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Tamoxifen (Nolvadex) is commonly prescribed for women with breast cancer. The National Cancer Institute has launched a study to give this synthetic hormone to thousands of symptom-free women who have a family history of the disease, to see if it can prevent the illness.
But new data from the drugıs manufacturer, Zeneca Pharmaceuticals (the new name for ICI Pharmaıs American division), has thrown serious doubt on the wisdom of that much-publicized NCI study. For, after two yearsı research, Zeneca scientists have shown that tamoxifen causes liver cancer in rats. It does so, said Dr. John Topham, a company toxicologist, not just when given in huge amounts, but at doses that overlap those found in some women taking the drug.
News of these disturbing findings began to circulate as early as 1986, Joachim G. Liehr, a University of Texas chemist told Science News (9/18/93). Liehr himself has shown that tamoxifen might pose a cancer risk. However, the Zeneca article, which appeared in Cancer Research this fall, is the first publication of the study's conclusions. These new data, says Science News, have "spawned considerable controversy over whether the federal government's cancer prevention trial might actually jeopardize some womenıs overall health."
Rats receiving just a tiny amount of the drug developed 20 to 34 times more liver tumors, many of these highly invasive. Even higher rates of such cancer were seen in rodents receiving the highest dose levels, with similarly elevated death rates. The manufacturer's own scientist concluded: "Tamoxifen must be regarded as a hepatic [i.e., liver] carcinogen in rats."
An NCI spokesperson, Susan G. Nayfield, said, "We are concerned about this" but claimed that such liver cancers are probably just "species-specific," i.e., have no relevance to the human situation. Nevertheless, she added that NCI is "very carefully" monitoring tamoxifen patients for liver problems, especially those in their 'chemoprevention' trial.
According to scientists at the American Health Foundation of Valhalla, New York, however, tamoxifen generates adducts--changes in DNA that are thought necessary for a tumor to begin. Tamoxifen's ability to alter DNA in this way "suggests that for adducts, there is no species specificity." Other studies have shown this potent drug to increase human endometrial cancer, as well as to cause circulatory and eye problems.