Synthesis and Characterization of Tritium-Labeled Neuropeptide Y Y₅ Receptor Ligands Derived From CGP71683A

Among the human neuropeptide Y (NPY) receptors, the Y₅ receptor (Y₅R) represents a potential therapeutic target for the treatment of obesity and stress-related diseases. As the Y₅R is expressed in various malignant tumors, it is also considered a potential target for cancer diagnosis and therapy. Although numerous non-peptidic Y₅R antagonists with high binding affinity have been reported, only labeled peptidic agonists are used in routine Y₅R competition binding assays, which are needed for the development and characterization of Y₅R ligands. In the present study, three radiolabeled derivatives of the high-affinity Y₅R antagonist CGP71683A (1) were synthesized ([³H]26, [³H]32, [³H]42) and studied in saturation binding assays using adherent or suspended HEC-1B-hY₅R cells or membrane preparations thereof. Their K_d values (1.5-13 nM) were consistent with the K_i values (1.3-11 nM) of the non-labeled analogs. However, all synthesized radioligands exhibited very high non-specific binding, primarily caused by binding of the radioligands to cellular membranes, which limits their use as tool compounds in radiochemical Y₅R binding assays. Nevertheless, a competition binding assay with [³H]32 and the endogenous Y₅R agonist hNPY afforded a K_i that was in agreement with literature data.