Progesterone-releasing vaginal rings for use in postpartum contraception. I. In vitro release rates of progesterone from core-loaded rings.

ABSTRACT

Vaginal rings of Dow Corning 382 Silastic polymer, having identical outside dimensions, were fabricated to contain cores of different diameters loaded with 25% w/w progesterone. Elution of rings was carried out in continuously flowing baths of isotonic saline at 37 degrees C and quantities of progesterone released in 24 h periods measured for up to 128 days. Release of the steroid was shown to be a membrane diffusion-controlled process, modified by the development of a gradually increasing zone of depletion at the core surface. Rings of a suitable core diameter were selected to give initial release of 5 mg/24 h progesterone and sterile batches of these rings, prepared for WHO-sponsored clinical studies in post-partum, lactating women, were shown to give highly consistent and reproducible rates of in vitro drug delivery. A comparison was made with the in vitro release rates of rings containing a homogeneous dispersion of progesterone.

ABSTRACT

PIP Chemists from London, England and a chemist from WHO in Geneva, Switzerland compared release rates of progesterone from vaginal rings with cores of different diameters (4, 5, 6, 6.7, and 7.24 mm). The manufacturer loaded each core with 25% w/w progesterone. The technique used to dissolve the progesterone from the silicone rubber core consisted of placing the rings in continuously flowing baths of isotonic saline at 37 degrees Celsius. The learned that a membrane diffusion controlled process, modified by the development of a gradually increasing zone of depletion at the core surface, did indeed release the progesterone. The used the UV absorption method to measure the amount of progesterone released in 24 hour periods for as much as 128 days. The vaginal ring with the 6 mm core released 3.6-5.5 mg progesterone/day in a 90 day period. The daily range of maximum and minimum values for each set of rings demonstrated good reproducibility. Progesterone release was inversely related to diffusion distance (between core surface and ring surface) for each day. Since, in their clinical trials in postpartum women, WHO wanted to use vaginal rings which initially released 5 mg progesterone/day and declines by about 0.5 mg/month under conditions of membrane limited diffusion as the depletion zone grew thicker, the study showed that the rings with a 6 mm core met the criteria. The chemists found that these rings and 4 sterile batches of these rings have highly consistent and reproducible rates in vitro drug delivery. They also compared the vaginal rings with a 6 mm core with rings with at homogeneous dispersion of progesterone throughout the polymer. The homogenous rings 1st released much progesterone then fell quickly from 10-20 mg/day during the 1st week to a gradual release of about 6 mg/day during the end of the 90 days.