Effects of modifications of residues in position 3 of dynorphin A(1-11)-NH₂ on κ receptor selectivity and potency

Tyrosine¹ and phenylalanine⁴ in dynorphin A (Dyn A) have been reported to be important residues for opioid agonist activity and for potency at κ receptors. The glycine residues in the 2 and 3 positions of dynorphin A may affect the relative orientation of the aromatic rings in positions 1 and 4, but their flexibility precludes careful analysis. To examine these effects on dynorphin A, we previously have synthesized the linear analogues [D-Ala³]Dyn A(1-11)-NH₂ (2) and [Ala³]Dyn A(1-11)-NH₂ (3) and reported their biological activities. Analogues 2 and 3 displayed affinities for the central κ opioid receptor (IC₅₀ = 0.76 and 1.1 nM, respectively) similar to that of Dyn A(1-11)-NH₂ (1) (IC₅₀ = 0.58 nM) and greatly enhanced selectivities for κ vs μ and κ vs δ receptors (IC₅₀ ratios of 350 and 1300 for 2, and 190 and 660 for 3, respectively). These results suggest that the structure and lipophilicity of the amino acid present in position 3 of Dyn A(1-11)-NH₂ as well as the conformational changes they induce in the message sequence of dynorphin have important effects on potency and selectivity for κ opioid receptors. To further investigate structure-activity relationships involving the residue at the 3 position of Dyn A(1-11)-NH₂, a series of Dyn A analogues with aromatic, charged, and aliphatic side chain substitutions at the 3 position was designed, synthesized, and evaluated for their affinities for κ, μ, and δ opioid receptors. It was found that analogues with lipophilic amino acids at the 3 position of Dyn A(1-11)-NH₂ generally displayed higher affinity but similar selectivities for the κ receptor than analogues with charged residues at the same position. It is suggested that the structural, configurational, and steric/lipophilic effects of amino acids at position 3 of Dyn A(1-11)-NH₂ may play an important role in potency and selectivity for the κ receptor.