METABOLISM OF DEXFENFLURAMINE IN HUMAN LIVER-MICROSOMES AND BY RECOMBINANT ENZYMES - ROLE OF CYP2D6 AND 1A2

Dexfenfluramine has been widely used as an appetite suppressant in the treatment of obesity. It was recently shown that the apparent non-ren al clearance of dexfenfluramine was significantly lower in poor metabo lizers than in extensive metabolisers of debrisoquine which suggested the involvement of the polymorphically expressed enzyme, CYP2D6, in de xfenfluramine metabolism, In this study, human liver microsomes and ye ast-expressed recombinant enzymes were used to examine dexfenfluramine metabolism in vitro, In human liver microsomes, the major product of dexfenfluramine was nordexfenfluramine with lesser amounts of a novel metabolite, N-hydroxynordexfenflaramine, and ketone and alcohol deriva tives being formed, Eadie-Hofstee plots (v against v/[s]) of nordexfen fluramine formation between 1 and 1000 mu M substrate concentration we re biphasic in three of four liver microsome samples examined, With me an K-m values of 3 and 569 mu M for the high and low affinity enzymes, respectively. At a substrate concentration (0.5 mu M) around the know n therapeutic plasma concentration there was negligible inhibition of microsomal dexfenfluramine N-dealkylation by sulphaphenazole and ketoc onazole, but between 33 and 100% inhibition by quinidine, and 0-58% in hibition by 7,8-naphthoflavone in seven liver samples. In human liver microsomes, there was also a significant correlation (r(s) = 0.79, n = 10, P < 0.01) between dextromethorphan O-demethylation and dexfenflur amine (at 1 mu M) N-dealkylation activities, Dexfenfluramine was a spe cific inhibitor (IC50 46 mu M) of CYP2D6-mediated dextromethorphan O-d emethylation in human liver microsomes but did not appreciably inhibit six other cytochrome P450 isoform-selective activities for CYP1A2, 2A 6, 2C9, 2C19, 2E1 and 3A activities in human liver microsomes, Yeast-e xpressed recombinant human CYP2D6 metabolized dexfenfluramine with hig h affinity (K-m 1.6 mu M, V-max 0.18 nmol min(-1) nmol P450(-1)) to no rdexfenfluramine which was the sole product observed. Recombinant CYP1 A2 was a lower affinity enzyme (K-m 301 mu M, V-max 1.12 nmol min(-1) nmol P450(-1)) and produced nordexfenfluramine with small amounts of N -hydroxynordexfenfluramine, This is the first detailed study to examin e the in-vitro metabolism of dexfenfluramine in human liver microsomes and by recombinant human P450s, We were able to identify CYP2D6 (high affinity) and CYP1A2 (low affinity) as the major enzymes catalysing t he N-dealkylation of dexfenfluramine in human liver microsomes. Pharma cogenetics 8:423-432 (C) 1998 Lippincott Williams & Wilkins.