Synthesis and structure-activity relationship of [Nle¹⁰] neurokinin A (4-10) analogs with constraint in the backbone and at position six

Steric requirements of binding [Nle¹⁰]NKA(4-10) to NK-2 receptor were studied by introducing conformationally constrained amino acid analogs into its sequence. Two series of [Nle¹⁰]NKA(4-10) analogs were synthesized to investigate (i) the significance of a putative β-turn in the receptor-ligand interaction by insertion of either (S)- or (R)-Gly ⁸{ANC-2}Leu⁹ γ-lactams to mimic a β-turn constraint, and (ii) the effect of hindered rotation in the Φ, χ¹ and χ² dihedral angle space of the crucially important Phe⁶ which was replaced systematically with d-Phe, d- and l-Tyr, as well as with their conformationally constrained analogs, Tic, HOTic and β-MePhe. Competition binding experiments with [³H]NKA were performed using cloned human NK-2 receptors expressed in CHO cells. The analog possessing only an (R)-Gly⁸{ANC-2}Leu⁹ constraint, had the same binding affinity as that of the parent peptide. The rank order of potency of the other analogs showed a cumulative effect of different structural modifications in decreasing the binding affinity, i.e., when changing the configuration of the lactam ring to S, replacing Phe⁶ with constrained analogs, Tic or β-MePhe, changing the configuration of the amino acid at position six to d, and introducing a hydroxyl group on the aromatic ring.