Tetrabranched Hetero-Conjugated Peptides as Bifunctional Agonists of the NOP and Mu Opioid Receptors

Analgesic Peptides: From Natural Diversity to Rational Design

Pain affects one-third of the global population and is a significant public health issue. The use of opioid drugs, which are the strongest painkillers, is associated with several side effects, such as tolerance, addiction, overdose, and even death. An increasing demand for novel, safer analgesic agents is a driving force for exploring natural sources of bioactive peptides with antinociceptive activity. Since the G protein-coupled receptors (GPCRs) play a crucial role in pain modulation, the discovery of new peptide ligands for GPCRs is a significant challenge for novel drug development. The aim of this review is to present peptides of human and animal origin with antinociceptive potential and to show the possibilities of their modification, as well as the design of novel structures. The study presents the current knowledge on structure-activity relationship in the design of peptide-based biomimetic compounds, the modification strategies directed at increasing the antinociceptive activity, and improvement of metabolic stability and pharmacodynamic profile. The procedures employed in prolonged drug delivery of emerging compounds are also discussed. The work summarizes the conditions leading to the development of potential morphine replacements.

(L)-Monomethyl Tyrosine (Mmt): New Synthetic Strategy via Bulky 'Forced-Traceless' Regioselective Pd-Catalyzed C(sp2)-H Activation

The enormous influence in terms of bioactivity, affinity, and selectivity represented by the replacement of (L)-2,6-dimethyl tyrosine (Dmt) instead of Phenylalanine (Phe) into Nociceptin/orphanin (N/OFQ) neuropeptide analogues has been well documented in the literature. More recently, the non-natural amino acid (L)-2-methyl tyrosine (Mmt), with steric hindrance included between Tyr and Dmt, has been studied because of the modulation of steric effects in opioid peptide chains. Here, we report a new synthetic strategy to obtain Mmt based on the well-known Pd-catalyzed ortho-C(sp2)-H activation approach, because there is a paucity of other synthetic routes in the literature to achieve it. The aim of this work was to force only the mono-ortho-methylation process over the double ortho-methylation one. In this regard, we are pleased to report that the introduction of the dibenzylamine moiety on a Tyr aromatic nucleus is a convenient and traceless solution to achieve such a goal. Interestingly, our method provided the aimed Mmt either as N-Boc or N-Fmoc derivatives ready to be inserted into peptide chains through solid-phase peptide synthesis (SPPS). Importantly, the introduction of Mmt in place of Phe1 in the sequence of N/OFQ(1-13)-NH2 was very well tolerated in terms of pharmacological profile and bioactivity.

Novel Mixed NOP/Opioid Receptor Peptide Agonists

The nociceptin/orphanin FQ (N/OFQ)/N/OFQ receptor (NOP) system controls different biological functions including pain and cough reflex. Mixed NOP/opioid receptor agonists elicit similar effects to strong opioids but with reduced side effects. In this work, 31 peptides with the general sequence [Tyr/Dmt¹,Xaa⁵]N/OFQ(1-13)-NH₂ were synthesized and pharmacologically characterized for their action at human recombinant NOP/opioid receptors. The best results in terms of NOP versus mu opioid receptor potency were obtained by substituting both Tyr¹ and Thr⁵ at the N-terminal portion of N/OFQ(1-13)-NH₂ with the noncanonical amino acid Dmt. [Dmt^1,5]N/OFQ(1-13)-NH₂ has been identified as the most potent dual NOP/mu receptor peptide agonist so far described. Experimental data have been complemented by in silico studies to shed light on the molecular mechanisms by which the peptide binds the active form of the mu receptor. Finally, the compound exerted antitussive effects in an in vivo model of cough.

Pharmacological characterization of naloxegol: In vitro and in vivo studies

Opioid-induced constipation is the most prevalent adverse effect of opioid drugs. Peripherally acting mu opioid receptor antagonists (PAMORAs), including naloxegol, are indicated for the treatment of opioid-induced constipation. The aim of this study was the in vitro and in vivo pharmacological characterization of naloxegol in comparison with naloxone. In vitro experiments were performed to measure calcium mobilization in cells coexpressing opioid receptors and chimeric G proteins and mu receptor interaction with G protein and β-arrestin 2 using bioluminescence resonance energy transfer. In vivo experiments were performed in mice to measure pain threshold using the tail withdrawal assay and colonic transit using the bead expulsion assay. In vitro, naloxegol behaved as a selective and competitive mu receptor antagonist similarly to naloxone, being 3-10-fold less potent. In vivo, naloxone was effective in blocking fentanyl actions when given subcutaneously (sc), but not per os (po). In contrast, naloxegol elicited very similar effects with sc or po administration counteracting in a dose dependent manner the constipating effects of fentanyl without interfering with the fentanyl mediated analgesia. Thus, a useful PAMORA action could be obtained with naloxegol both after po and sc administration.

Biased Agonism at Nociceptin/Orphanin FQ Receptors: A Structure Activity Study on N/OFQ(1-13)-NH2

Nociceptin/orphanin FQ (N/OFQ) controls different biological functions via selective stimulation of the N/OFQ peptide (NOP) receptor. The pleiotropic actions of N/OFQ may limit the development of NOP ligands as innovative drugs in different therapeutic areas. The pharmacological concept of functional selectivity (aka biased agonism) might be useful for amplifying beneficial actions and/or counteracting side effects. Thus, molecules with large bias factors toward G protein or β arrestin are required for investigating the translational value of NOP biased modulation. Herein, the biased behavior of a heterogeneous library of NOP-targeting peptide derivatives was evaluated in vitro with the aim to provide possible insights into the structural determinants that govern the selective activation of G protein versus β-arrestin. Our results demonstrate that lipidation of N/OFQ(1-13)-NH2 is a useful strategy for obtaining G protein biased agonists for the NOP receptor.