Direct determination of endothelin receptor antagonist levels in plasma using a scintillation proximity assay

An assay using scintillation proximity bead technology has been developed s uitable for the quantitation of endothelin (ET) receptor antagonists in pre clinical and clinical samples of plasma. The assay measures the competitive inhibition of radiolabelled ET-1 binding to ETA receptor membranes bound t o wheat germ agglutinin (WGA)-coated scintillation proximity assay (SPA) be ads in the presence of plasma containing A-127722, a potent orally active, ETA selective ET antagonist. The assay requires as little as 50 mu l plasma and no extraction procedure is needed. The SPA methodology eliminates the need for the separation of bound from free ligand. Using this method, A-127 722 could be directly quantified in rat plasma with a detection limit of 1 ng/ml.