Determination of Free Extracellular Concentrations of Piperacillin by Microdialysis

Abstract

The tissue penetration and distribution of antibiotics is of great importance, since most of the infections occur in the tissue. At the infection site, the free, unbound fraction of the antibiotic is responsible for the antiinfective effect. These free extracellular concentrations can be measured by microdialysis. It was the aim of the study to correlate free levels of the beta-lactam antibiotic piperacillin in blood with those in tissue. In vivo microdialysis sampling was used to study the tissue distribution patterns of piperacillin in anesthetized rats after single dose iv administration of the drug. The pharmacokinetics of piperacillin in plasma were consistent with a two-compartment body model. Comparisons between calculated free concentrations in the peripheral compartment and measured free extracellular concentrations revealed excellent agreement. Microdialysis is a suitable method to evaluate unbound drug concentrations in the tissues. In case of piperacillin, predictions of the concentration time profiles of free drug in the peripheral compartment can be made on the basis of plasma data.

PK-PD modeling of β-lactam antibiotics: In vitro or in vivo models?

“…Nolting et al 4 studied this relationship for the same bacteria and drug in healthy Wistar rats and were able to predict the free tissue levels based on total plasma concentration without the need of a proportionality factor, indicating that this drug distribution is governed by passive diffusion process. In this study, however, the proportionality factor was necessary to allow the prediction of free tissue concentrations based on plasma PK parameters, indicating that infection caused physiological changes in muscle such as increase in vascular permeability leading to increase in protein concentration at the infection site, increase in tissue temperature and pH alterations that would ultimately lead to an around 30% decrease in free interstitial levels of PIP.…”

“…20 Microdialysis probes in vitro relative recoveries were determined by retrodialysis using PIP 1 mg ml À1 in Ringer at a flow rate of 1.5 ml min À1 as previously described. 4 …”

“…The in vivo recovery was in the same order of magnitude of the average recovery reported in literature for the same probe and drug determined in healthy rat skeletal muscle (10.1±0.9%) (ref. 4).…”

“…1,2 For the PK part of the model, the knowledge of the free interstitial levels of anti-infective agents at the biophase has drawn the attention of researchers, as the free concentrations at the site of infection are responsible for the bacterial killing and the outcome of anti-infective therapy. 3 The feasibility of the prediction of free interstitial levels, determined by microdialysis, based on plasma PK parameters, has been demonstrated in literature for some b-lactam antibiotics such as piperacillin alone 4 or in combination with tazobactam, 5 ceftriaxone, 6 cefaclor 7 and amoxicillin 8 using healthy animals. However, some studies have demonstrated that infection changes the physiological characteristics of the infected tissue due to the inflammatory process developed by host aiming to eliminate the invasive organism, as observed in soft tissue infections, septic shock, pneumonia or vascular alterations associated to diabetes.…”

“…Furthermore, the comparison between the parameters generated using an in vitro and an in vivo infection could be used to evaluate the ability of the in vitro experiments to forecast the infection's outcome in vivo using the same drug/bacteria combination. Therefore, the aims of this study were to model the killing effect of piperacillin against Escherichia coli in immunocompromised infected rats using the same modified E maxmodel described previously, 13 and to compare the PK-PD parameters obtained in vivo with those determined by simulating in vitro against E.coli the free tissue levels of piperacillin expected at the infection site in humans, based on the assumption that the correlation between free skeletal muscle and total plasma concentrations established in healthy rodents 4 would hold true in humans. For this study, besides the in vivo PD evaluation, the PK data used to carry out the PK-PD modeling was obtained by microdialysis at the infection site of immunocompromised E. coli-infected rats.…”