The DNA-damaging potential of tamoxifen in breast cancer and normal cells

Arch Toxicol. 2007 Jul;81(7):519-27. doi: 10.1007/s00204-007-0188-3. Epub 2007 Feb 16.


Tamoxifen (TAM) is a non-steroidal anti-estrogen used widely in the treatment and chemoprevention of breast cancer. TAM treatment can lead to DNA damage, but the mechanism of this process is not fully understood and the experimental data are often inconclusive. We compared the DNA-damaging potential of TAM in normal human peripheral blood lymphocytes and MCF-7 breast cancer cells by using the comet assay. In order to assess whether oxidative DNA damage may contribute to TAM-induced lesions, we employed two DNA repair enzymes: endonuclease III (Endo III) and formamidopyrimidine-DNA glycosylase (Fpg). The kinetics of repair of DNA damage was also measured. In order to evaluate the involvement of free radicals in the genotoxicity of TAM we pre-treated the cells with nitrone spin traps: DMPO and POBN. The use of common antioxidants: vitamin C, amifostine and genistein, helped to assess the contribution of free radicals. TAM damaged DNA in both normal and cancer cells, inducing mainly DNA strand breaks but not alkali-labile sites. The drug at 5 and 10 microM induced DNA double strand breaks (DSBs) in lymphocytes and at 10 microM in MCF-7 cells. We observed complete repair of DSBs in cancer cells by contrast with incomplete repair of these lesions in lymphocytes. In both types of cells TAM induced oxidized purines and pyrimidines. Incubation of the cells with nitrone spin traps and antioxidants decreased, with exception of amifostine in MCF-7 cells, the extents of DNA damage in both kinds of cells, but the results were more distinct in cancer cells. Our results indicate that TAM can be genotoxic for normal and cancer cells by free radicals generation. It seems to have a higher genotoxic potential for normal cells, which can be the result of incomplete repair of DNA DSBs. Free radicals scavengers can modulate TAM-induced DNA damage interfering with its antitumour activity in cancer cells.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Amifostine / pharmacology
  • Ascorbic Acid / pharmacology
  • Breast Neoplasms / drug therapy*
  • Breast Neoplasms / genetics
  • Breast Neoplasms / metabolism
  • Cell Line, Tumor
  • Cell Survival / drug effects
  • Comet Assay / methods
  • DNA / drug effects*
  • DNA / metabolism
  • DNA Breaks, Double-Stranded
  • DNA Breaks, Single-Stranded
  • DNA Damage*
  • DNA Repair / drug effects
  • DNA-Formamidopyrimidine Glycosylase / metabolism
  • Dose-Response Relationship, Drug
  • Endodeoxyribonucleases / metabolism
  • Estrogen Receptor Modulators / toxicity*
  • Female
  • Free Radical Scavengers / pharmacology
  • Genistein / pharmacology
  • Humans
  • Hydrogen-Ion Concentration
  • Kinetics
  • Lymphocytes / drug effects*
  • Lymphocytes / metabolism
  • Male
  • Mutagens / toxicity*
  • Oxidative Stress / drug effects*
  • Reactive Oxygen Species / metabolism
  • Tamoxifen / toxicity*


  • Estrogen Receptor Modulators
  • Free Radical Scavengers
  • Mutagens
  • Reactive Oxygen Species
  • Tamoxifen
  • DNA
  • Genistein
  • Endodeoxyribonucleases
  • DNA-Formamidopyrimidine Glycosylase
  • Amifostine
  • Ascorbic Acid