Pharmacokinetics of penicillin G in plasma and interstitial fluid collected with dialysis fiber bundles in sheep.

A new method of collection of interstitial fluid (ISF) (the site of most bacterial infections) was developed for the determination of free (unbound) penicillin G concentrations in sheep. Dialysis fiber bundles for the collection of ISF were first characterized in vitro and subsequently implanted in the subcutaneous fascia of the dorsal thorax parallel to the vertebral column in sheep. The sheep were then dosed intravenously with 26.4 and 52.9 mg/kg of sodium penicillin G using a crossover experimental design. Plasma and ISF dialysate were collected after dosing for determination of penicillin G concentrations using high pressure liquid chromatography (HPLC). The concentration of penicillin G in the ISF dialysate was calculated with the recovery ratio determined for each fiber bundle. The decline of penicillin G concentrations in ISF dialysate paralleled the disappearance of the drug from plasma providing evidence for the rapid diffusion of penicillin G into the fiber bundles. Pharmacokinetic analysis determined that the disposition of penicillin G was best described by a two-compartment open model with penicillin concentrations in plasma (Cp) defined by two biexponential equations, Cp = 170.64e-7.16t + 31.04e-1.56t for the low dose and Cp = 418.19e-1.56t for the high dose.