Pharmacokinetic model of presystemic metabolism.

A pharmacokinetic perfusion model is presented that describes the disposition of drugs that are subject to both first-pass hepatic and intestinal epithelial metabolism. Equations are developed to estimate hepatic and intestinal epithelial clearances by determining drug concentrations as a function of time in both portal and peripheral vein blood after oral and iv administration, and by determining or estimating the flow rate of blood perfusing the gastrointestinal tissues. A method that can be used, under the stated conditions, to approximate the systemic availability after oral administration of drugs subject to presystemic metabolism is also developed.