EFFECTS OF TAMOXIFEN ON ESTROGEN AND PROGESTERONE RECEPTORS IN HUMAN-BREAST CANCER

Abstract

Patients (20) with primary breast cancer were treated with tamoxifen (10 mg per os twice a day) for 1-4 wk. Before and after the tamoxifen administration, tumor specimens were obtained and assayed for estrogen receptors and progesterone receptors (PGR). Total cytosol estrogen receptor (ERC) and occupied nuclear estrogen receptor (ERN) were measured by hydroxylapatite assay; unoccupied PGR was measured by the dextran-coated characoal assay. ERC, ERN and PGR were detectable in 11, 8 and 6 tumors, respectively, before tamoxifen administration. After tamoxifen treatment, ERC decreased in 10 of 11 ERC-positive tumors. Occupied ERN increased in 3 of 5 ERN-positive tumors treated with tamoxifen for a short period (1-2 wk); they decreased in all 3 ERN-positive tumors after longer administration (3-4 wk). PGR increased in 3 of 5 ERN-positive tumors after short-term tamoxifen treatment, but they decreased in all 3 tumors treated by the drug for a longer period. Increased PGR responses were accompanied by an increase of ERN in 2 of 3 ERN-positive tumors. Tamoxifen probably interacts with the estrogen receptor system in human breast cancer tissue and may be estrogenic during short treatment; longer treatment probably results in an antiestrogenic response.