Development of a radioreceptor assay for the D2-selective dopamine agonist N-0437.

N-0437 is a recently developed dopamine (D2) agonist, theoretically attractive in the therapy of Parkinson's disease and glaucoma. Since its high potency allows small doses of the compound in clinical use and as extensive metabolism occurs in animals, a highly sensitive assay method was required for drug-monitoring purposes. To this end we developed a radioreceptor assay (RRA), a sensitive tool for the assessment of the sample's (dopaminergic) bioactivity. The RRA is based on competition between N-0437 and its tritium-labeled analogue for binding to dopamine receptors. The assay has been optimized for the preparation of the receptor suspension and the incubation conditions. Direct application of the assay for biological samples was impossible because of matrix interferences. Therefore, a solid-phase extraction method was developed in which the combination of a polar Si column and dichloromethane as eluent resulted in an effective elimination of the interferences. Recoveries were better than 90 and 95% for plasma and urine, respectively, even at concentrations at the determination limit of the method (300 pg/ml). Relative standard deviations were less than 15%. Because RRAs are stereoselective, the method discriminates between active and inactive species.