Carcinogenesis, Vol. 23, No. 11, 1897-1902,
November 2002
© 2002 Oxford University Press
CARCINOGENESIS |
Identification of human CYP forms involved in the activation of tamoxifen and irreversible binding to DNA
MRC Molecular Endocrinology Group, Department of Obstetrics and Gynaecology, Robert Kilpatrick Building, University of Leicester, Leicester LE2 7LX, UK,
1 Cancer Biomarkers and Prevention Group, University of Leicester LE1 7RH, UK and
2 Biology and Biotechnology Research Program, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
This study investigates which CYP forms are responsible for the conversion of tamoxifen to its putative active metabolite 
-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 -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 -hydroxytamoxifen and the only one that results in appreciable levels of irreversible binding of tamoxifen to DNA.
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  W. Chu, A. Fyles, E. M. Sellers, D. R. McCready, J. Murphy, T. Pal, and S. A. Narod Association between CYP3A4 genotype and risk of endometrial cancer following tamoxifen use Carcinogenesis, October 1, 2007; 28(10): 2139 - 2142. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 K. Brown, E. M. Tompkins, D. J. Boocock, E. A. Martin, P. B. Farmer, K. W. Turteltaub, E. Ubick, D. Hemingway, E. Horner-Glister, and I. N.H. White Tamoxifen Forms DNA Adducts in Human Colon after Administration of a Single [14C]-Labeled Therapeutic Dose Cancer Res., July 15, 2007; 67(14): 6995 - 7002. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 R. Obligacion, M. Murray, and I. Ramzan Drug-Metabolizing Enzymes and Transporters: Expression in the Human Prostate and Roles in Prostate Drug Disposition J Androl, March 1, 2006; 27(2): 138 - 150. [Full Text] [PDF]
|
|
|
|
 |
|
 D. H. Phillips, A. Hewer, M. R. Osborne, K. J. Cole, C. Churchill, and V. M. Arlt Organ specificity of DNA adduct formation by tamoxifen and {alpha}-hydroxytamoxifen in the rat: implications for understanding the mechanism(s) of tamoxifen carcinogenicity and for human risk assessment Mutagenesis, July 1, 2005; 20(4): 297 - 303. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 Aspen Cancer Conference Fellows Toxicol Pathol, October 1, 2004; 32(6): 749 - 761. [PDF]
|
|
|
|
|
|
M.E. Heres-Pulido, I. Duenas-Garcia, L. Castaneda-Partida, A. Sanchez-Garcia, M. Contreras-Sousa, A. Duran-Diaz, and U. Graf Genotoxicity of tamoxifen citrate and 4-nitroquinoline-1-oxide in the wing spot test of Drosophila melanogaster Mutagenesis, May 1, 2004; 19(3): 187 - 193. [Abstract] [Full Text] [PDF]
|
|