IN-VITRO CHARACTERIZATION OF THE CYTOCHROME-P450 ISOENZYMES INVOLVED IN THE BACK OXIDATION AND N-DEALKYLATION OF REDUCED HALOPERIDOL

In-vitro studies were performed using human liver microsomes and c-DNA -expressed human P450 isoforms to identify the cytochrome P450 isoenzy me(s) involved in the back oxidation and N-dealkylation of reduced hal operidol, Back oxidation and N-dealkylation of reduced haloperidol wer e assessed by measuring the formation of haloperidol and 4-(4-chloroph enyl)-4-hydroxypiperidine (CPHP), respectively. The Haloperidol and CP HP formation rates as a function of substrate concentration, measured in three livers, followed monophasic enzyme kinetics. For haloperidol formation K-m values ranged from 51-59 mu M, and V-max values from 190 -334 pmol mg(-1) min(-1); for CPHP formation K-m values were 44-49 mu M, and V-max values 74-110 pmol mg(-1) min(-1). haloperidol and CPHP f ormation rates in the nine liver preparations were significantly corre lated with dextromethorphan N-demethylase activity (a marker of CYP3A4 activity), but not with the CYP2D6, CYP1A2 and CYP2C9 activity. Ketoc onazole and troleandomycin, inhibitors of CYP3A4, inhibited competitiv ely both haloperidol and CPHP formation, with a K-i value lower than 0 .2 mu M for ketoconazole and lower than 0.3 mu M for troleandomycin. S ulphaphenazole (CYP2C9), furafylline (CYP1A2) and quinidine and paroxe tine (CYP2D6) gave only little inhibition (IC50 > 60 mu M), CPHP and h aloperidol formation were, moreover, enhanced by alpha-naphthoflavone, an effect known for CYP3A4 mediated reactions, Anti-CYP3A4 antibodies strongly inhibited haloperidol and CPHP formation, whereas CYP2D6 ant ibodies did not, Among the recombinant human CYP isoforms tested, CYP3 A4 exhibited the highest activity with respect to haloperidol and CPHP formation rates, with no detectable effect of CYP1A2, CYP2D6 and CYP2 C9, These results strongly suggest that back oxidation and N-dealkylat ion of reduced haloperidol in human liver microsomal preparations are mediated by CYP3A4. Pharmacogenetics 8:383-389 (C) 1998 Lippincott Wil liams & Wilkins.