Determination of Dermal Pharmacokinetics by Microdialysis Sampling in Rats.

The evaluation of absorption and availability at the site of action of a drug candidate is an important element of drug discovery and development, as clinical response is a function of the bioavailability of the active agent and its continued presence at the site of action. Evaluation of dermal pharmacokinetics facilitates the selection of new compounds or chemical structures for advancement as possible clinical candidates. An advantage of microdialysis is that it allows the measurement of compound concentrations at the site of action without disturbing the tissue milieu, making it possible to determine the relationship between this important variable and plasma concentrations of the agent. Described in this unit are laboratory protocols for performing dermal microdialysis experiments in rat for the purpose of defining the pharmacokinetics parameters of test agents. © 2019 by John Wiley & Sons, Inc.

Dermal pharmacokinetics of pyrazinamide determined by microdialysis sampling in rats.

Studies have demonstrated the efficacy of pyrazinamide (PZA) against stages of the Leishmania parasite that causes cutaneous leishmaniasis. Although PZA is widely distributed in most body fluids and tissues, the amount of drug reaching the skin is unknown. This study aimed to investigate the pharmacokinetics of PZA in rat dermal tissue by dermal microdialysis. Skin pharmacokinetics was assessed by implanting a linear microdialysis probe in the dermis of ten rats. In addition, blood samples were collected to assess plasma pharmacokinetics. Unbound microdialysate (N = 280) and plasma (N = 120) concentrations following single intravenous doses of 25 mg/kg or 50 mg/kg PZA were quantified by a validated HPLC method. Probe calibration was performed by retrodialysis. Non-compartmental analysis and non-linear mixed-effects modelling were performed using WinNonlin and NONMEM v.7.3. PZA rapidly permeated into the dermis and reached high levels, with mean maximum concentrations (Cmax) of 22.4 ± 7.1 µg/mL and 48.6 ± 17.3 µg/mL for the two doses studied. PZA showed significant distribution to the skin (fAUCdermal/fAUCplasma = 0.82 ± 0.31 and 0.84 ± 0.25 for 25 mg/kg and 50 mg/kg doses, respectively). Active unbound concentrations in dermal tissue reached lower levels than free plasma concentrations, indicating that free PZA levels in plasma were in equilibrium with tissue levels. These results showed equivalent unbound drug tissue concentrations and corresponding unbound plasma levels. This study shows that PZA distributes rapidly into dermal interstitial fluid space in rats and therefore may be a potential agent in the treatment of cutaneous leishmaniasis.