CLOMIPRAMINE N-DEMETHYLATION METABOLISM IN HUMAN LIVER-MICROSOMES

AIM: To study the effect of cytochrome P-450 (CYP450) inhibitors on cl omipramine (Clo) N-demethylation in vitro. METHODS: The kinetic parame ters of Clo N-demethylation in human liver microsomes were obtained by the Michaelis-Menten equation. The parameters after pretreatment with putative inhibitors of various CYP450 isoforms were compared with con trols. RESULTS: K-m1, K-m2, V-max1, V-max2, V-max1/K-m1, and V-max2/K- m2 were (0.11 +/- 0.06), (24 +/- 14) mu mol.L-1, (114 +/- 47), (428 +/ - 188) nmol . g(-1) . min(-1), (1.8 +/- 1.6), and (0.019 +/- 0.005) L . g(-1) . min(-1), respectively. The interindividual variations for th e last 4 parameters reached up to 2.5-, 7.3-, 3.4-, and 1.8-fold. At 5 mu mol . L-1 of Clo, troleandomycin (Tro), furafylline (Fur), ditioca rb sodium (Dit), and S-mephenytoin (Mep) produced a marked inhibition on Clo N-demethylation while sulfaphenazole (Sul) and quinidine (Qui) had only slight effects. The inhibitory rates by Dit 30, Mep 500, Fur 10, Tro 10, Fur 80, Tro 200 and Fur 80 + Tro 200 mu mol . L-1 were 27. 0 %, 32.9 %, 42.8 %, 40.5 %, 63.9 %, 66.4 %, and 78.3 %, respectively. The IC50 (95 % confidence limits) for Fur and Tro were 27.7 (19.1 - 3 6.3) and 42.1 (20.9 - 63.3) mu mol.L-1, respectively. CONCLUSIONS: The N-demethylation of Clo exhibited a biphasic behavior. This reaction w as mediated mainly by both CYP1A2 and CYP3A4, to a minor extent by CYP 2C19 at the low concentration of Clo in vitro.