Novel Pharmacokinetic-Pharmacodynamic Model forPrediction of Outcomes with an Extended-Release FormulationofCiprofloxacin

Abstract

The pharmacokinetics of an extended-release (XR) formulation of ciprofloxacin has been compared to that of the immediate-release (IR) product in healthy volunteers. The only significant difference in pharmacokinetic parameters between the two formulations was seen in the rate constant of absorption, which was approximately 50% greater with the IR formulation. The geometric mean plasma ciprofloxacin concentrations were applied to an in vitro pharmacokinetic-pharmacodynamic model exposing three different clinical strains of Escherichia coli (MICs, 0.03, 0.5, and 2.0 mg/liter) to 24 h of simulated concentrations in plasma. A novel mathematical model was derived to describe the time course of bacterial CFU, including capacity-limited replication and first-order rate of bacterial clearance, and to model the effects of ciprofloxacin concentrations on these processes. A "mixture model" was employed which allowed as many as three bacterial subpopulations to describe the total bacterial load at any moment. Comparing the two formulations at equivalent daily doses, the rates and extents of bacterial killing were similar with the IR and XR formulations at MICs of 0.03 and 2.0 mg/liter. At an MIC of 0.5 mg/liter, however, the 1,000-mg/day XR formulation showed a moderate advantage in antibacterial effect: the area under the CFU-time curve was 45% higher for the IR regimen; the nadir log CFU and 24-h log CFU values for the IR regimen were 3.75 and 2.49, respectively; and those for XR were 4.54 and 3.13, respectively. The mathematical model explained the differences in bacterial killing rate for two regimens with identical AUC/MIC ratios.Ciprofloxacin (Cipro), the first broad-spectrum oral fluoroquinolone, was approved by the Food and Drug Administration in 1987. An intravenous formulation was approved in 1991, and an oral suspension formulation followed in 1997. Recently, an extended-release (XR) formulation of ciprofloxacin has been developed by Bayer Healthcare (Cipro XR). Pharmacokinetic studies of XR ciprofloxacin in healthy volunteers have demonstrated a significant difference in peak concentrations in plasma between the XR and immediate-release (IR) formulations: peak concentrations were 40 to 50% higher with the XR formulation, while areas under the concentrationtime curve (AUCs) were comparable to those observed with the IR formulation (H. Stass, J. Nagelschmitz, E. Brandel, et al., Abstr. Am. Fed. Med. Res. Cong., abstr. 24 and 25, 2002).The objectives of this analysis were threefold. The first objective was to characterize the pharmacokinetics of multiple oral doses of ciprofloxacin when given as an IR and an XR formulation in daily doses of either 500 mg or 1,000 mg in healthy male volunteers. The second objective was to apply the geometric mean plasma concentration profiles from the healthy-volunteer study to an in vitro pharmacokinetic-pharmacodynamic (PK-PD) model exposing three different clinical strains of Escherichia coli to 24 h of simulated concentrations in plasma. Finally, the third objective was to devel...

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