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

Abstract 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-NH 2 ) exhibited potent affinity for both μ - and δ -receptors ( K i μ  = 0.47 nmol/L, K i δ  = 1.63 nmol/L).