Pharmacodynamic modeling of ciprofloxacin resistance in Staphylococcus aureus

Abstract

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.

FIG. 1.

Schematic of the two-population pharmacodynamic model with unique growth (g) and ciprofloxacin killing rate (k) constants for susceptible (S) and resistant (R) subpopulations (model 2). The legend for remaining symbols is given in Materials and Methods.

FIG. 2.

Fit of the pharmacodynamic model to experimental data from the in vitro system. Observed viable counts of MRSA 8043 (closed symbols, top panel), MRSA 8043C0-1 (open symbols, top panel), MRSA 8282 (closed symbols, bottom panel), and MRSA 8282C0-1 (open symbols, bottom panel) following exposure to pharmacokinetic profiles produced by various simulated ciprofloxacin dosage regimens are indicated. Observed viable counts for growth control experiments are also depicted. The model-predicted viable count versus time profiles are shown by the solid (MRSA 8043 and MRSA 8282) and dashed (MRSA 8043C0-1 and MRSA 8282C0-1) curves. The legend for both panels appears in the top panel.

FIG. 3.

Validation of pharmacodynamic model predictions in the in vitro system. Ciprofloxacin concentrations (left panels) and MRSA 8043 and MRSA 8282 viable count profiles (right panels) for an experimental ciprofloxacin dosage regimen (single 6,250-mg dose followed by 400 mg every 12 h) designed to prevent the emergence of resistant subpopulations likely to be present in a culture of 10⁷ CFU/ml (top) and for a conventional ciprofloxacin dosage regimen of 400 mg every 12 h with a culture of 10⁵ CFU/ml not likely to contain resistant subpopulations (bottom). The concentration-time panels show measured central (closed symbols) and peripheral (open symbols) compartment concentrations, predicted central compartment pharmacokinetic profiles (lines) from the one-compartment pharmacokinetic model, and MICs (dotted line) and MPCs for MRSA 8043 (solid line) and MRSA 8282 (dash-dotted line). The viable count-time panels show pharmacodynamic model-predicted (curves) and observed (symbols) viable counts for MRSA 8043 (squares) and MRSA 8282 (diamonds). The reliable limit of detection in the experiments was 100 CFU/ml.

FIG. A1.

Time-kill curves for parent strains MRSA 8043 and MRSA 8282 (top), grlA mutants MRSA 8043C0-1 (S80Y) and MRSA 8282C0-1 (A116P) (middle), and grlA/gyrA double mutants MRSA 8043C96-1 (S80Y/S84L) and MRSA 8282C96-1 (A116P/S84L) (bottom) following exposure to ciprofloxacin at concentrations of 0 (♦), 1 (▪), 2 (▴), 4 (•), and 8 (×) times the corresponding MIC. Results are means ± standard deviations of the results from three separate experiments.