Endomorphin analogues with balanced affinity for both μ- and δ-opioid receptors

Analogues of endomorphin and tripeptides modified at positions 4 and 3, respectively, with various phenylalanine analogues were synthesized and their affinities for opioid receptors were evaluated. Most of the peptides exhibited potent μ-receptor affinity and selectivity, among them, compound 7 (Dmt-Pro-Tmp-Tmp-NH₂) exhibited potent affinity for both μ- and δ-receptors (K_iμ=0.47 nmol/L, K_iδ=1.63 nmol/L).