Novel ligands balance estrogen receptor β and α agonism for safe and effective suppression of the vasomotor response in the ovariectomized female rat model of menopause.

Abstract

Vasomotor thermo-dysregulation (hot flashes) are an often debilitating symptom of menopause. Effective treatment is achieved primarily through activation of the estrogen receptor (ER)α with estrogens but is also associated with increased risk for breast and uterine cancer. In this study, we have tested novel compounds lacking the B ring of 17-hydroxy-β-estradiol (E2) (A-CD compounds) with differing ratios of ERα:ERβ binding affinities for the ability to reduce diurnal/nocturnal tail-skin temperatures (TSTs) in the ovariectomized female rat menopausal hot flash model. Normal mammary tissue expresses the predominantly antiproliferative ERβ. Therefore, we hypothesized that a preferential ERβ agonist with fractional ERα activity would safely reduce TSTs. The A-CD compound, L17, is a preferential ERβ agonist that has a ratio of ERβ:ERα binding affinity relative to E2 of 9.3 (where ERβ:ERα for E2, 1.0). In the ovariectomized rat, daily administration of low doses (1 mg/kg) of the A-CD compound TD81 (ERα:ERβ relative affinity, 15.2) was ineffective in temperature regulation, whereas L17 showed a trend toward TST reduction. Both E2 and the A-CD compound, TD3 (ERβ:ERα relative affinity, 5.0), also reduced TSTs but had marked proliferative effects on mammary and uterine tissues. At 2 mg/kg, L17 strongly reduced TSTs even more effectively than E2 but, importantly, had only minimal effect on uterine weight and mammary tissues. Both E2- and L17-treated rats showed similar weight reduction over the treatment period. E2 is rapidly metabolized to highly reactive quinones, and we show that L17 has 2-fold greater metabolic stability than E2. Finally, L17 and E2 similarly mediated induction of c-fos expression in neurons within the rat thermoregulatory hypothalamic median preoptic nucleus. Thus, the A-CD compound, L17, may represent a safe and effective approach to the treatment of menopausal hot flashes.