Further characterization of the DNA adducts formed in rat liver after the administration of tamoxifen, N-desmethyltamoxifen or N,N-didesmethyltamoxifen

• Published in: Carcinogenesis (New York) 1999-10, Vol.20 (10), p.2011-2016
• Main Authors: Brown, Karen, Heydon, Robert T., Jukes, Rebekah, White, Ian N.H., Martin, Elizabeth A.
• Format: Article
• Language: English
• Subjects: a-Acetoxytamoxifen; Animals; Biological and medical sciences; Cattle; Chromatography, High Pressure Liquid; Deoxyguanine Nucleotides - metabolism; DNA Adducts - metabolism; Drug toxicity and drugs side effects treatment; Female; Liver - drug effects; Liver - metabolism; Mass Spectrometry; Medical sciences; N,N-Didesmethyltamoxifen; N-Desmethyltamoxifen; Pharmacology. Drug treatments; Phosphorus Radioisotopes; Rats; Rats, Inbred F344; tamoxifen; Tamoxifen - administration & dosage; Tamoxifen - analogs & derivatives; Tamoxifen - pharmacology; Toxicity: digestive system
• ISSN: 0143-3334; 1460-2180
• DOI: 10.1093/carcin/20.10.2011

The present study compares the formation of DNA adducts, determined by 32P-postlabelling, in the livers of rats given tamoxifen and the N-demethylated metabolites N-desmethyltamoxifen and N,N-didesmethyltamoxifen. Results show that after 4 days treatment (0.11 mmol/kg i.p.), similar levels of DNA damage were seen after treatment with either tamoxifen or N-desmethyltamoxifen [109 ± 40 (n = 3) and 100 ± 33 (n = 4) adducts/108 nucleotides, respectively], even though the concentration of tamoxifen in the livers of tamoxifen-treated rats was about half that of N-desmethyltamoxifen in the N-desmethyltamoxifen-treated animals (51 ± 16 and 100 ± 8 nmol/g, respectively). Administration of N,N-didesmethyltamoxifen to rats resulted in a 5-fold lower level of damage (19 adducts/108 nucleotides, n = 2). Following 32P-postlabelling and HPLC, hepatic DNA from rats treated with tamoxifen and its metabolites showed distinctive patterns of adducts. Treatment of rats with N,N-didesmethyltamoxifen gave a major product that co-eluted with one of the minor adduct peaks seen in the livers of rats given tamoxifen. Following dosing with N-desmethyltamoxifen, the major product co-eluted with one of the main peaks seen following treatment of rats with tamoxifen. This suggests that tamoxifen can be metabolically converted to N-desmethyltamoxifen prior to activation. However, analysis of the 32P-postlabelled products from the reaction between α-acetoxytamoxifen and calf thymus DNA showed two main peaks, the smaller one of which (~15% of the total) also co-eluted with that attributed to N-desmethyltamoxifen. This indicates that N-desmethyltamoxifen and N,N-didesmethyltamoxifen are activated in a similar manner to tamoxifen leading to a complex mixture of adducts. Since an HPLC system does not exist that can fully separate all these 32P-postlabelled adducts, care has to be taken when interpreting results and determining the relative importance of individual adducts and the metabolites they are derived from in the carcinogenic process.