Studies of Tamoxifen/Toremifene DNA interactions in monkeys

Institution: University of California, Davis
Investigator(s): Uta  Hellmann-Blumberg , Ph.D. -
Award Cycle: 1996 (Cycle II) Grant #: 2FB-0223 Award: $75,000
Award Type: Postdoctoral Fellowship
Research Priorities
Pathogenesis>Mistakes on the master blueprint: molecular genetics and gene regulations



Initial Award Abstract (1996)
The antiestrogen tamoxifen has been the drug of choice since the early 1980’s for the treatment of women with estrogen-receptor (ER) positive breast cancer and is sometimes hailed as a "wonder drug’. While it does prolong disease-free survival for several years after initial surgery, uncertainty exists about the optimum length of tamoxifen treatment, since it can actually cause cancer of the endometrium (lining of the uterus). Tamoxifen has also been shown to cause liver cancer in laboratory animals (rats). Experiments on rats demonstrate that tamoxifen binds to DNA and forms so-called adducts. DNA adducts are modifications of the DNA that lead to mutations and, if sufficient mutations accumulate, to cancer. An alternative drug, toremifene, also prevents the recurrence of breast cancer but does not form DNA adducts in rats. Conclusive data in humans, however, are not available at this point because of the difficulty of obtaining data on tamoxifen and toremifene DNA adducts relevant to human exposure. Because rats differ in many aspects from humans, a study will be initiated to test the effects of tamoxifen and toremifene treatment in monkeys. This data will be much more relevant to effects of these drugs on humans.

We propose to determine whether and how much DNA adducts are formed in liver, endometrium and spleen of the monkeys at different time points during treatment with either drug. High levels of DNA adducts will show that the treatment is likely to cause new tumors. The long-term goal of this study is to obtain data that lead to new treatments for breast cancer.


Final Report (1998)
This project addressed the question whether the antiestrogenic breast cancer drug tamoxifen can cause other cancers such as endometrial cancer during long-term therapy or if used for chemoprevention and whether the similar drug toremifene is safer. It is based on the hypothesis that some of the tamoxifen metabolites produced in the human body can bind to DNA and damage critical genes. By testing the main human metabolites of tamoxifen in a test tube, we identified one metabolite that does bind to DNA. The corresponding metabolite of toremifene which, in contrast to tamoxifen, has not been linked to endometrial cancer in breast cancer patients, did not bind to DNA under the same conditions. However, preliminary experiments indicate that the reactivity of these drugs is altered when other drugs are present. This is an important finding that we plan to pursue in future studies - whether other drugs or environmental carcinogens given simultaneously with tamoxifen or toremifene increase the ability of tamoxifen and toremifene to bind to DNA and cause cancer.

We also tested samples of liver and endometrial tissue from monkeys treated with either tamoxifen or toremifene for evidence of DNA binding. We did not detect any DNA binding except in one liver sample of a monkey treated with tamoxifen. However, the evidence was weak and the liver samples from the monkeys were generally variable in quality and quantity. While monkeys are the best model for testing drugs because of their similarity to humans, they are very expensive and subject to strict guidelines regarding the sampling of tissues and other invasive procedures. When we realized that substantial DNA damage would occur only during long term treatment and possibly in the presence of other factors, we tried to find different model system. Rats are not suitable, because they readily develop liver cancer when treated with tamoxifen. This is presumably due to DNA damage caused by a metabolite that is not important in humans. However, reports from another laboratory showing that several tamoxifen metabolites cause DNA damage in mice lead us to believe that one of them is the human metabolite we found to bind to DNA. We are currently testing this hypothesis.

In summary, we conclude that tamoxifen and possibly toremifene can cause DNA damage during long term treatment (chemoprevention), but without additional risk factors toremifene appears to be safer.


Symposium Abstract (2005)
S. S. Matkar1), V.J. Kozina2), L.A. Wrischnik2) and U. Hellmann-Blumberg1)

1) Department of Chemistry and 2) Department of Biology, University of the Pacific, 3601 Pacific Avenue, Stockton, CA 95211

Cisplatin is one of several therapeutic drugs used in the treatment of advanced (metastatic) breast cancer. It is usually given in combination with other drugs which enhance its efficacy. Benzo[a]pyrene (BaP) is a ubiquitous environmental pollutant produced during combustion of organic matter. It is also present in significant amounts in cigarette smoke and charbroiled food. Exposure to elevated levels of BaP has been linked to lung cancer and may contribute to other cancers. The carcinogenic effects of BaP are ascribed primarily to the binding of metabolites to DNA. Likewise, the anticancer drug cisplatin is known to bind to DNA; however, DNA binding alone is not sufficient to kill tumor cells. Thus, cisplatin is usually given with various other drugs. Among the compounds that have been investigated for modulation of cisplatin response are plant chemicals such as caffeine (a methylxanthine) and genistein (a flavonoid). Surprisingly, no studies on modulation of cisplatin efficacy by BaP are available. We found that BaP alone can kill human MCF-7 breast cancer cells that are treated for one week with 1 microM BaP.

Information obtained from investigating the synergistic effects of cisplatin and BAP in human breast cancer cells is expected to provide answers to two questions (a) whether exposure to elevated levels of BaP (from first- or secondhand smoke, for example) is likely to affect cisplatin treatment of metastatic breast cancer and (b) whether compounds such as BaP are useful in anticancer treatment protocols that contain cisplatin or other combinations.
Intrinsic reactivity of tamozifen and Toremifene metabolites with DNA
Periodical:Breast Cancer Research and Treatment
Index Medicus: Breast Cancer Res Treat
Authors: Hellmann-Blumber U, Carter MG, Wauz GT, and DeGregorio MW
Yr: 1998 Vol: 50 Nbr: 2 Abs: Pg:135-141
Tamoxifen metabolite Bx causes DNA adducts
Periodical:Breast Cancer Research and Treatment
Index Medicus: Breast Cancer Res Treat
Authors: Hellmann-Blumberg U, and DeGregorio MW
Yr: 1997 Vol: 46 Nbr: Abs: Pg:85