O- and N-demethylation of venlafaxine in vitro by human liver microsomes and by microsomes from cDNA-transfected cells: Effect of metabolic inhibitors and SSRI antidepressants

The biotransformation of venlafaxine (VF) into its two major metabolites, O -desmethylvenlafaxine (ODV) and N-desmethylvenlafaxine (NDV) tons studied i n vitro with human liver microsomes and with microsomes containing individu al human cytochromes from cDNA-transfected human lymphoblastoid cells. VF w as coincubated with selective cytochrome P450 (CYP) inhibitors and several selective serotonin reuptake inhibitors (SSRIs) to assess their inhibitory effect on VF metabolism. Formation rates for ODV incubated with human micro somes were consistent with Michaelis-Menten kinetics for a single-enzyme me diated reaction with substrate inhibition. Mean parameters determined by no n-linear regression were: V-max = 0.36 nmol/min/mg protein, K-m = 42 mu M, and K-s 22901 mu M (K-s represents a constant which reflects the degree of substrate inhibition). Quinidine (QUI) was a potent inhibitor of ODV format ion with a K-i of 0.04 mu M, and paroxetine (PX) was the most potent SSRI a t inhibiting ODV formation with a mean K-i value of 0.17 mu M. Studies usin g expressed cytochromes showed that ODV was formed by CYP2C9, -2C29, and -2 D6. CYP2D6 teas dominant with the lowest K-m, 23.2 mu M,and highest intrins ic clearance (V-max/K-m ratio). No unique model teas applicable to the form ation of NDV for all four fivers tested. Parameters determined by applying a single-enzyme model were V-max = 2.24 nmol/min/mg protein, and K-m = 2504 mu M. Ketoconazole was a potent inhibitor of NDV production, although its inhibitory activity was not as great as observed with pure 3A substrates. N DV formation tons also reduced by 42% by a polyclonal rabbit antibody again st rat liver CYP3A1. Studies using expressed cytochromes showed that NDV wa s formed by CYP2C9, -2C19, and -3A4. The highest intrinsic clearance was at tributable to CYP2C19 and the lowest to CYP3A4. However the high in vivo ab undance of 3A isoforms will magnify the importance of this cytochrome. Fluv oxamine (FX), at a concentration of 20 mu M, decreased NDV production by 46 % consistent with the capacity of FX to inhibit CYP3A, 2C9, and 2C19. These results are consistent with previous studies that show CYP2D6 and -3A4 pla y important roles in the formation of ODV and NDV, respectively. In additio n we have shown that several other CYPs have important roles in the biotran sformation of VF. (C) 1999 American College of Neuropsychopharmacology. Pub lished by Elsevier Science Inc.