N-demethylation accompanies alpha-hydroxylation in the metabolic activation of tamoxifen in rat liver cells.
Previous work has shown that a major route of activation of tamoxifen to DNA-binding products in rat liver cells is via alpha-hydroxylation leading to modification of the N(2)-position of guanine in DNA and to a lesser extent the N(6)-position of adenine. Improved resolution by HPLC has now identified two major adducts in rat liver DNA, one of them the aforementioned tamoxifen-N(2)-guanine adduct and the other the equivalent adduct in which the tamoxifen moiety has lost a methyl group. Treatment of rats or rat hepatocytes with N-desmethyltamoxifen gave rise to the second adduct, whereas treatment with tamoxifen or alpha-hydroxytamoxifen gave rise to both. Furthermore, N,N-didesmethyltamoxifen was found to be responsible for an additional minor DNA adduct formed by tamoxifen, alpha-hydroxytamoxifen and N-desmethyltamoxifen. The involvement of metabolism at the alpha position was confirmed in experiments in which [alpha-D(2)-ethyl]tamoxifen, but not [beta-D(3)-ethyl]tamoxifen, produced reduced levels of DNA adducts. Tamoxifen N-oxide and alpha-hydroxytamoxifen N-oxide also gave rise to DNA adducts in rat liver cells, but the adduct patterns were very similar to those formed by tamoxifen and alpha-hydroxytamoxifen, indicating that the N-oxygen is lost prior to DNA binding. These and earlier results demonstrate that in rat liver cells in vivo and in vitro, Phase I metabolic activation of tamoxifen involves both alpha-hydroxylation and N-demethylation, which is followed by Phase II activation at the alpha-position to form a highly reactive sulphate. Detection of tamoxifen-related DNA adducts by (32)P-postlabelling is achieved with >90% labelling efficiency.
Phillips
D H
DH
Institute of Cancer Research, Haddow Laboratories, Sutton SM2 5NG, UK. davidp@icr.ac.uk
Hewer
A
A
Horton
M N
MN
Cole
K J
KJ
Carmichael
P L
PL
Davis
W
W
Osborne
M R
MR
D000818:Animals; D001711:Biotransformation; D002478:Cells, Cultured; D002851:Chromatography, High Pressure Liquid; D004247:DNA / Q000378:metabolism; D018736:DNA Adducts; D005260:Female; D006900:Hydroxylation; D008099:Liver / Q000378:metabolism*; D008745:Methylation; D010084:Oxidation-Reduction; D010761:Phosphorus Radioisotopes; D051381:Rats; D011916:Rats, Inbred F344; D013629:Tamoxifen / Q000493:pharmacokinetics*
D018736:DNA Adducts; D010761:Phosphorus Radioisotopes; D013629:Tamoxifen; D004247:DNA