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Tamoxifen-citrate counteracts the antitumor effects of cytotoxic drugs in vitro.

Hormones and cytotoxic drugs are often combined in the treatment of patients with breast carcinoma to broaden the antitumor spectrum of the therapy. We found that, in vitro, the most commonly used endocrine agent, tamoxifen citrate, attenuates the cytotoxic potential of 5-fluorouracil (5-FU) and of doxorubicin. The effect was observed on estrogen receptor positive and on estrogen receptor negative breast tumor cells. Combinations of growth inhibitory hormones and cytotoxic drugs may therefore be counter-productive. For the treatment of hormone-independent tumors they may even be harmful since in these tumors tamoxifen exerts no independent cell kill that compensates for its modifying effect on the cytotoxicity of drugs.