Preclinical pharmacokinetics and metabolism of a potent non-nucleoside inhibitor of the hepatitis C virus NS5B polymerase

Abstract

The disposition of compound A, a potent inhibitor of the hepatitis C virus (HCV) NS5B polymerase, was characterized in animals in support of its selection for further development. Compound A exhibited marked species differences in pharmacokinetics. Plasma clearance was 44 ml min⁻¹ kg⁻¹ in rats, 9 ml min⁻¹ kg⁻¹ in dogs and 16 ml min⁻¹ kg⁻¹ in rhesus monkeys. Oral bioavailability was low in rats (10%) but significantly higher in dogs (52%) and monkeys (26%). Compound A was eliminated primarily by metabolism in rats, with biliary excretion accounting for 30% of its clearance. Metabolism was mainly mediated by cyclohexyl hydroxylation, with N-deethylation and acyl glucuronide formation constituting minor metabolic pathways. Qualitatively, the same metabolites were identified using in vitro systems from all species studied, including humans. The low oral bioavailability of compound A in rats was mostly due to poor intestinal absorption. This conclusion was borne out by the findings that hepatic extraction in the rat was only 30%, intraperitoneal bioavailability was good, and compound A was poorly absorbed from the rat isolated intestinal loop, with no detectable intestinal metabolism. Compound A was not an inhibitor of major human cytochrome P450 enzymes, indicating minimal potential for clinical drug–drug interactions. The metabolic clearance of compound A in rat, dog and monkey hepatocytes correlated with the systemic clearance observed in these species. Since compound A was very stable in human hepatocytes, the results suggest that it will be a low clearance drug in humans.

Keywords :

• Hepatitis C virus
• NS5B polymerase
• antivirals
• non-nucleoside inhibitor

Acknowledgements

The authors thank Francesca Naimo, Maria Verdirame, Mark Jairaj, Massimiliano Fonsi, Antonella Cellucci, Massimo Aquilina, Fabrizio Colaceci and Giuseppe Mesiti for support and advice. They also thank Frank Tang (Merck Research Laboratories, Rahway, NJ, USA) for radiochemical synthesis of tritiated compound A; Magang Shou (Merck Research Laboratories, West Point, PA, USA) for kindly providing CYP-neutralizing antibodies; and Thomayant Prueksaritanont for helpful discussions. The work was supported in part by a grant from the Ministero dell'Istruzione, dell'Università e della Ricerca.