Synthesis and characterization of selective fluorescent ligands for the neurokinin NK2 receptor

Article abstract of DOI:10.1021/jm00039a012

Several fluorescent probes for the NK₂ receptor were designed, synthesized, and pharmacologically characterized. These fluorescent ligands are analogues of the selective NK₂ heptapeptide antagonist N-α-benzoyl- Ala-Ala-D-Trp-Phe-D-Pro-Pro-Nle-NH₂ (1, GR94800). They were obtained by substitution of 2,n-diaminoalkyl amino acid (n=3-6) for Ala¹ and the subsequent coupling of the fluorophore NBD (7-nitrobenz-2-oxa-1,3-diazol-4- yl) or fluoresceinthiocarbamyl to the N-ω amino group. The fluorescent derivatives retained high binding affinities for the NK₂ receptor in transfected CHO cells. In contrast, fluorescent derivatives made by replacing the N-α-benzoyl group of 1 by NBD or fluorescein were considerably less active. The effect on ligand potency of varying the length of the spacer arm between the peptide moiety and the fluorescent group was also studied. The most potent fluorescent antagonists were N-α-benzoyl-Dab(γ-NBD)-Ala-D-Trp- Phe-D-Pro-Pro-Nle-NH₂ (5B), pK(i) = 8.87 for NK₂; N-α-benzoyl-Orn(δ- NBD)-Ala-D-Trp-Phe-D-Pro-Pro-Nle-NH₂ (4B), pK(i) = 8.84; and N-α-benzoyl- Lys(ε-NBD)-Ala-D-Trp-Phe-D-Pro-Pro-Nle-NH₂ (3B), pK(i) = 8.83. These three compounds were highly selective for NK₂ over NK₃ and NK₁ receptors. We show that these fluorescent ligands are useful tools for the detection of NK₂ receptor expression by flow cytometry. Additionally, these fluorescent probes should prove valuable for fluorescence microscopy and study of ligand- receptor interaction by spectrofluorimetry.