United Kingdom⁴

Received 25 March 2006/ Returned for modification 28 May 2006/ Accepted 27 August 2006

Drug exposure or pharmacodynamic breakpoints refer to a magnitude of drug exposure which separates a population into groups with high and low probabilities of attaining a desired outcome. We used a pharmacodynamic model of disseminated candidiasis to define an in vivo drug exposure breakpoint for flucytosine (5FC) against Candida albicans. The results were bridged to humans by using population pharmacokinetics and Monte Carlo simulation. An in vivo drug exposure breakpoint for 5FC was apparent when serum levels were above the MIC for 45% of the dosing interval. The Monte Carlo simulations suggested that using a human dose of 100 mg/kg of body weight/day in four divided doses, 5FC resistance was defined at an MIC of 32 mg/liter. Target attainment rates following administration of 25, 50, and 100 mg/kg/day were similar, suggesting that the use of a lower dose of 5FC is possible. Using six isolates of C. albicans with MICs ranging from 0.06 to >64 mg/liter, we also explored the influence that the MIC, the fraction of the dosing interval that the serum levels of 5FC remained above the MIC (T>MIC), the 5FC resistance genotype, and the in vivo growth rate had on the response to 5FC. The MIC and T>MIC were both critical measures affecting the generation of a drug effect but had no bearing on the magnitude of the maximal kill induced by 5FC. The in vivo growth rate was a critical additional determinant of the exposure-response relationship. There was a relationship between the 5FC resistance genotype and the exposure-response relationship.

Published ahead of print on 5 September 2006.