Development of a radioreceptor assay for measuring an oxytocin antagonist in blood.

The purpose of the present study was to develop a radioreceptor assay to monitor the pharmacokinetics of the oxytocin antagonist, TT-235, in the blood of the pregnant rat and baboon. The receptors used for this assay were prepared from the pregnant rat uterus at delivery. The assay using blood from pregnant rats and baboons was performed on filter plates and analyzed for radioactivity in a gamma counter. The assay was sensitive to 10 pg/well with a range from 10 to 1,000 pg. The average recovery was 86%. A new radioreceptor assay was developed for the oxytocin antagonist, TT-235. TT-235 had a much longer half-life than oxytocin in the pregnant rat and baboon. This longer half-life may partially explain the prolonged in vivo tocolytic activity of TT-235 in these animals.