Preclinical pharmacokinetics of MK-0974, an orally active calcitonin-gene related peptide (CGRP)-receptor antagonist, mechanism of dose dependency and species differences

The underlying mechanism for low oral bioavailability of MK-0974, a potent calcitonin-gene related peptide (CGRP)-receptor antagonist, in monkeys and for species-dependent non-linear pharmacokinetics in monkeys and rats were investigated. In monkeys, MK-0974 displayed moderate clearance (14-20 ml min(-1) kg(-1)), while oral bioavailability was 6%. The pharmacokinetics of MK-0974 remained linear across 0.5-10 mg kg(-1) intravenous dose in monkeys, but the oral area under the plasma concentration-time curve (AUC) increase (5-30 mg kg(-1)) was 15-fold over dose-proportional. Based on a comparison of AUC following hepatic portal vein administration and cephalic vein infusion, MK-0974 exhibited a low-to-moderate hepatic extraction ratio (36%) in monkeys. Following oral dose of [14C]MK-0974 to monkeys, the hepatic portal AUC ratio of MK-0974 versus total radioactivity was 0.32, and the total radioactivity recovered in bile and urine was 45-83%. MK-0974 undergoes significant oxidative metabolism (Cytochrome P450 (CYP) 3A) in monkey intestinal microsomes. In contrast, oral AUC of MK-0974 in rats was near dose-proportional (15-100 mg kg(-1)). Following oral administration of [14C]MK-0974 to rats, the hepatic portal AUC ratio of MK-0974 to total radioactivity (0.67) was higher than in monkeys. Additionally, the metabolic rate of MK-0974 was slower in rat than in monkey intestinal microsomes. Collectively, intestinal first-pass metabolism played a significant role in the low oral bioavailability in monkeys and contributed to the species-dependent non-linear oral pharmacokinetics in rats and monkeys of MK-0974.