Estrogen and progesterone receptors in the endometrium of postmenopausal breast cancer patients treated with tamoxifen and progestogens.

Postmenopausal breast cancer patients who were treated with tamoxifen and progestogens showed a uniform decidual reaction of the endometrium. It is well established that progestogens antagonize the estrogen effect on the endometrium by reducing its receptors in the endometrium. To assess in vivo such a possible effect of progestogens on endometrium primarily exposed to tamoxifen, we analyzed estrogen and progesterone receptors (ER, PR) on endometrial specimens showing decidualization from nine postmenopausal breast cancer patients on tamoxifen and progestogen treatment and on endometrial polyps with areas of decidualization from two other similar patients. ER was weakly detected in the endometrial glands of four (36.4%) patients and in the endometrial stroma of one (9.1%) patient. PR was detected in the endometrial gland of only one (9.1%) patient. No PR was detected in the endometrial stroma. There was no correlation between the length of tamoxifen treatment, the tamoxifen dosage, or the length of progestogen treatment and the ER or PR content, although progestogens were administered for more than 3 consecutive months in all patients. This relatively very low ER and PR content may be attributed to the antagonistic effect of progestogens on the "priming" estrogen-like effect of tamoxifen on the endometrium.