Changes in the hormone dependency of DMBA-induced rat mammary tumors with reference to the effect of tamoxifen.

Tamoxifen was administered over a period of 4 weeks to 19 female Sprague-Dawley rats bearing 20 DMBA-induced mammary tumors. Consequently, 13 tumors (65.0 per cent) regressed and showed a reduction in their estradiol receptor (ER) and progesterone receptor (PgR) concentrations. In spite of subsequent treatment given to the rats bearing the regressed tumors, 6 new tumors occurred. The ER and PgR concentrations of these tumors were lower than those of the tumors before tamoxifen therapy, and histochemical study showed these tumors to be composed mostly of ER-negative cells. These results suggest that: (a) the hormone dependency of breast cancer is reduced by tamoxifen therapy; (b) breast cancers recurring during tamoxifen therapy show reduced hormone dependency.