Baseline variations in in vivo cytochrome P450 3A-dependent phenotypes evaluated using concentrations of endogenous or deuterium-labeled cortisol and their 6β-hydroxylated metabolites in 103 cynomolgus monkeys and 20 humanized-liver mice transplanted with hepatocytes.

Drug exposure, in terms of plasma concentrations, may be affected by individual differences in intrinsic drug-metabolizing enzymes as much as by exogenous drug interactions. However, baseline variations in drug-metabolizing enzyme activities in normal subjects and acceptable ranges for these variations remain unclear. To elucidate these baseline variations, CYP3A-dependent endogenous cortisol and/or deuterium-labeled cortisol and their 6β-hydroxylated metabolite concentrations were determined in vivo as biomarkers in 103 cynomolgus monkeys and 20 immunodeficient mice transplanted with human hepatocytes. After a single intravenous administration of cortisol-d4 (0.50 mg/kg) in monkeys, the mean maximum plasma level of cortisol-d4 was 1020 ± 160 ng/mL (n = 3) at 0.083 hours and was similar to the mean background cortisol level at the same time point, although background levels remained roughly constant throughout the day. Using spot plasma samples from an additional 100 monkeys, ratios of endogenous 6β-hydroxycortisol to cortisol concentration indicated variability: the median value was 1.4% (5% and 95% percentiles, 0.82% and 2.3%, respectively). The lowest and highest in vivo clearance values for cortisol-d4 in humanized-liver mice with hepatocytes from 4 different donors had a 2.6-fold range across the 4 groups. The endogenous cortisol levels in mouse plasma samples were below the detection limit (<1 ng/mL). Currently, drug interaction study alerts are triggered at 1.25- to 2-fold increases in object drug clearance or metabolite/parent concentration ratio; however, evaluation should consider multiple factors, and these could be within the range of intrinsic baseline variations. SIGNIFICANCE STATEMENT: The mean 6β-hydroxycortisol plasma concentration in 100 cynomolgus monkeys was 4.4 ng/mL (coefficient of variation, 36%). The lowest and highest clearances of cortisol-d4 in humanized-liver mice indicated a 2.6-fold range among the 4 sources of hepatocytes. This large baseline range may inform drug interaction guidance, which currently triggers alerts at a 1.25-fold increase in blood concentrations.