Synthesis and Characterization of Tritium‐Labeled Neuropeptide Y Y ₅ Receptor Ligands Derived From CGP71683A

ABSTRACT

Among the human neuropeptide Y (NPY) receptors, the Y ₅ receptor (Y ₅ R) represents a potential therapeutic target for the treatment of obesity and stress‐related diseases. As the Y ₅ R is expressed in various malignant tumors, it is also considered a potential target for cancer diagnosis and therapy. Although numerous non‐peptidic Y ₅ R antagonists with high binding affinity have been reported, only labeled peptidic agonists are used in routine Y ₅ R competition binding assays, which are needed for the development and characterization of Y ₅ R ligands. In the present study, three radiolabeled derivatives of the high‐affinity Y ₅ R antagonist CGP71683A ( 1 ) were synthesized ([ ³ H] 26 , [ ³ H] 32 , [ ³ H] 42 ) and studied in saturation binding assays using adherent or suspended HEC‐1B‐hY ₅ R 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 Y ₅ R binding assays. Nevertheless, a competition binding assay with [ ³ H] 32 and the endogenous Y ₅ R agonist hNPY afforded a K _i that was in agreement with literature data.