| Liver is a major site of metabolism of many xenobiotics, since the great majority of drug metabolizing enzymes are localized in the hepatocytes. Recently, several hepatic uptake and efflux transporters were found on the sinusoidal and canalicular membranes of hepatocytes, respectively. Furthermore, the substrate specificities of enzymes and transporters overlap significantly. The goal of this thesis research is to investigate the interplay of hepatic enzymes and transporters and the resulting potential clinical implications.; In specific aim one, we hypothesized that hepatic microsome studies are insufficient to characterize in vivo hepatic metabolic clearance; rather, studies in freshly isolated hepatocytes are needed because intact hepatic uptake and efflux transporters, which are not found in microsomes, modulate drug disposition and metabolism. Using the model compound digoxin, hepatic metabolic clearance determined using hepatocytes, but not using microsomes, correlated well with in vivo data.; In specific aim two, we hypothesized that metabolic drug-drug interactions at the hepatic uptake or efflux transporter levels, independent of any change in enzymatic activity, would alter the metabolism and pharmacokinetics of substrate compounds. In hepatocytes, inhibition of uptake transporters reduced substrate metabolism compared to controls, whereas inhibition of efflux transporters resulted in increased drug metabolism relative to controls. Co-administration of uptake or efflux transporter inhibitors with a model compound, erythromycin, in rats resulted in similar pharmacokinetic profiles. However the changes versus control resulted from very different mechanisms, inhibition of hepatic uptake and inhibition of biliary excretion, confirming our prediction.; In specific aim three, we hypothesized that because rifampin induces hepatic enzymes and inhibits uptake transporters, dosing of a drug that is a dual substrate of enzymes and uptake transporters on the final day of an inducing regimen should exhibit less inductive effect than dosing on the following day in the absence of rifampin. We tested this hypothesis in vitro and in vivo in rats. Concomitant dosing of rifampin and a model compound, digoxin, completely diminished the induction effect in hepatocytes and in whole animals. Therefore, to observe the full induction effect, an uptake transporter substrate should be administered one day following the completion of an inducing regimen. |
