Journal Article

Identification of human CYP forms involved in the activation of tamoxifen and irreversible binding to DNA

David J. Boocock, Karen Brown, Anthony H. Gibbs, Esther Sanchez, Kenneth W. Turteltaub and Ian N.H. White

in Carcinogenesis

Volume 23, issue 11, pages 1897-1902
Published in print November 2002 | ISSN: 0143-3334
Published online November 2002 | e-ISSN: 1460-2180 | DOI:
Identification of human CYP forms involved in the activation of tamoxifen and irreversible binding to DNA

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This study investigates which CYP forms are responsible for the conversion of tamoxifen to its putative active metabolite α-hydroxytamoxifen and irreversible binding to DNA. We have used eight different baculovirus expressed recombinant human CYP forms and liquid chromatography-mass spectrometry to show that only CYP3A4 is responsible for the NADPH-dependent α-hydroxylation of tamoxifen. Surprisingly, this CYP did not catalyse the formation of 4-hydroxytamoxifen. We demonstrate for the first time, by means of accelerator mass spectrometry, that CYP3A4 also catalysed the activation of [14C]tamoxifen to intermediates that irreversibly bind to exogenous DNA. Incubation of [14C]tamoxifen (20.6 kBq, 100 μM) with CYP3A4, in the presence of NADPH for 60 min led to levels of DNA binding of 39.0±9.0 adducts/108 nucleotides (mean ± SE, n = 6). While CYP3A4 converted tamoxifen to N-desmethyltamoxifen (38.3 ± 7.20 pmol/20 min/pmol CYP, n = 4), the polymorphic CYP2D6 showed the highest activity for producing this metabolite (48.6±1.52pmol/20 min/pmol CYP). CYP2D6 was also the most active in catalysing 4-hydroxylation of tamoxifen, although an order of magnitude lower level was also detected with CYP2C19. With tamoxifen as substrate, no 3,4-dihydroxytamoxifen could be detected with any CYP form. CYP2B6 did not catalyse the metabolism or the binding of tamoxifen to DNA. It is concluded that CYP3A4 is the only P450 of those tested that converts tamoxifen to α-hydroxytamoxifen and the only one that results in appreciable levels of irreversible binding of tamoxifen to DNA.

Keywords: AMS, accelerator mass spectrometry; LC-ESIMS, liquid chromatography-electrospray secondary ion mass spectrometry.

Journal Article.  3364 words.  Illustrated.

Subjects: Clinical Cytogenetics and Molecular Genetics

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