Increase of progesterone receptor by tamoxifen as a hormonal challenge test in breast cancer.

In 25 cases of postmenopausal breast cancer, estradiol receptor (ER) and progesterone receptor (PR) were measured in cutaneous metastatic nodules before and after administration of 30 mg of tamoxifen per day for 1 week. No response was recorded in ER-poor cases. However, in tumors containing greater than 10 fmol ER per mg cytosol protein, 6 of 14 cases showed an increase in PR of greater than 30 fmol/mg cytosol protein. The presence or absence of PR before administration of tamoxifen did not discriminate systematically between hormone-responsive and nonresponsive tumors. These findings demonstrate in vivo that biochemical changes brought about by an agent binding to ER can be observed only in ER-positive cases. In addition they suggest that, in these ER-positive cases responding to tamoxifen by increase of PR, the simultaneous or sequential administration of both antiestrogen (rescuing PR) and progestagen (decreasing PR) may allow better hormonal control of the disease.