Comparative effect of estriol and equine conjugated estrogens on the uterus and the vagina.

Estriol has been reported to act selectively on the vagina and cervix without causing endometrial proliferations. Studies comparing this effect of estriol to that of equine conjugated estrogens after intravaginal administration have not been reported. In this study, intravaginally administered Ortho- Gynest which contains estriol, and Premarin which contains equine conjugated estrogens, were evaluated for their ability to stimulate vaginal maturation and uterine growth in rats. Approximately 15 times more conjugated estrogens than estriol was needed to induce the same degree of vaginal cornification in castrated rats. In contrast, estriol was less potent than the conjugated estrogens in causing uterine growth in immature rats after subcutaneous administration in sesame oil and in mature rats after intravaginal administration in cream preparations. In studies evaluating the vaginal irritation potential of estriol vaginal cream, the irritation exhibited by treated rabbits was found to be comparable to that of sham-treated control rabbits. Estriol was well-tolerated in an oral acute safety study in rats.