Comparative effects of tibolone and conjugated equine estrogens with and without medroxyprogesterone acetate on the reproductive tract of female cynomolgus monkeys.

OBJECTIVE: To measure the effects of 2 years' treatment with tibolone on the reproductive tract of female monkeys (Macaca fascicularis) in comparison with conventional hormone replacement therapy.
DESIGN: Ovariectomized adult female monkeys were randomized for 2 years of treatment into five groups: controls (n = 31); tibolone at 0.05 mg/kg (LoTIB group; n = 30); tibolone at 0.2 mg/kg (HiTIB group; n = 31); conjugated equine estrogens (CEE) at 0.042 mg/kg (CEE group; n = 28); or CEE + medroxyprogesterone acetate (MPA) at 0.167 mg/kg (CEE + MPA group; n = 29). Endpoints included vaginal cytology; uterine weight; histopathologic evaluation of the uterus, vagina, and cervix; histomorphometry of the endometrium; and immunohistochemical detection of the proliferation marker Ki67 and progesterone receptor in endometrial tissue.
RESULTS: Endometrial atrophy was found in 29 of 30 and 23 of 31 animals in the LoTIB and HiTIB groups, respectively, compared with 0 of 28 and 11 of 29 in the CEE and CEE + MPA groups, respectively. All ovariectomized control animals had atrophic endometria. No complex or atypical hyperplasia was seen. Simple endometrial hyperplasia of a significant degree was seen in 3 of 31 HiTIB-treated animals, 1 of 30 LoTIB-treated animals, 26 of 28 CEE-treated animals, and 16 of 29 CEE + MPA-treated animals, and in none of the control animals. Marked simple endometrial hyperplasia and Ki-67 expression was induced by CEE and partially antagonized by MPA. LoTIB and HiTIB slightly increased endometrial thickness, whereas CEE and CEE + MPA induced a marked increase of 350% and 200%, respectively. Neither LoTIB nor HiTIB increased endometrial proliferation (Ki67 labeling) or induced vaginal keratinization. Endometrial bleeding was not seen in tibolone-treated animals but was present in 10 of 29 animals given CEE + MPA.
CONCLUSIONS: The effect of tibolone on the uterus and lower reproductive tract was minimal. The lack of a proliferative response of the endometrium to tibolone, coupled with the lower incidence of endometrial bleeding, suggests that tibolone may have advantages over CEE and CEE + MPA regarding endometrial safety and efficacy.