Continuous dosing of a novel contraceptive vaginal ring releasing Nestorone® and estradiol: pharmacokinetics from a dose-finding study

Background: As part of a program to develop a novel estradiol-releasing contraceptive vaginal ring (CVR), we evaluated the pharmacokinetic (PK) profile of CVRs releasing segesterone acetate (Nestorone® (NES)) combined with one of three different estradiol (E₂) doses.

Study design: A prospective, double-blind, randomized, multi-centered study to evaluate a 90-day CVR releasing NES [200mcg/day] plus E₂, either 10mcg/day, 20mcg/day, or 40mcg/day in healthy reproductive-age women with regular cycles. Participants provided blood samples twice weekly for NES and E₂ levels during the first 60 days (ring 1) and the last 30 days (ring 2) of use. A subset underwent formal PK assessments at ring initiation, ring exchange (limited PK), and study completion.

Results: The main study enrolled 197 women; 22 participated in the PK substudy. Baseline characteristics between the main and PK participants were comparable, with an average BMI of 25.8 kg/m² (SD 4.3). In the PK substudy, all three rings showed similar NES PK: mean area under the curve (AUC_(0-72)) 34,181 pg*day/mL; concentration maximum (C_max) 918 pg/mL; time to maximum concentration (T_max) 3.5 h. For E_2, the C_max occurred at 2 h, and was significantly higher with the 20 mcg/day ring (mean 390 pg/mL); 10 mcg/day, 189 pg/mL, p=.003; 40 mcg/day, 189 pg/mL, p<.001), and declined rapidly to≤50 pg/mL for all doses by 24 h. For all subjects, the median E₂ levels remained under 35 pg/mL during treatment.

Conclusion: PK parameters of NES were not affected when paired with different doses of E₂, but E₂ levels from all three doses were lower than anticipated and no dose response was observed.

Implications: While these novel estradiol-releasing combination contraceptive vaginal rings provided sustained release of contraceptive levels of Nestorone over 90 days, the E₂ levels achieved were not consistent with bone protection, and a dose-response was not observed.