Induction of tamoxifen-dependent rat mammary tumors.

We have observed previously that animals given tamoxifen (TAM) and the carcinogen dimethylbenzanthracene develop exclusively hormone-independent tumors. Since our data implied that the TAM-associated tumors were different from control tumors, we undertook studies to examine the role of TAM in the induction and growth of these tumors. Following cessation of TAM administration, almost one-third [29.0 +/- 1.7% (SEM)] of the tumors regressed and more tumors appeared. Resumption of TAM administration resulted in regrowth of some tumors and regression of the new tumors. These studies demonstrate that some of the TAM-associated tumors are actually dependent upon TAM for growth, while the appearance of new tumors suggests that TAM does not totally prevent tumor formation but may only delay it.