PT  - JOURNAL ARTICLE
AU  - Hayley S. Brown
AU  - Alison J. Wilby
AU  - Jane Alder
AU  - J. Brian Houston
TI  - Comparative Use of Isolated Hepatocytes and Hepatic Microsomes for Cytochrome P450 Inhibition Studies: Transporter-Enzyme Interplay
AID  - 10.1124/dmd.110.035824
DP  - 2010 Dec 01
TA  - Drug Metabolism and Disposition
PG  - 2139--2146
VI  - 38
IP  - 12
4099  - http://dmd.aspetjournals.org/content/38/12/2139.short
4100  - http://dmd.aspetjournals.org/content/38/12/2139.full
SO  - Drug Metab Dispos2010 Dec 01; 38
AB  - 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 cytochrome P450 (P450) inhibitors (clarithromycin, enoxacin, nelfinavir, and saquinavir) with known hepatic transporter involvement and a range of uptake (cell/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 that in microsomes (10- to 20-fold), and this result was consistent with their high cell/medium concentration ratios. For nelfinavir and saquinavir (high-clearance, extensively metabolized drugs), the opposite trend was seen in the Ki values: despite very high cell/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.