Bivalent Argininamide‐Type Neuropeptide Y Y 1 Antagonists Do Not Support the Hypothesis of Receptor Dimerisation

Bivalent ligands are potential tools to investigate the dimerisation of G‐protein‐coupled receptors. Based on the ( R )‐argininamide BIBP 3226, a potent and selective neuropeptide Y Y 1 receptor (Y 1 R) antagonist, we prepared a series of bivalent Y 1 R ligands with a wide range of linker lengths (8–36 atoms). Exploiting the high eudismic ratio (>1000) of the parent compound, we synthesised sets of R , R ‐, R , S ‐ and S , S ‐configured bivalent ligands to gain insight into the “bridging” of two Y 1 Rs by simultaneous interaction with both binding sites of a putative receptor dimer. Except for the S , S isomers, the bivalent ligands are high‐affinity Y 1 R antagonists, as determined by Ca 2+ assays on HEL cells and radioligand competition assays on human Y 1 R‐expressing SK‐N‐MC and MCF‐7 cells. Whereas the R , R enantiomers are most potent, no marked differences were observed relative to the corresponding meso forms. The difference between R , R and R , S diastereomers was most pronounced (about sixfold) in the case of the Y 1 R antagonist containing a spacer of 20 atoms in length. Among the R , R enantiomers, linker length and structural diversity had little effect on Y 1 R affinity. Although the bivalent ligands preferentially bind to the Y 1 R, the selectivity toward human Y 2 , Y 4 , and Y 5 receptors was markedly lower than that of the monovalent argininamides. The results of this study neither support the presence of Y 1 R dimers nor the simultaneous occupation of both binding pockets by the twin compounds. However, as the interaction with Y 1 R dimers cannot be unequivocally ruled out, the preparation of a bivalent radioligand is suggested to determine the ligand–receptor stoichiometry. Aiming at such radiolabelled pharmacological tools, prototype twin compounds were synthesised, containing an N‐propionylated amino‐functionalised branched linker ( K i ≥18 n M ), a tritiated form of which can be easily prepared.