Pharmacokinetic and pharmacodynamic investigations with vaginal devices releasing levonorgestrel at a constant, near zero order rate

The pharmacokinetic and pharmacodynamic effects of vaginal devices releasing levonorgestrel (L-NOG) at a constant rate (in vitro release: approximately 20 micrograms/day) were studied in a group of 20 normally menstruating women during a period of 90 days of continuous use. Peripheral blood samples were withdrawn two or three times (in the great majority of subjects, on Mondays, Wednesdays and Fridays) during a pretreatment (control) cycle, during 90 days of exposure and during the first week following the removal of the device, and the levels of L-NOG, estradiol and progesterone were analyzed by radioimmunoassay techniques. In 8 of these subjects, endometrial biopsies were also taken during days 23 to 25 of the control cycle, and then 6 and 10 weeks following the insertion of the devices. In addition, the initial absorption rate and removal half-life of L-NOG were assessed in 7 subjects using the devices for a period of 8 days only. Following insertion of the devices, the levels of L-NOG rose very rapidly, and reached the final "plateau" in some 30 minutes' time. This was followed by a limited period of "burst" with doubling the levels for a few hours, after which the levels remained stable and diminished very slowly in a linear fashion with an average decline of 23-26% during 90 days, corresponding to a daily decrease of 0.2 to 0.3 per cent. The removal half-life (first compartment) after 90 days of exposure in 19 subjects was 16.1 (13.7-18.6) hours. Sixty-nine treatment segments of 30 days were studied with frequent hormone assays; of these, 20 (or 29%) were anovulatory, 13 (19%) exhibited inadequate luteal function, and 32 (52%) had normal ovulatory-like cycles. All endometrial biopsies obtained during the pretreatment cycle were normal secretory; of the 16 biopsies obtained during treatment, 4 were suppressed, 2 proliferative, 6 secretory, and 4 could not be dated because of bleeding. An assessment of the bleeding profiles during exposure to L-NOG revealed almost a doubling of the number of days with bleeding and spotting (35% compared to 18% during the pretreatment cycle). However, significantly more frequent bleeding was found in the 20 anovulatory segments (37.3%) than in the 36 normal ovulatory-like ones (27.7%). It is concluded that differences in the frequency of bleeding and spotting with low-dose progestogens may reflect differences in the frequency of ovulation inhibition just as much as differences in the hormonal profiles of the compounds administered.