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Antimicrobial Agents and Chemotherapy, January 2005, p. 209-219, Vol. 49, No. 1
0066-4804/05/$08.00+0     doi:10.1128/AAC.49.1.209-219.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Pharmacodynamic Modeling of Ciprofloxacin Resistance in Staphylococcus aureus

Jeffrey J. Campion,¹ Patrick J. McNamara,² and Martin E. Evans^1*

Division of Infectious Diseases, Department of Internal Medicine, College of Medicine,¹ Division of Pharmaceutical Sciences, College of Pharmacy, University of Kentucky, Lexington, Kentucky²

Received 7 May 2004/ Returned for modification 23 August 2004/ Accepted 17 September 2004

Three pharmacodynamic models of increasing complexity, designed for two subpopulations of bacteria with different susceptibilities, were developed to describe and predict the evolution of resistance to ciprofloxacin in Staphylococcus aureus by using pharmacokinetic, viable count, subpopulation, and resistance mechanism data obtained from in vitro system experiments. A two-population model with unique growth and killing rate constants for the ciprofloxacin-susceptible and -resistant subpopulations best described the initial killing and subsequent regrowth patterns observed. The model correctly described the enrichment of subpopulations with low-level resistance in the parent cultures but did not identify a relationship between the time ciprofloxacin concentrations were in the mutant selection window (the interval between the MIC and the mutant prevention concentration) and the enrichment of these subpopulations. The model confirmed the importance of resistant variants to the emergence of resistance by successfully predicting that resistant subpopulations would not emerge when a low-density culture, with a low probability of mutants, was exposed to a clinical dosing regimen or when a high-density culture, with a higher probability of mutants, was exposed to a transient high initial concentration designed to rapidly eradicate low-level resistant grlA mutants. The model, however, did not predict or explain the origin of variants with higher levels of resistance that appeared and became the predominant subpopulation during some experiments or the persistence of susceptible bacteria in other experiments where resistance did not emerge. Continued evaluation of the present two-population pharmacodynamic model and development of alternative models is warranted.

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* Corresponding author. Mailing address: Division of Infectious Diseases, Department of Internal Medicine, Room MN672, University of Kentucky Medical Center, 800 Rose St., Lexington, KY 40536-0298. Phone: (859) 323-8178. Fax: (859) 323-8926. E-mail: Martin.Evans{at}uky.edu.

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Antimicrobial Agents and Chemotherapy, January 2005, p. 209-219, Vol. 49, No. 1
0066-4804/05/$08.00+0     doi:10.1128/AAC.49.1.209-219.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

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