The cis-4-Amino-l-proline Residue as a Scaffold for the Synthesis of Cyclic and Linear Endomorphin-2 Analogues

Endomorphin-2 (EM-2: Tyr-Pro-Phe-Phe-NH₂) is an endogenous tetrapeptide that combines potency and efficacy with high affinity and selectivity toward the μ opioid receptor, the most responsible for analgesic effects in the central nervous system. The presence of the Pro² represents a crucial factor for the ligand structural and conformational properties. Proline is in fact an efficient stereochemical spacer, capable of inducing favorable spatial orientation of aromatic rings, a key factor for ligand recognition and interaction with receptors. Here the Pro² has been replaced by 4­(S)-NH₂-2­(S)-proline (cAmp), a proline/GABA cis-chimera residue. This bivalent amino acid maintains the capacity to influenc the tetrapeptide conformation and offers the opportunity to generate new linear models and unusually constrained cyclic analogues characterized by an N-terminal Tyr bearing a free α-amino group. The results indicate that the new analogues do not show affinity for both δ and κ opioid receptors and bind only poorly to the μ receptors (for cyclopeptide 9: K_i^μ = 660 nM; GPI (IC₅₀) = 1.4% at 1 μM; for linear tetrapeptide acid 13: K_i^μ = 2000 nM; GPI (IC₅₀) = 0% at 1 μM; for linear tetrapeptide amide 15: K_i^μ = 310 nM; GPI (IC₅₀) = 894 nM).