Ovulation inhibition by estetrol in an in vivo model

Background Currently, the synthetic steroid ethinylestradiol is the preferred estrogen in combined oral contraceptives. The aim of the present study was to evaluate the effectiveness of the natural steroid estetrol (E₄) as an ovulation inhibitor in rats when compared to ethinylestradiol. Study design Regularly cycling female rats were treated orally twice daily for four consecutive days, starting on the day of estrus, with E₄ (0.03, 0.1, 0.3, 1.0 or 3.0 mg/kg), ethinylestradiol (0.0003, 0.001, 0.003, 0.01 or 0.03 mg/kg) or vehicle control (eight animals per group). In a second experiment conducted under the same experimental protocol, 2.0 mg/kg of E₄ was administered as a single daily dose or as a dose of 1.0 mg/kg twice daily. In both studies, the primary endpoint was the number of ovulated oocytes in the genital tract. Results Estetrol at the twice-daily dose of 0.3 mg/kg and above inhibited ovulation. This effect was statistically significant (p < 0.05). The comparator, ethinylestradiol, significantly inhibited ovulation (p < 0.05) at the highest dose (0.03 mg/kg) administered twice daily. An E₄ dose of 2.0 mg/kg administered once daily for four consecutive days inhibited ovulation in four of eight rats. In contrast, when the same dose was administered in two separate doses, that is 1.0 mg/kg twice daily, ovulation was inhibited in eight of eight rats. The ED₅₀ for the ethinylestradiol and the E₄ dose-response curves shows that ethinylestradiol is 18 times more potent than E₄. Conclusion Twice-daily administration of E₄ effectively inhibits ovulation in cycling rats. The effect is dose-dependent. The relative potency of E₄ is about 18 times less compared to that of ethinylestradiol. We conclude that, based on these data, combined with available pharmacological and clinical data on the safety and efficacy of E₄, the human fetal estrogenic steroid estetrol is a potential candidate to replace ethinylestradiol in combined oral contraceptives.