alpha-hydroxytamoxifen, a genotoxic metabolite of tamoxifen in the rat: identification and quantification in vivo and in vitro

The metabolic formation of alpha-hydroxytamoxifen, a reactive metabolite of tamoxifen in rat liver, was characterized and quantified in vitro (hepatic microsomal incubations) and in vivo (bile-duct cannulated animals). This m inor metabolite was identified by chromatographic and mass spectral compari sons with the authentic compound. The rates of formation of alpha-hydroxyta moxifen in incubations (30 min) of tamoxifen (25 mu M) with liver microsoma l preparations from women (pool of six), female CD1 mice or female Sprague- Dawley rats, as quantified by liquid chromatography-mass spectrometry (LC-M S), were 1.15 +/- 0.03, 0.30 +/- 0.05 and 2.70 +/- 0.35 pmol/min/mg protein , respectively. Selective inhibition of microsomal P450 indicated that alph a-hydroxylation was catalysed predominantly by CYP3A in humans. Bile-duct c annulated and anaesthetized female rats and mice given [C-14]tamoxifen (43 mu mol/kg, i.v.) excreted, respectively, 24 and 21% of the administered rad ioactivity in bile over 5 and 3.5 h. The major radiolabelled biliary metabo lite in rats, characterized by LC-MS after enzymic hydrolysis of conjugates , was the glucuronide of 4-hydroxytamoxifen (10% of dose) and only 0.1% of the dose was recovered as alpha-hydroxytamoxifen, After administration of a lpha-hydroxytamoxifen (43 mu mol/kg, i.v.) to rats, only 1.19% of the admin istered compound was recovered from a glucuronide metabolite in bile, indic ating a possible 0.84% alpha-hydroxylation of tamoxifen in vivo. There was, however, no indication of the presence in bile of either O-sulphonate or g lutathione conjugates derived from alpha-hydroxytamoxifen. This study shows for the first time that alpha-hydroxytamoxifen can be glucuronylated in ra t liver. Whereas sulphonation results in electrophilic genotoxic intermedia tes, glucuronidation may represent a means of detoxifying alpha-hydroxytamo xifen.