Major inter-species differences in the rates of O-sulphonation and O-glucuronylation of alpha-hydroxytamoxifen in vitro: a metabolic disparity protecting human liver from the formation of tamoxifen-DNA adducts

Tamoxifen is a hepatic genotoxin in rats and mice but a hepatocarcinogen on ly in rats, It is not associated with DNA adducts and liver tumours in pati ents, The proposed major pathway for its bioactivation in rats involves alp ha -hydroxylation, O-sulphonation and generation of carbocation that reacts with DNA, Rat liver microsomes catalyse a-hydroxylation at similar to2- an d 4-fold the rate achieved by human and murine liver microsomes, respective ly, O-glucuronylation will deactivate alpha -hydroxytamoxifen and compete w ith sulphonation, Rates of O-sulphonation of alpha -hydroxytamoxifen in hep atic cytosol have been determined by a HPLC assay of substrate-dependent 3' -phosphoadenosine 5'-phosphate production, The rank order of O-glucuronylat ion in hepatic microsomes was estimated by HPLC-mass spectrometry, The rate of sulphonation of trans-alpha -hydroxytamoxifen (25 muM) in cytosol from adult female Sprague-Dawley rats and CD1 mice was 5.3 +/- 0.8 and 3.9 +/- 0 .5 pmol/min/mg protein (mean +/- SD, n = 3), respectively, In cytosol fract ions from women aged 40-65 years, the rate was 1.1 +/- 0.4 pmol/min/mg prot ein (mean +/- SD, n = 6). The K-m for trans-alpha -hydroxytamoxifen in rat, mouse and human cytosol was 84.6 +/- 3.8, 81.4 +/- 4.6 and 104.3 +/- 5.6 m uM (mean +/- SD, n = 3), respectively; the corresponding V-max values were 22.4 +/- 3.4, 17.1 +/- 3.1 and 6.3 +/- 1.9 pmol/min/mgprotein, These K-m we re similar to a value obtained by others using purified rat liver hydroxyst eroid sulphotransferase a. Turnover of the cis epimer was too slow for accu rate determination of rates, Sulphonation of trans-alpha -hydroxytamoxifen in human uterine cytosol was undetectable, The rank order of O-glucuronylat ion of trans-alpha -hydroxytamoxifen in liver microsomes was human >> mouse > rat, In combination, lower rates of alpha -hydroxylation and O-sulphonat ion and a higher rate of O-glucuronylation in human liver would protect pat ients from the formation of tamoxifen-DNA adducts.