0090-9556/97/2508-0978-0984$02.00/0
DRUG METABOLISM AND DISPOSITION
Copyright © 1997 by The American Society for Pharmacology and Experimental Therapeutics
Vol. 25, No. 8

In Vitro and In Vivo Evaluations of the Metabolism, Pharmacokinetics, and Bioavailability of Ester Prodrugs of L-767,679, A Potent Fibrinogen Receptor Antagonist
An Approach for the Selection of a Prodrug Candidate

Thomayant Prueksaritanont, Lynn M. Gorham, Michael J. Breslin, John H. Hutchinson, George D. Hartman, Kamlesh P. Vyas, and Thomas A. Baillie

Departments of Drug Metabolism (T.P., L.M.G., K.P.V., T.A.B.) and Medicinal Chemistry (M.J.B., J.H.H., G.D.H.), Merck Research Laboratories

The present study demonstrates the utility of an in vitro-in vivo correlative approach in the selection of an optimum prodrug candidate of L-767,679 (N-{[7-(piperazin-1-yl)-3,4-dihydro-1(1H)-isoquinolinone-2-yl]acetyl}-3(S)-(ethynyl)--alanine), a potent fibrinogen receptor antagonist. As an initial screening step, a comparative in vitro hepatic metabolism study was conducted for L-767,679 and a series of aliphatic and aromatic ester prodrugs in dogs, monkeys, and humans. In all species, the active acid L-767,679, but not the ester prodrugs, was resistant to metabolism. Only the methyl, ethyl, and isopropyl esters were converted exclusively to the active acid in liver microsomal preparations from dogs and humans, and thus were selected for further studies. In the preparations from monkeys, all of the esters investigated were metabolized efficiently to both the active acid and several other products. The absolute formation rates of L-767,679 from the esters followed the rank order: methyl ~ ethyl > isopropyl in all species, and in humans > dogs for the three esters. The three ester prodrugs did not undergo appreciable hydrolysis in blood or upon incubation with intestinal S9 from any of the studied species.

In vivo evaluation of the previous three aliphatic esters in dogs and monkeys supported the in vitro findings. L-767,679 was metabolically stable in both dogs and monkeys. After intravenous administration of the prodrugs to either species, the extent of acid formation was higher in dogs than in monkeys. In addition, the extent of L-767,679 formed from these prodrugs followed the rank order: methyl ~ ethyl > isopropyl. Similar results were obtained after oral dosing of the prodrugs, such that the bioavailability of L-767,679 was higher in dogs than in monkeys, and the bioavailability was higher after the ethyl ester than after the isopropyl prodrug in both species. In either species, both ethyl and isopropyl ester prodrugs were better absorbed than L-767,679.

Overall, the results suggested that the bioavailability of the active acid after administration of an ester prodrug was dictated primarily by two factors, viz.: 1) the relative rates of ester hydrolysis versus competing metabolic reactions and 2) the absolute rates of ester hydrolysis. In the case of L-767,679 prodrugs, absorption was not a limiting factor. Consequently, the bioavailability of L-767,679 after oral administration of the ester prodrugs would likely be greater in humans than in dogs, and in humans would be higher with the ethyl ester than with the isopropyl ester. On this basis, the ethyl ester was considered as a promising candidate for clinical evaluation as a fibrinogen receptor antagonist prodrug.