Tamoxifen tolerance and steroid receptors in a tumor model.

In a rat mammary adenocarcinoma model, prolonged exposure to the antiestrogen Tamoxifen results in a Tamoxifen-tolerant tumor cell line which is readily transplantable and grows under continuous oral intake of the drug. The solid tumor contains steroid receptors; however, the level of Tamoxifen needed for effective displacement of diethylstilbestrol cannot be achieved with therapeutical doses; resistance to Tamoxifen is not the result of diminished or absent estrogen receptors.