Scaling factors to relate drug metabolic clearance in hepatic microsomes, isolated hepatocytes, and the intact liver: studies with induced livers involving diazepam.

Microsomal protein recovery and hepatocellularity have been determined and investigated as scaling factors for interrelating clearance by hepatic microsomes, freshly isolated hepatocytes and whole liver from untreated (UT) rats and rats treated with either the cytochrome P450 inducer phenobarbital (PB) or dexamethasone (DEX). Hepatocellularity in UT rats (1.1 x 10(8) hepatocytes/g liver) was not significantly different after either PB or DEX induction (1.1 and 1.3 x 10(8) hepatocytes/g liver, respectively). However the microsomal protein recovery index, which provides a scaling factor that is inversely related to the efficiency of the microsomal preparation procedure, was 47 mg/g liver in both PB and DEX microsomes and differs from UT rats (60 mg/g liver). These contrasting findings are consistent with the interlaboratory trends in the literature, indicating that, although hepatocellularity estimates are in good accord, microsomal recovery can vary 2-fold; this has implications for scaling. The oxidation of diazepam to its three primary metabolites was measured in PB and DEX microsomes and hepatocytes and the scaling factors were applied to these data and previously reported UT data. Marked changes in kinetics occur on induction resulting in a shift in the major pathway. In particular, 3-hydroxylation is induced over 20-fold by DEX. Diazepam CL(int) was determined in vivo after administration of a bolus dose into the hepatic portal vein of UT, PB, and DEX rats; values of 127, 191, and 323 ml/min/SRW (where SRW is a standard rat weight of 250 g), respectively, were obtained. Using these scaling factors, the hepatocyte predictions of CL(int) were excellent (99, 144, and 297 ml/min/SRW for UT, PB, and DEX, respectively), whereas only the DEX prediction (248 ml/min/SRW) was accurate for the microsomal system, with a substantial underprediction for UT and PB (46 and 68 ml/min/SRW, respectively). Evidence is presented for product inhibition, resulting from accumulation of primary metabolites within the microsomal preparation, as the mechanism responsible for this underprediction. These results illustrate that the scaling factor approach is applicable to induced livers in which both cytochrome P450 complement and zonal distribution are altered. These data, together with our previous studies, demonstrate that CL(int) in cells (2.4-297 ml/min/SRW), microsomes (2.7-248 ml/min/SRW), and in vivo (1.5-323 ml/min/SRW) are related in a linear fashion and hence inherently both in vitro systems are of equal value in predicting in vivo CL(int).