Further characterization of the DNA adducts formed in rat liver after the administration of tamoxifen, N-desmethyltamoxifen or N, N-didesmethyltamoxifen.
The present study compares the formation of DNA adducts, determined by (32)P-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/10(8) 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/10(8) nucleotides, n = 2). Following (32)P-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 (32)P-postlabelled products from the reaction between alpha-acetoxytamoxifen and calf thymus DNA showed two main peaks, the smaller one of which ( approximately 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 (32)P-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.
Brown
K
K
MRC Toxicology Unit, Hodgkin Building, Lancaster Road, Leicester LE1 9HN, UK.
Heydon
R T
RT
Jukes
R
R
White
I N
IN
Martin
E A
EA
D000818:Animals; D002417:Cattle; D002851:Chromatography, High Pressure Liquid; D018736:DNA Adducts / Q000378:metabolism*; D003848:Deoxyguanine Nucleotides / Q000378:metabolism; D005260:Female; D008099:Liver / Q000187:drug effects* / Q000378:metabolism; D013058:Mass Spectrometry; D010761:Phosphorus Radioisotopes; D051381:Rats; D011916:Rats, Inbred F344; D013629:Tamoxifen / Q000008:administration & dosage / Q000031:analogs & derivatives* / Q000494:pharmacology*
D018736:DNA Adducts; D003848:Deoxyguanine Nucleotides; C122072:N,N-didesmethyltamoxifen; D010761:Phosphorus Radioisotopes; D013629:Tamoxifen; C034009:2'-deoxyguanosine 3'-phosphate; C028787:N-desmethyltamoxifen