ABSTRACT:Predicting drug-drug interactions requires an assessment of the drug concentration available to the enzyme active site, both in vivo, and within an in vitro incubation. These predictions are confounded when the inhibitor accumulates within the liver, either as a result of active transport processes or intracellular binding (including lysosomal trapping). In theory, hepatocytes should provide a more accurate estimation of inhibitory potency compared with microsomes for those compounds that undergo hepatic accumulation. However, they are not routinely used for K i determination and there is limited comparative information available. Therefore, the aims of this study were to compare K i values determined in rat microsomes and freshly isolated hepatocytes using six cytochrome P450 inhibitors (miconazole, fluconazole, ketoconazole, quinine, fluoxetine, and fluvoxamine) with a range of uptake properties (cell-to-medium concentration ratios 4.2-6000). Inhibition studies were performed using four probe substrates for CYP2C, CYP2D, and CYP3A enzymes (tolbutamide and phenytoin, dextromethorphan and midazolam, respectively). Comparison of unbound K i values (range 0.05-30 M) showed good agreement between microsomes and hepatocytes for inhibition of 18 pathways of metabolism. In addition to this, there was no relationship between the cell-to-medium concentration ratios (covering over 3 orders of magnitude) and the microsomal to hepatocyte K i ratio of these inhibitors. These data suggest that the hepatic accumulation of these inhibitors results from intracellular binding rather than the involvement of uptake transporters and indicate that microsomes and hepatocytes appear to be equivalent for determining the inhibitory potency of the six inhibitors investigated in the present study.Quantitative prediction of drug-drug interactions (DDIs) requires an accurate estimation of drug concentration at the enzyme active site, both in vivo within the liver and within an in vitro incubation. Since it is impossible to directly measure this liver concentration in humans, plasma concentrations have been used as an in vivo surrogate, but with varying degrees of success (Ito et al., 2002(Ito et al., , 2004. Although the use of plasma inhibitor concentration has accurately predicted the degree of in vivo DDIs for several compounds (Brown et al., 2005(Brown et al., , 2006Obach et al., 2006), even the use of total plasma concentrations may underestimate hepatocellular concentrations for inhibitors that are accumulated within the liver.Drug can be sequestered within the hepatocyte via several mechanisms; for example, lysosomal uptake, intracellular protein binding, or the involvement of hepatic uptake transporters. Lysosomes represent 1% of the hepatocyte volume; therefore, trapping is an important distribution process for weak bases, which can accumulate within this compartment due to the pH gradient between the acidic lysosome compartment (pH 5), the hepatic cytosol (pH 7.2), and the plasma (pH 7.4) (Daniel, 2003). Drug accumulation...