Pharmacokinetics and Tolerability of a Novel 17β-Estradiol and Progesterone Intravaginal Ring in Sheep

This study reports the preparation, in vitro release, pharmacokinetics, and local tolerability of novel ethylene-vinyl acetate intravaginal rings (IVRs) delivering 17β-estradiol (E₂) and progesterone (P), in drug-naïve ovariectomized female Dorset crossbred sheep. After preparation and assessment of in vitro release of E₂ and P, animals were randomized to treatment groups 1 or 2 (comparator rings releasing 50 or 100 μg/d E₂, respectively), groups 3 or 4 (ethylene-vinyl acetate IVRs, 160 μg/d E₂ with 4 [160/4 IVR] or 8 mg/d P [160/8 IVR], respectively), or group 5 (160 μg E₂ and 10 mg P administered intravenously). IVRs were placed on day 1 and remained in place through day 29. Animals underwent daily examinations to confirm ring placement, and vaginal irritation was scored from 0 (none) to 4 (severe). Blood samples were taken at scheduled times for pharmacokinetic analysis. Postmortem examinations performed on groups 1-4 were macroscopic and microscopic evaluations, including irritation scoring and histopathology. IVRs were retained over 28 days in all but 1 animal (group 4). In all animal groups, clinical observations showed no significant abnormal findings. Pharmacokinetic analysis in the animals showed sustained release of E₂ and P over a 28-day period. Irritation scores and microscopic assessments were consistent with foreign object placement. A novel 2-drug IVR delivery system was well tolerated in a sheep model and pharmacokinetic release was as expected over a 28-day release period. These results will guide future human clinical studies.