Objective: To evaluate the effects of conjugated equine estrogens, medroxyprogesterone acetate (MPA), conjugated equine estrogens combined with MPA, tamoxifen, and soybean estrogens on vaginal cytology in surgically postmenopausal cynomolgus macaques (Macaca fascicularis).
Design: Randomized long-term experimental trial.
Setting: Cytologic samples were taken from animals in two long-term randomized studies of the effects of hormonal and dietary effects on atherosclerosis.
Patients: Surgically postmenopausal cynomolgus macaques.
Interventions: Conjugated equine estrogens at the doses given do not exert an estrogenic effect on the with MPA, tamoxifen, and soybean estrogens were given via the diet, at doses scaled from those given to women.
Main outcome measure: Vaginal cytologic maturation index.
Results: Conjugated equine estrogens elicited a marked maturation effect, which was antagonized partially by the addition of MPA. Tamoxifen produced a lesser estrogenic response. The cytologic pattern in animals given soybean estrogens or MPA alone did not differ from that of controls.
Conclusion: Soybean estrogens at the doses given do not exert an estrogenic effect on the vagina of macaques. Conjugated equine estrogens are potent inducers of vaginal keratinization in this model; tamoxifen has a lesser effect. Medroxyprogesterone acetate partially antagonizes the effects of conjugated equine estrogens, and has no effect when given alone. The results support the possibility that soybean estrogens may be a "tissue-selective" estrogen with minimal effects on the reproductive tract.