Selective activation of the -opioid receptor (DOP) has great potential for the treatment of chronic pain, benefitting from ancillary anxiolytic and antidepressant-like effects. Moreover, DOP agonists show reduced adverse effects as compared to -opioid receptor (MOP) agonists that are in the spotlight of the current "opioid crisis." Here, we report the first crystal structures of the DOP in an activated state, in complex with two relevant and structurally diverse agonists: the potent opioid agonist peptide KGCHM07 and the small-molecule agonist DPI-287 at 2.8 and 3.3 Å resolution, respectively. Our study identifies key determinants for agonist recognition, receptor activation, and DOP selectivity, revealing crucial differences between both agonist scaffolds. Our findings provide the first investigation into atomic-scale agonist binding at the DOP, supported by site-directed mutagenesis and pharmacological characterization. These structures will underpin the future structure-based development of DOP agonists for an improved pain treatment with fewer adverse effects. RESULTS Activation-related changes in the DOPBoth agonist-bound structures are in an activated state. Unless otherwise indicated, we will use the higher-resolution BRIL-DOP-KGCHM07 structure for comparison with previously published inactive-state antagonist-bound DOP structures [Protein Data Bank (PDB) 4N6H and 4RWD] (16, 17) and with active-state structures of the MOP (PDB 5C1M and 6DDF) (18,20) and KOP (PDB 6B73) (19). First, the agonist-bound DOP structures display large outward movements of the intracellular parts of helices V (4.5 Å) and VI (9.4 to 11.2 Å), and a 3.9 Å inward movement of helix VII ( Fig. 2A), which is a common feature of the active conformational states of GPCRs (21). The shift of helix VII at the level of residue N314 7.49 [superscripts according to the Ballesteros and Weinstein numbering (22)] (Fig. 3A), which leads to a collapse of the allosteric sodium-binding pocket in active-state GPCR structures (23), is even more pronounced in the determined DOP structures as compared to the active MOP and KOP (Fig. 3B and fig. S1). However, this greater shift of N314 7.49 in the DOP might be affected by three mutations in the sodium-binding pocket (N90 2.45 S, D95 2.50 G, N131 3.35 S) that were introduced during construct design. The activation-related outward movement of helix VI at the level of residue F270 6.44 is greater in the agonist-bound DOP than in the MOP and KOP. On the contrary, the very tips of helix VI (at position 6.28 as a reference) are more tilted by 4 to 6 Å in the active-state MOP and KOP ( fig. S1), likely due to the bound G protein or nanobody, respectively, pushing helix VI tips further outward (24). Elucidating the active -opioid receptor crystal structure with peptide and smallmolecule agonists. Sci. Adv. 5, eaax9115 (2019).

A reported mixed opioid agonist - neurokinin 1 receptor (NK1R) antagonist 4 (Dmt-D-Arg-Aba-Gly-(3’,5’-(CF3)2)NMe-benzyl) was modified to identify important features in both pharmacophores. The new dual ligands were tested in vitro and subsequently two compounds (lead structure 4 and one of the new analogues 22, Dmt-D-Arg-Aba-β-Ala-NMe-Bn) were selected for in vivo behavioral assays, which were conducted in acute (tail-flick) and neuropathic pain models (cold plate and von Frey) in rats. Compared to the parent opioid compound 33 (without NK1R pharmacophore), hybrid 22 was more active in the neuropathic pain models. Attenuation of neuropathic pain emerged from NK1R antagonism as demonstrated by the pure NK1R antagonist 6. Surprisingly, despite a lower in vitro activity at NK1R in comparison with 4, compound 22 was more active in the neuropathic pain models. Although potent analgesic effects were observed for 4 and 22, upon chronic administration, both manifested a tolerance profile similar to that of morphine and cross tolerance with morphine in a neuropathic pain model in rat.

The melanocortin receptors (MC1R–MC5R) belong to class A G-protein-coupled receptors (GPCRs) and are known to have receptor-specific roles in normal and diseased states. Selectivity for MC4R is of particular interest due to its involvement in various metabolic disorders, including obesity, feeding regulation, and sexual dysfunctions. To further improve the potency and selectivity of MC4R (ant)­agonist peptide ligands, we designed and synthesized a series of cyclic peptides based on the recent crystal structure of MC4R in complex with the well-characterized antagonist SHU-9119 (Ac-Nle4-c­[Asp5-His6-DNal­(2′)7-Arg8-Trp9-Lys10]-NH2). These analogues were pharmacologically characterized in vitro, giving key insights into exploiting binding site subpockets to deliver more selective ligands. More specifically, the side chains of the Nle4, DNal(2′)7, and Trp9 residues in SHU-9119, as well as the amide linkage between the Asp5 and Lys10 side chains, were found to represent structural features engaging a hMC4R/hMC3R selectivity switch.

Constraining the conformation of flexible peptides is a proven strategy to increase potency, selectivity, and metabolic stability. The focus has mostly been on constraining the backbone dihedral angles; however, the correct orientation of the amino acid side chains (χ-space) that constitute the peptide pharmacophore is equally important. Control of χ-space utilizes conformationally constrained amino acids that favor, disfavor, or exclude the gauche (-), the gauche (+), or the trans conformation. In this review we focus on cyclic aromatic amino acids in which the side chain is connected to the peptide backbone to provide control of χ- and χ-space. The manifold applications for cyclized analogues of the aromatic amino acids Phe, Tyr, Trp, and His within peptide medicinal chemistry are showcased herein with examples of enzyme inhibitors and ligands for G protein-coupled receptors.

To address the need for highly potent, metabolically stable, and selective agonists, antagonists, and inverse agonists at the melanocortin receptor subtypes, conformationally constrained indolo- and benzazepinone residues were inserted into the α-MSH pharmacophore, His(6)-Phe(7)-Arg(8)-Trp(9)-domain. Replacement of His(6) by an aminoindoloazepinone (Aia) or aminobenzazepinone (Aba) moiety led to hMC4R and hMC5R selective agonist and antagonist ligands, respectively (tetrapeptides 1 to 3 and 4, respectively). In peptides 1 to 3 and depending on the para-substituent of the d-Phe residue in position 2, the activity goes from allosteric partial agonism (1, R = H) to allosteric full agonism (2, R = F) and finally allosteric partial agonism (3, R = Br).