Transporter-Enzyme Interplay in Predicting Drug Absorption and Disposition


Leslie Z. Benet, Ph.D.

Department of Bioengineering and Therapeutic Sciences

Schools of Pharmacy and Medicine

University of California




        In 2005, Wu and Benet (2005) noted that a Biopharmaceutics Drug Disposition Classification System (BDDCS) could serve as the basis for predicting the importance of transporters in determining drug bioavailability and disposition.  They reasoned that for highly soluble, highly permeable Class 1 compounds, metabolism would be the major route of elimination and that transporter effects on availability and disposition would be negligible. In contrast for the poorly permeable, high solubility Class 3 compounds and the poorly permeable, poorly soluble Class 4 compounds, metabolism would only play a minor role in drug elimination, with renal and biliary excretion of unchanged drug being the predominant routes of elimination.  Uptake transporters would be major determinants of the bioavailability of these poorly permeable drugs and both uptake and efflux transporters could be important for drug elimination. 


        Highly permeable, poorly soluble, extensively metabolized Class 2 compounds constitute the majority of new molecular entities (NMEs) (~70%) and present the most complicated relationship in defining the impact of transporters due to the marked effects of transporter-enzyme interplay. Uptake transporters are unimportant for gut bioavailability, but can play a major role in hepatic elimination.  Efflux transporters have major effects on bioavailability, metabolism and elimination of Class 2 drugs.  Drug efflux by intestinal P-glycoprotein (P-gp) is known to decrease the bioavailability of many CYP3A4 (3A4) substrates.  We have demonstrated that the interplay between P-gp and 3A4 at the apical intestinal membrane can increase the opportunity for drug metabolism by determining bidirectional extraction ratios across 3A4 transfected Caco-2 cells for three Class 2 dual P-gp/3A4 substrates: K77 (an experimental cysteine protease inhibitor), sirolimus and saquinavir, as well as three negative control 3A4 substrates, midazolam, verapamil and felodipine, although in MDCK-MDR1 cells, verapamil and midazolam can be shown to be P-gp substrates.  Liver and intestinal perfusion studies in rats with the Class 2 drugs K77, tacrolimus, digoxin and atorvastatin, confirm the P-gp and CYP3A interplay in the gut, and emphasize the additional importance of hepatic uptake transporters, OATPs. In these studies, midazolam and felodipine served as the controls.


           More recently we have carried out whole animal studies in rats studying control versus inhibition of hepatic uptake and hepatic efflux for atorvastatin and erythromycin following both i.v. and oral dosing. For Oatp/OATP inhibition studies we have used rifampin as the inhibitor. We showed that induction studies using rifampin can underestimate the extent of induction if rifampin is present in the systemic circulation due to the counter-effects of Oatp inhibition. Our human studies investigating the effect of inhibition of OATP1B on the disposition of atorvastatin and its two active hydroxyl metabolites, as well as glyburide and its active metabolite will be reviewed. These results serve as a template for predicting enzyme-transporter (both absorptive and efflux) interactions in the intestine and the liver and have the potential to explain why one frequently observes discordances between in vivo and in vitro metabolic results, as well as why drug interactions involving 3A4 and P-gp in the intestine for Class 2 compounds cannot be adequately predicted based on intravenous in vivo or microsomal studies. Recent reviews (Shugarts and Benet, 2009; Benet, 2009) describe this work.