Pharmacokinetic observations on ST-1435 administered subcutaneously and intravaginally

Plasma levels of ST-1435 were determined by radioimmunoassay in 36 women and three men during treatment with subdermal capsules releasing ST-1435. The total lengths of the capsules used were 180 mm, 90 mm, 30 mm, 15 mm and 7.5 mm. A linear correlation between the mean plasma concentrations of ST-1435 and capsule length was found during the treatment period studied. However, great interindividual variation in plasma levels of ST-1435 between study subjects having similar capsules was observed. In comparison with the mean plasma concentrations in the initial months, only less than half of these plasma concentrations of ST-1435 were measured after six months of treatment, which indicates a short lifetime for this contraceptive system. In two women the disappearance of ST-1435 from the circulation was studied by measuring ST-1435 concentrations in the plasma after removal of a contraceptive vaginal ring (CVR) releasing ST-1435. Triphasic disappearance curves of ST-1435 were found, with half-lives 0.7-1.0 h, 3.1-4.0 h, and 10.7.3-11.8 h.