N-Demethylation accompanies α-hydroxylation in the metabolic activation of tamoxifen in rat liver cells

• Published in: Carcinogenesis (New York) 1999-10, Vol.20 (10), p.2003-2009
• Main Authors: Phillips, David H., Hewer, Alan, Horton, Martin N., Cole, Kathleen J., Carmichael, Paul L., Davis, Warren, Osborne, Martin R.
• Format: Article
• Language: English
• Subjects: (Z)-1-[4-[2-(dimethylamino)ethoxy]phenyl]-1; 2-diphenyl-1-butene; 3-2H2]-(Z)-1-[4-[2-(dimethylamino)-ethoxy]phenyl]-1; 4-2H3]-(Z)-1-[4-[2-(dimethylamino)ethoxy]phenyl]-1; 4-2H5]-(Z)-1-[4-[2-(dimethylamino)ethoxy]phenyl]-1; [D5-ethyl]tamoxifen; [α-D2-ethyl]tamoxifen; [β-D3-ethyl]tamoxifen; Animals; Biological and medical sciences; Biotransformation; Cells, Cultured; Chromatography, High Pressure Liquid; DNA - metabolism; DNA Adducts; Drug toxicity and drugs side effects treatment; Female; Hydroxylation; Liver - metabolism; Medical sciences; Methylation; Oxidation-Reduction; PEI; Pharmacology. Drug treatments; Phosphorus Radioisotopes; polyethyleneimine; Rats; Rats, Inbred F344; tamoxifen; Tamoxifen - pharmacokinetics; Toxicity: digestive system
• ISSN: 0143-3334; 1460-2180
• DOI: 10.1093/carcin/20.10.2003

Previous work has shown that a major route of activation of tamoxifen to DNA-binding products in rat liver cells is via α-hydroxylation leading to modification of the N2-position of guanine in DNA and to a lesser extent the N6-position of adenine. Improved resolution by HPLC has now identified two major adducts in rat liver DNA, one of them the aforementioned tamoxifen–N2-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 α-hydroxytamoxifen gave rise to both. Furthermore, N,N-didesmethyltamoxifen was found to be responsible for an additional minor DNA adduct formed by tamoxifen, α-hydroxytamoxifen and N-desmethyltamoxifen. The involvement of metabolism at the α position was confirmed in experiments in which [α-D2-ethyl]tamoxifen, but not [β-D3-ethyl]tamoxifen, produced reduced levels of DNA adducts. Tamoxifen N-oxide and α-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 α-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 α-hydroxylation and N-demethylation, which is followed by Phase II activation at the α-position to form a highly reactive sulphate. Detection of tamoxifen-related DNA adducts by 32P-postlabelling is achieved with >90% labelling efficiency.