Cytochrome P-450 3A and 2D6 catalyze ortho hydroxylation of 4-hydroxyamoxifen and 3-hydroxytamoxifen (droloxifene) yielding tamoxifen catechol: Involvement of catechols in covalent binding to hepatic proteins

Earlier study suggested that 3,4-dihydroxytamoxifen (tam catechol), a tamox ifen metabolite, is proximate to the reactive intermediate that binds coval ently to proteins and possibly to DNA (Dehal and Kupfer, 1996). The current study demonstrates that rat and human hepatic cytochrome P-450s (CYPs) cat alyze tam catechol formation from tamoxifen (tam), 9-hydroxy-tam (Droloxife ne), and I-hydroxy-tam (4-OH-tam), Higher levels of catechol were formed fr om COH-tam and 3-hydroxy-tam than from tam, Evidence that human hepatic CYP 3A4 and 2D6 catalyze the formation of tam catechol from 4-OH-tam and suppor tive data that the catechol is proximate to the reactive intermediate, was obtained: 1) There was a good correlation (r = 0.82; p less than or equal t o .0004) between steroidal 6 beta-hydroxylase (CYP3A activity) and ortho hy droxylation of 4-OH-tam in human liver microsomes; 2) monospecific antibodi es against CYP3A4 strongly inhibited catechol formation from 4-OH-tam and i ts covalent binding to proteins in human liver microsomes; 3) low levels of ketoconazole inhibited catechol tam accumulation and covalent binding of 4 -OH-tam to human liver proteins; 4) among human P-450s expressed in insect cells (supersomes), only CYP3A4 and 2D6 noticeably catalyzed catechol forma tion, and cytochrome b(s) markedly stimulated the CYP3A4 Catalysis; and 5) human livers with high CYP3A and low or high CYP2D6 activity exhibited high catechol formation and those with tow 3A and 2D6 activities formed only li ttle catechol, These findings demonstrate that CYP3A4 and to a lesser exten t 2D6 catalyze tam catechol formation and support the participation of tam catechol in covalent binding to proteins.