Pharmacokinetic-pharmacodynamic modelling of the in vitro antiinfective effect of piperacillin-tazobactam combinations.

PURPOSE: The aim of the study was to investigate the in vitro antiinfective effect of piperacillin-tazobactam (PIP-TZB) combinations on Escherichia coli in simulations of free concentration time profiles of both drugs, similar to those obtained in human tissue after i.v. bolus administrations.

METHODS: An in vitro dilution model was used to expose E. coli ATCC 35218 (beta-lactamase producer) to various piperacillin-tazobactam concentration profiles obtained after i.v. bolus multiple dose, using different dose ratio combinations (1:4, 1:8, 1:16) and dosing regimens, ranging from once-a-day to 4 times a day. The antimicrobial effect was evaluated by determination of the number of bacteria over time. The concentration of PIP in the model was determined by HPLC.

RESULTS: A modified Emax model was used to describe the pharmacodynamic effect. The model was linked with the piperacillin concentrations determined experimentally to provide a pharmacokinetic-pharmacodynamic (PK-PD) model. The EC50 for piperacillin alone averaged 5.66 +/- 0.29 micrograms/ml. The EC50 for all doses of piperacillin combined with 0.5 g of tazobactam were dose-dependent and averaged 1.70 +/- 0.56, 3.95 +/- 1.02, and 6.14 +/- 1.24 micrograms/ml for PIP 2, 4, and 8 g, respectively. By increasing the dose of TZB in combination with a fixed dose of PIP, a decreased EC50 was observed.

CONCLUSIONS: The PK-PD model allowed a detailed evaluation of the dosing regimens investigated. The results suggested that for these combinations, 3 times a day administration is as effective as 4 times a day. Pharmacodynamic activity of the combinations can be prolonged by sufficiently high inhibitor concentrations.