Endometrial effects of bazedoxifene acetate, a novel selective estrogen receptor modulator, in postmenopausal women.

OBJECTIVE: To assess the endometrial effects of bazedoxifene acetate in healthy postmenopausal women. METHODS: The endometrial effects of bazedoxifene 2.5, 5.0, 10, 20, 30, and 40 mg/d were evaluated in a 2-part, 6-month, double-blind, randomized, active- and placebo-controlled study among a total of 497 healthy postmenopausal women. Conjugated estrogens (0.625 mg)/medroxy-progesterone acetate (2.5 mg) served as the active control. Patients underwent transvaginal ultrasonography to measure double-wall endometrial thickness and endometrial biopsy at baseline and at 6 months of treatment. The incidence of amenorrhea was assessed from self-reported daily diaries. RESULTS: Bazedoxifene treatment at 2.5-20 mg/d resulted in mean changes from baseline in endometrial thickness that were no different than those seen with placebo treatment. Changes in endometrial thickness for the bazedoxifene 30 and 40 mg groups were significantly smaller than for placebo. The change from baseline in endometrial thickness was significantly and inversely related to dose (P ≤.001). None of the endometrial biopsy specimens demonstrated endometrial hyperplasia. Subjects in the 2.5-20 mg bazedoxifene groups experienced amenorrhea rates of 57-74%, comparable with the 59% seen in placebo. Over 90% of subjects treated with bazedoxifene 30 or 40 mg/d were amenorrheic at 6 months. CONCLUSION: Bazedoxifene at dosages up to 40 mg/d was well tolerated and did not stimulate the endometrium. The significant decreases in endometrial thickness and decreased uterine bleeding observed with doses of 30 and 40 mg/d as compared with placebo suggest endometrial antagonism, representing a novel characteristic not previously associated with any selective estrogen receptor modulator.