JAC Advance Access originally published online on April 19, 2006
Journal of Antimicrobial Chemotherapy 2006 57(6):1116-1121; doi:10.1093/jac/dkl135
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Selection of ciprofloxacin resistance in Escherichia coli in an in vitro kinetic model: relation between drug exposure and mutant prevention concentration
1 Antibiotic Research Unit, Department of Medical Sciences, Clinical Bacteriology and Infectious Diseases, Uppsala University S-751 22 Uppsala, Sweden 2 Microbiology Programme, Department of Cell and Molecular Biology, Biomedical Center, Uppsala University S-751 24 Uppsala, Sweden
Received 19 July 2005; returned 16 October 2005; revised 9 February 2006; accepted 21 March 2006
*Corresponding author. Tel: +46-18-611-5640; Fax: +46-18-611-5650; E-mail: otto.cars{at}smi.ki.se
Objectives: To evaluate the mutant prevention concentrations (MPCs) of ciprofloxacin for two susceptible and one first-step gyrA resistant mutant Escherichia coli strains in an in vitro kinetic model and to identify the pharmacodynamic index that best predicts prevention of resistance emergence.
Methods: An in vitro kinetic model was used to measure MPC with static antibiotic concentrations and to test different dosing profiles to study pharmacokinetics/pharmacodynamics indices important to prevent the growth of resistant mutants. In one set of kinetic experiments the starting concentration was equal to the MPC and the T > MPC was varied before antibiotic dilution was begun. In a second set of kinetic experiments Cmax was varied and dilution of the antibiotic was started at time zero.
Results: From the static experiments we calculated MPC values of 0.128 mg/L for both the susceptible strains (16x MIC) and 0.188 mg/L (4x MIC) for the first-step resistant (gyrA) strain. The kinetic experiments showed that the T > MPC needed to prevent the growth of resistant bacteria was shorter with an increased Cmax. When resistance was selected, several subpopulations with different levels of susceptibility to ciprofloxacin emerged.
Conclusions: Neither T > MPC nor Cmax proved to be single correlates for preventing resistance development. For the two investigated wild-type strains, an AUC/MPC ratio of
22 was the single pharmacodynamic index that predicted prevention of resistant mutant development.
Keywords: antibiotic resistance , MPC , PK/PD , E. coli
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