Determination of microsome and hepatocyte scaling factors for in vitro/in vivo extrapolation in the rat and dog

Abstract

1. In vivo clearance predictions from in vitro assays require scaling factors to relate the concentrations of hepatocytes or microsomal protein to the intact liver.2. The aims were to measure the variability in scaling factors for Wistar rat and beagle dog for which the literature is particularly scarce and determine any sex differences.3. Scaling factors were determined by comparing the cytochrome P450 (P450) content in hepatocytes or microsomes against the P450 content of fresh liver homogenate. The use of fresh homogenate is recommended as freezing can increase contamination and affect the P450 assay.4. Meangeo hepatic microsomal concentrations in Wistar rats were 61 mg g−1 liver (95% confidence interval (CI); 47–75 mg g−1 liver) and in beagle dogs 55 mg g−1 liver (95% CI = 48–62 mg g−1 liver). Meangeo hepatocellularity was 163 × 106 cells g−1 liver for Wistar rats (95% CI = 127–199 × 106 cells g−1 liver) and 169 × 106 cells g−1 liver (95% CI = 131–207 × 106 cells g−1 liver) for beagle dogs. The data generated in this study indicate a consistency in scaling factors between rat and dog. No sex differences were observed.