Long-term tamoxifen therapy to control or to prevent breast cancer: laboratory concept to clinical trials.

The successful evaluation of tamoxifen as an antiestrogenic therapy for advanced breast cancer in the early 1970's, has resulted in its availability in more than 70 countries around the world. Currently the drug development process is focusing attention upon long-term adjuvant therapy and the future prospect of chemosuppression. Progress at this stage, however, must be cautious. Trials conducted using women with stage I disease have a high proportion of women who may never have a recurrence. At this point, the risk is justified because the toxicity of tamoxifen is low and disease recurrence is invariably very difficult or impossible to control. Future studies in the general population must be carefully weighed to ensure that the hazards do not exceed the benefits. The pharmacology of tamoxifen seems to be a balance of estrogenic and antiestrogenic effects. Longer treatments with the drug must be carefully monitored. Uterine tissue should be examined to ensure that excessive stimulation does not occur. This is particularly true in the light of the recent report that a human uterine carcinoma, transplanted into athymic mice, can grow more rapidly during tamoxifen therapy (Satyaswaroop et al., 1984; Clark and Satyaswaroop, 1985). In fact, we have recently confirmed this observation in a collaborative study with Dr. Satyaswaroop. We have demonstrated that when athymic mice are transplanted in one axilla with an MCF-7 breast tumor and the human endometrial tumor in the other, tamoxifen causes the endometrial tumor to grow, but not the breast tumor. This again illustrates the target site specific effects of tamoxifen. If, in the long run, the estrogenic side effects of tamoxifen are too severe then there is a case for the development of a non-estrogenic antiestrogen. Clearly this may provide benefit for short term (1-2 years) therapy and avoid any estrogen-like stimulation of tumor growth. Similarly, the concern about antithrombin III depression will be avoided. On the negative side, however, the concerns about atherosclerosis and osteoporosis will again have to be addressed with a new generation of agents.