Comparison of binding affinity of oxytocin antagonists to human and rat uterine oxytocin receptors and their correlation to the rat uterine oxytocic bioassay.

One of the primary methods used to screen the development of oxytocin antagonists (OTAs) is the rat oxytocic bioassay. The purpose of this study was to determine whether the rat oxytocic bioassay is a good predictor of binding affinity to human and rat uterine oxytocin receptors (OTr). The binding affinities of five OTAs to human and rat uterine OTr were determined and correlated with pA2 values derived from the rat uterine oxytocic bioassay. Human uterine myometrial tissue was obtained from patients at the time of cesarean section. Rat uterine tissue for the OTr assay was taken at Day 21 of pregnancy. OTr assays were accomplished by isolating uterine cell membranes and performing saturation analysis with cold OTAs and tritium-labeled oxytocin. The association constants (Ka; 10(+8)/M) were calculated by nonlinear curve-fitting techniques. The Ka for the five OTAs (Mpa1-OT, Antag I, L366948, Antag II, and Antag III), as estimated from the human OTr assay, were 0.55, 0.60, 2.27, 1.91, and 47.20, respectively. Ka estimates obtained through use of rat uterine membranes were 0.51, 1.16, 5.89, 2.03, and 24.40, respectively. Correlation of the log10 of the rat oxytocic bioassay results with those of the rat and human OTr assay was 0.94 and 0.98, respectively (p < 0.01). Antag III was approximately 55, 48, and 90 times more potent than Mpa1-OT as determined by the rat bioassay and rat and human uterine OTr assays, respectively. Mpa1-OT is an OTA that is currently undergoing clinical evaluation.(ABSTRACT TRUNCATED AT 250 WORDS)