0090-9556/97/2507-0840-0844$02.00/0
DRUG METABOLISM AND DISPOSITION
Copyright © 1997 by The American Society for Pharmacology and Experimental Therapeutics
Vol. 25, No. 7

In vitro Identification of the P450 Enzymes Responsible for the Metabolism of Ropinirole

J. C. Bloomer, S. E. Clarke, and R. J. Chenery

Department of Drug Metabolism and Pharmacokinetics, SmithKline Beecham Pharmaceuticals

The in vitro metabolism of ropinirole was investigated with the aim of identifying the cytochrome P450 enzymes responsible for its biotransformation. The pathways of metabolism after incubation of ropinirole with human liver microsomes were N-despropylation and hydroxylation. Enzyme kinetics demonstrated the involvement of at least two enzymes contributing to each pathway. A high affinity component with a K_M of 5-87 µM and a low affinity component with a K_M of approximately two orders of magnitude greater were evident. The high affinity component could be abolished by pre-incubation of the microsomes with furafylline. Additionally, incubation of ropinirole with microsomes derived from CYP1A2 transfected cells readily produced the N-despropyl and hydroxy metabolites. Some inhibition of ropinirole metabolism was also observed with ketoconazole, indicating a minor contribution by CYP3A. Multivariate correlation data were consistent with the involvement of the cytochrome P450 enzymes 1A2 and 3A in the metabolism of ropinirole. Thus, it could be concluded that the major P450 enzyme responsible for ropinirole metabolism at lower (clinically relevant) concentrations is CYP1A2 with a contribution from CYP3A, particularly at higher concentrations.