Treatment with bazedoxifene, a selective estrogen receptor modulator, causes regression of endometriosis in a mouse model.

Endometriosis is a common estrogen-dependent disorder. Medical treatments currently consist of progestins or GnRH agonists; however, neither is fully effective and both entail significant side effects. Selective estrogen receptor (ER) modulators (SERM) have tissue-selective actions, acting as an ER agonist in some tissues and ER antagonist in others. The SERM bazedoxifene (BZA) effectively antagonizes estrogen-induced uterine endometrial stimulation without countering estrogenic effects in bone or central nervous system. These properties make it an attractive candidate for use in the treatment of endometriosis. Experimental endometriosis was created in reproductive-age CD-1 mice. After 8 wk, 10 animals received i.p. injections of BZA (3 mg/kg·d) for 8 wk, whereas 10 received vehicle control. Mice were killed, and implant size was assessed. The mean size of the implants after treatment was 60 mm(2) in the control group and 21 mm(2) in the BZA treatment group (P = 0.03). Quantitative PCR and immunohistochemical analysis were used to determine the effect on endometrial gene expression. PCNA, ERα, and LIF mRNA and protein expression were significantly decreased in endometrium of the treated group. Caspase 3 mRNA expression was increased. Expression of PR and Hoxa10 were not significantly altered by treatment. There was no evidence of ovarian enlargement or cyst formation. Decreased PCNA and ER expression demonstrated that the regression of endometriosis likely involved decreased estrogen-mediated cell proliferation. BZA may be an effective novel agent for the treatment of endometriosis due to greater endometrial-specific estrogen antagonism compared with other SERM.