Syntheses, opioid binding affinities, and potencies of dynorphin A analogues substituted in positions 1, 6, 7, 8 and 10

Structural, stereochemical, stereoelectronic and conformational requirements for biological activity of dynorphin A_1–11‐NH₂ analogues at opioid receptors were explored by substitution of Tyr¹, Arg⁶, Arg⁷, Ile⁸ and Pro¹⁰ with other amino acid residues. Interestingly, substitution of Tyr¹ with N^α‐Ac‐Tyr^l, D‐Tyr¹, Phe¹ or p‐BrPhe¹ led to analogues that were quite potent at κ opioid receptors, and additional substitution of Ile⁸with D‐Ala⁸ and/or Pro¹⁰ with D‐Pro¹⁰ retained high potency in brain binding assay: [N^α‐Ac‐Tyr¹]‐ (1), [D‐Tyr¹]‐ (2) [Phe¹]‐ (3), [Phe¹. D‐Ala⁸]‐ (5), [p‐BrPhe¹, D‐Ala^s]‐ (6), [Phe¹, D‐Pro¹⁰]‐ (7) and [Phe¹, D‐Ala⁸, D‐Pro¹⁰]‐Dyn A_1–11‐NH₂ (8) had IC₅₀(nM) binding affinities of 13.2, 18.6, 1.64, 1.26, 1.84, 2.44 and 1.62 nM, respectively. The D‐Phe¹ analogue 4, however, was only weakly active (610 nM). All of the analogues except 4 were modestly selective for κ vs. μ guinea pig brain opioid receptor (11‐ to 88–fold) and quite selective for κ vs. δ receptors (65–576). However, all of the analogues appeared to have very low or essentially no activity in the guinea pig ileum and mouse vas deference functional bioassays, and one analogue, 5, appeared to have weak antagonist activities. On the other hand, if constrained amino acids such as β‐methylphenylalanine or 1,2,3,4‐tetrahydroisoquinoline carboxylic acid, and hydroxyproline were placed in the 1 position, inactive analogues or analogues with greatly reduced potency and biological activity were obtained (compounds 12–14). It had previously been suggested that the Arg⁶ and Arg⁷ residues were critical for biological activity. However, when we replace either one of these residues, [Nle⁶]Dyn A_1–11 (9) and [Nle⁷]Dyn A_1–11‐NH₂ (10) were both highly potent binders in κ receptor binding studies (IC₅₀= 0.95 and 0.43 nM, respectively), and interestingly also were potent in μ and δ binding studies. Furthermore, both of the analogues were modestly potent in the GPI and MVD assays (94, 65 nM; 31, 81 nM, respectively). These results demonstrate that basic residues at positions 6 and 7 in dynorphin are not very important for binding to κ opioid receptors. Finally, many of the compounds reported here showed high selectivity for central vs. peripheral κ opioid receptors, with compound 4 being the most selective (63 000‐fold).