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Apelin is an endogenous peptide that is involved in many diseases such as cardiovascular diseases, obesity, and cancer, which has made it an attractive target for drug discovery. Herein, we explore the penultimate and final sequence positions of [Pyr 1 ]-apelin-13 (Ape13) via C-terminal N α -alkylated amide bonds and the introduction of positive charges, potentially targeting the allosteric sodium pocket, by assessing the binding affinity and signaling profiles at the apelin receptor (APJ). Synthetic analogues modified within this segment of Ape13 showed high affinity ( K i 0.12-0.17 nM vs Ape13 K i 0.7 nM), potent Gα i1 activation (EC 50 Gα i1 0.4-0.9 nM vs Ape13 EC 50 1.1 nM), partial agonist behavior disfavoring β-arrestin 2 recruitment for positively charged ligands (e.g., 49 ( SBL-AP-058 ), EC 50 β-arr2 275 nM, E max 54%) and high plasma stability for N -alkyl ligands ( t 1/2 > 7 h vs Ape13 t 1/2 0.5 h). Combining the benefits of the N α -alkylated amide bond with the guanidino substitution in a constrained ligand led to 63 ( SBL-AP-049 ), which displayed increased plasma stability ( t 1/2 5.3 h) and strong reduction of β-arrestin 2 signaling with partial maximal efficacy (EC 50 β-arr 864 nM, E max 48%), significantly reducing the hypotensive effect in vivo ., Competing Interests: The authors declare no competing financial interest., (© 2023 American Chemical Society.)

Side-chain-constrained amino acids are useful tools to modulate the biological properties of peptides. In this study, we applied side-chain constraints to apelin-13 (Ape13) by substituting the Pro12 and Phe13 positions, affecting the binding affinity and signaling profile on the apelin receptor (APJ). The residues 1Nal, Trp, and Aia were found to be beneficial substitutions for Pro12, and the resulting analogues displayed high affinity for APJ ( K i 0.08-0.18 nM vs Ape13 K i 0.7 nM). Besides, constrained (d-Tic) or α,α-disubstituted residues (Db z g; d-α-Me-Tyr(OBn)) were favorable for the Phe13 position. Compounds 47 (Pro12-Phe13 replaced by Aia-Phe, K i 0.08 nM) and 53 (Pro12-Phe13 replaced by 1Nal-Db z g, K i 0.08 nM) are the most potent Ape13 analogues activating the Gα 12 pathways ( 53 , EC 50 Gα 12 2.8 nM vs Ape13, EC 50 43 nM) known to date, displaying high affinity, resistance to ACE2 cleavage as well as improved pharmacokinetics in vitro ( t 1/2 5.8-7.3 h in rat plasma) and in vivo .

Herein, the synthesis of novel conformationally constrained amino acids, 4-amino-8-bromo-2-benzazepin-3-one (8-Br-Aba), 3-amino-3,4-dihydroquinolin-2-one, and regioisomeric 4-amino-naphthoazepinones (1- and 2-Ana), is described. Introduction of these constricted scaffolds into the N -terminal tetrapeptide of dermorphin (i.e., H-Tyr-d-Ala-Phe-Gly-NH 2 ) induced significant shifts in binding affinity, selectivity, and in vitro activity at the μ- and δ-opioid receptors (MOP and DOP, respectively). A reported constrained μ-/δ-opioid lead tetrapeptide H-Dmt-d-Arg-Aba-Gly-NH 2 was modified through application of various constrained building blocks to identify optimal spatial orientations in view of activity at the opioid receptors. Interestingly, when the aromatic moieties were turned toward the C -terminus of the peptide sequences, (partial) (ant)agonism at MOP and weak (ant)agonism at DOP were noticed, whereas the incorporation of the 1-Ana residue led toward balanced low nanomolar MOP/DOP binding and in vitro agonism.