Quantification and prediction of skin pharmacokinetics of amoxicillin and cefuroxime

The purpose of this project was to develop and validate a pharmacokinetic model and to quantify the rate and extent of distribution between plasma and skin of two beta-lactam antibiotics, amoxicillin (AMX) and cefuroxime (CFX), which are frequently administered systemically to treat skin and skin structure infections. Dosing regimens are usually based on plasma concentration, however, concentrations at the target site are better correlated with the effect. For each antibiotic, three different i.v. bolus doses were administered to three female rabbits according to a randomized cross-over design and plasma samples were collected serially. Skin concentrations were obtained by continuous microdialysis. Skin and unbound plasma concentrations were fitted simultaneously using a semi-physiological model and the transfer constants plasma/skin (K(in)) and skin/plasma (K(out)) were estimated. K(in) and K(out) were then used to predict skin concentrations from the plasma levels obtained from an oral administration of AMX or from an i.v. bolus of CFX. The predicted skin profiles were similar to those measured by microdialysis during the actual experiments. In conclusion, this study shows that it is possible to generate a reasonable prediction of skin pharmacokinetics from any plasma level once a careful characterization of the transfer process between plasma and skin has been made.