THE DEUTERIUM-ISOTOPE EFFECT FOR THE ALPHA-HYDROXYLATION OF TAMOXIFENBY RAT-LIVER MICROSOMES ACCOUNTS FOR THE REDUCED GENOTOXICITY OF [D-5-ETHYL]TAMOXIFEN

This study describes the application of on line HPLC-electrospray ioni zation MS in the structural determination of the metabolites formed fo llowing incubation with rat liver microsomes of an equimolar mixture o f the anticancer drug tamoxifen and its [D-5-ethyl]-analogue. The rati o of Ca 3:1 between unlabelled and D-4-labelled alpha-hydroxytamoxifen , indicating a large isotope effect for this metabolic process, accoun ted for the previously observed lower yield of DNA adducts formed in t he livers of rats treated with D-5-tamoxifen compared with unlabelled drug, The loss of one deuterium atom on metabolism discriminated hydro xyethylated metabolites from others and enabled two further such metab olites to be detected, namely alpha-hydroxytamoxifen N-oxide and alpha -hydroxy-N-desmethyltamoxifen of which the latter is novel, Furthermor e, the use of [alpha-D-2-ethyl]- and [beta-D-3-ethyl] tamoxifens discr iminated alpha- from beta-hydroxylated metabolites and proved that the metabolites described here were alpha-hydroxylated. In contrast to th e alpha-hydroxylated metabolites, the other metabolites identified, na mely tamoxifen N-oxide, N-desmethyltamoxifen, 4-hydroxytamoxifen and t heir deuterated counterparts were not depleted in the deuterated compo nents, The use of on line HPLC-electrospray ionization MS combined wit h isotopic labelling is a powerful technique for probing the structure s of metabolites, and, applied to tamoxifen, has provided further evid ence that alpha-hydroxylation is an important pathway for the conversi on of the drug into a DNA-reactive metabolite.