Tamoxifen forms DNA adducts in human colon after administration of a single [14C]-labeled therapeutic dose.
Tamoxifen is widely prescribed for the treatment of breast cancer and is also licensed in the United States for the prevention of this disease. However, tamoxifen therapy is associated with an increased occurrence of endometrial cancer in women, and there is also evidence that it may elevate the risk of colorectal cancer. The underlying mechanisms responsible for tamoxifen-induced carcinogenesis in women have not yet been elucidated, but much interest has focused on the role of DNA adduct formation. We investigated the propensity of tamoxifen to bind irreversibly to colorectal DNA when given to 10 women as a single [(14)C]-labeled therapeutic (20 mg) dose, approximately 18 h before undergoing colon resections. Using the sensitive technique of accelerator mass spectrometry, coupled with high-performance liquid chromatography separation of enzymatically digested DNA, a peak corresponding to authentic dG-N(2)-tamoxifen adduct was detected in samples from three patients, at levels ranging from 1 to 7 adducts/10(9) nucleotides. No [(14)C]-radiolabel associated with tamoxifen or its major metabolites was detected. The presence of detectable CYP3A4 protein in all colon samples suggests that this tissue has the potential to activate tamoxifen to alpha-hydroxytamoxifen, in addition to that occurring in the systemic circulation, and direct interaction of this metabolite with DNA could account for the binding observed. Although the level of tamoxifen-induced damage displayed a degree of interindividual variability, when present, it was approximately 10 to 100 times higher than that reported for other suspect human colon carcinogens such as 2-amino-1-methyl-6-phenyimidazo[4,5-b]pyridine. These findings provide a mechanistic basis through which tamoxifen could increase the incidence of colon cancers in women.
Brown
Karen
K
Cancer Biomarkers and Prevention Group, Reproductive Sciences, Department of Cancer Studies and Molecular Medicine, University of Leicester, Leicester, United Kingdom. kb20@le.ac.uk
Tompkins
Elaine M
EM
Boocock
David J
DJ
Martin
Elizabeth A
EA
Farmer
Peter B
PB
Turteltaub
Kenneth W
KW
Ubick
Esther
E
Hemingway
David
D
Horner-Glister
Emma
E
White
Ian N H
IN
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D018931:Antineoplastic Agents, Hormonal / Q000494:pharmacology; D002250:Carbon Radioisotopes / Q000737:chemistry; D002851:Chromatography, High Pressure Liquid; D003106:Colon / Q000473:pathology; D003110:Colonic Neoplasms / Q000188:drug therapy*; D051544:Cytochrome P-450 CYP3A; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D018736:DNA Adducts; D016889:Endometrial Neoplasms / Q000209:etiology / Q000473:pathology; D005260:Female; D006801:Humans; D008875:Middle Aged; D013629:Tamoxifen / Q000008:administration & dosage*
D018931:Antineoplastic Agents, Hormonal; D002250:Carbon Radioisotopes; D018736:DNA Adducts; D013629:Tamoxifen; D003577:Cytochrome P-450 Enzyme System; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human