Eight inhibitory monoclonal antibodies define the role of individual P-450s in human liver microsomal diazepam, 7-ethoxycoumarin, and imipramine metabolism.

Eight inhibitory monoclonal antibodies (MAbs) individually specific to human cytochrome P-450 (P-450) 1A1, 1A2, 2A6, 2B6, 2C subfamily (2C8, 2C9, 2C18 and 2C19), 2D6, 2E1, and 3A4/5 were used to define the role of single P-450s in the metabolism of diazepam (DZ), 7-ethoxycoumarin (7-EC), and imipramine (IMI) in human liver microsomes (HLM). The MAbs were added combinatorially to six HLM samples. With DZ as a substrate, more than 80% of temazepam (TMZ) formation was inhibited in all six samples by the addition of MAb to 3A4/5, indicating an 80% contribution of 3A4/5 to TMZ formation. Nordiazepam formation was inhibited with MAbs to 2B6 (6-23%), 2C subfamily (12-61%) and 3A4/5 (14-45%). The MAbs to 1A1, 1A2, 2A6, 2D6, and 2E1 did not inhibit TMZ or nordiazepam formation; this indicates their noninvolvement in DZ metabolism. The MAb-defined P-450 contribution to 7-EC Odeethylation in six HLM samples was 17 to 60% for 2E1, 15 to 46% for 2A6, and 5 to 22% for 1A2, reflecting the role and variation of each P-450 in this activity. MAbs to 1A1, the 2C subfamily, 2D6, and 3A4/5 did not affect 7-EC metabolism in the HLM samples. IMI is metabolized mainly to 2-hydroxyimipramine by expressed 2C19 and 2D6, and desipramine (DIM) by expressed 1A2, 2C18, 2C19 and 2D6. Expressed 1A1, 2C9, and 3A4 showed low activities for the formation of DIM. Of six HLM samples, five showed IMI hydroxylation activity (0.35-2.6 nmol/min/nmol P-450) while one (HL43) lacked hydroxylation activity. All six HLM samples showed N-deethylation activity (0.74-1.4 nmol/min/nmol P-450). The MAb-determined contribution of 2D6 and 2C19 to 2-hydroxyimipramine formation ranged from 47 to 90% and from 0 to 49%, respectively, while HL43 did not show 2-hydroxylation. The role of P-450s involved in DIM formation varied for 2C19 (13-50%), 1A2 (23-41%), and 3A4 (8-26%). These studies demonstrate a system for identifying the quantitative metabolic role of single P-450s and their interindividual variability in a tissue containing multiple P-450s. The system using inhibitory MAbs is simple, precise, and applicable to any P-450-mediated catalytic activity including that for drugs, carcinogens, mutagens, toxic chemicals and endobiotics.