TY - JOUR T1 - COMPARATIVE USE OF ISOLATED HEPATOCYTES AND HEPATIC MICROSOMES FOR P450 INHIBITION STUDIES: TRANSPORTER-ENZYME INTERPLAY JF - Drug Metabolism and Disposition JO - Drug Metab Dispos DO - 10.1124/dmd.110.035824 SP - dmd.110.035824 AU - Hayley S Brown AU - Alison J Wilby AU - Jane E Alder AU - J Brian Houston Y1 - 2010/09/16 UR - http://dmd.aspetjournals.org/content/early/2010/09/16/dmd.110.035824.abstract N2 - Accurate assignment of the concentration of victim drug/inhibitor available at the enzyme active site, both in vivo and within an in vitro incubation is an essential requirement in rationalizing and predicting drug-drug interactions. Inhibitor accumulation within the liver, whether as a result of active transport processes or intracellular binding, may best be accounted for using hepatocytes rather than hepatic microsomes to estimate in vitro inhibitory potency. The aims of this study were to compare Ki values determined in rat liver microsomes and freshly isolated rat hepatocytes of four P450 inhibitors (clarithromycin, enoxacin, nelfinavir and saquinavir) with known hepatic transporter involvement and a range of uptake (cell-to-medium concentration ratios 20-3000) and clearance (10-1200 μL/min/106 cells) properties. Inhibition studies were performed using two well-established P450 probe substrates (theophylline and midazolam). Comparison of unbound Ki values showed marked differences between the two in vitro systems for inhibition of metabolism. In two cases, (clarithromycin and enoxacin - both low clearance drugs) inhibitory potency in hepatocytes markedly exceeded microsomes (10 to 20-fold) and this was consistent with their high cell-to-medium concentration ratios. For nelfinavir and saquinavir (high clearance, extensively metabolised drugs) the opposite trend was seen in the Ki values, despite very high cell-to-medium concentration ratios; stronger inhibition was evident within microsomal preparations. Hence the consequences of hepatic accumulation resulting from uptake transporters vary according to the clearance of the inhibitor. This study demonstrates that transporter-enzyme interplay can result in differences in inhibitory potency between microsomes and hepatocytes, and hence drug-drug interaction predictions, that are not always intuitive. ER -