Recent progress in non-opioid analgesic peptides.

Pain is a prevalent complex medical problem, characterized by physically debilitating and mentally destabilizing conditions. Current pain therapeutics mainly include non-steroidal anti-inflammatory drugs and narcotics (opioids), but they exhibit limitations in efficacy, unwanted side effects and the problem of drug abuse. To overcome these issues, the discovery of different molecular players within pain pathways could lead to new opportunities for therapeutic intervention. Among other strategies, peptides could be powerful pharmaceutical agents for effective opioid-free medications for pain treatment. This review is a compendium of representative non-opioid analgesic peptides acting directly or indirectly at different ion channels and receptors distributed in nociceptive pathways. They include peptides targeting Ca2+, Na+ and K+ voltage-gated ion channels, the neuronal nicotinic receptors (nAChR), transient receptor potential channels (TRP), and different non-opioid G-protein coupled receptors (GPCRs), like the calcitonin gen-related peptide (CGRP), cannabinoid, bradykinin and neurotensin receptors, among others. Peptides engineered from protein-protein interactions among pain-related receptors and regulatory proteins also led to new therapeutic approaches for pain management. Following some successful examples, already in the clinics or under clinical trials, the improved understanding of pain mechanisms, and the advances in peptide permeation and/or delivery, could afford new analgesic peptides in the near future.

Disulfide bonds versus TrpTrp pairs in irregular beta-hairpins: NMR structure of vammin loop 3-derived peptides as a case study.

Structural studies on model peptides have led to a good understanding of the rules behind the formation and stability of regular beta-hairpins. To test their applicability to the successful design of irregular beta-hairpins with long loops and/or beta-bulges at the strands, we mimicked loop 3 of vammin, a 4:6 beta-hairpin with a non-Gly beta-bulge. The most stabilising cross-strand pairs, disulfide bonds or/and TrpTrp pairs, were incorporated at non-hydrogen-bonded sites in peptides spanning the 69-80 region of vammin. According to NMR data, these modified peptides adopt beta-hairpin conformations as intended by design. The Trp-containing peptides reproduce even the unusual positive phi angle for the Gln residue, with the indole rings in the preferred edge-to-face orientation. For the first time the beta-hairpin-stabilising capacities of a disulfide bond and a TrpTrp pair are compared in the same model system. We found that the contribution to stability of the noncovalent indole-indole interaction is larger than that of the covalent disulfide bond, and that their combination gives rise to an even more stable beta-hairpin.

Further evidence for 2-alkyl-2-carboxyazetidines as gamma-turn inducers.

Reverse turns, a common motif in proteins and peptides, have attracted attention due to their relevance in a wide variety of biological processes. In an attempt to artificially imitate and stabilize these turns in short peptides, we have developed versatile synthetic methodologies for the preparation of 2-alkyl-2-carboxyazetidines and incorporated them into the i + 1 position of model tetrapeptides, where they have shown a tendency to induce gamma-turns. However, to ascertain the general utility of these restricted amino acids as gamma-type reverse turn inducers, it was then required to study the conformational preferences when located at other positions. To this end, model tetrapeptides R-CO-Ala-Xaa-NHMe, containing differently substituted azetidine moieties (Xaa = Aze, 2-MeAze, 2-BnAze) at the i + 2 position, were synthesized and subjected to a thorough conformational analysis. The theoretical and experimental results obtained, including the X-ray diffraction structure of a dipeptide derivative containing this skeleton, provide evidence that the 2-alkyl-2-carboxyazetidine scaffold is able to efficiently induce gamma-turns when incorporated into these short peptides, irrespective of their localization in the peptide chain.

Synthesis and biological properties of beta-turned Abeta(31-35) constrained analogues.

A series of constrained pentapeptide analogues of the fragment Abeta(31-35) has been prepared using solid phase synthesis protocols. The results of conformational studies and surface plasmon resonance (SPR) experiments seem to indicate that the affinity of these constrained analogues for immobilized Abeta(25-35) peptide could be related to their ability to adopt a Leu34N-Ile31O beta-turn-like folded conformation.

Exceptional stereoselectivity in the synthesis of 1,3,4-trisubstituted 4-carboxy beta-lactam derivatives from amino acids.

[reaction: see text] The base-promoted cyclization of optically pure N-(p-methoxybenzyl)-N-(2-chloro)propionyl amino acid derivatives resulted in a diastereo- and enantioselective approach to valuable 1,3,4,4-tetrasubstituted beta-lactams. The stereochemical outcome of the reaction is exclusively governed by the configuration of the N-(2-chloro)propionyl moiety.

Synthesis, high-throughput screening and pharmacological characterization of ß-lactam derivatives as TRPM8 antagonists.

The mammalian transient receptor potential melastatin channel 8 (TRPM8), highly expressed in trigeminal and dorsal root ganglia, mediates the cooling sensation and plays an important role in the cold hypersensitivity characteristic of some types of neuropathic pain, as well as in cancer. Consequently, the identification of selective and potent ligands for TRPM8 is of great interest. Here, a series of compounds, having a ß-lactam central scaffold, were prepared to explore the pharmacophore requirements for TRPM8 modulation. Structure-activity studies indicate that the minimal requirements for potent ß-lactam-based TRPM8 blockers are hydrophobic groups (benzyl preferentially or t Bu) on R1, R2, R3 and R5 and a short N-alkyl chain (≤3 carbons). The best compounds in the focused library (41 and 45) showed IC50 values of 46 nM and 83 nM, respectively, in electrophysiology assays. These compounds selectively blocked all modalities of TRPM8 activation, i.e. menthol, voltage, and temperature. Molecular modelling studies using a homology model of TRPM8 identified two putative binding sites, involving networks of hydrophobic interactions, and suggesting a negative allosteric modulation through the stabilization of the closed state. Thus, these ß-lactams provide a novel pharmacophore scaffold to evolve TRPM8 allosteric modulators to treat TRPM8 channel dysfunction.

Transient Receptor Potential Melastatin 8 Channel (TRPM8) Modulation: Cool Entryway for Treating Pain and Cancer.

TRPM8 ion channels, the primary cold sensors in humans, are activated by innocuous cooling (<28 °C) and cooling compounds (menthol, icilin) and are implicated in sensing unpleasant cold stimuli as well as in mammalian thermoregulation. Overexpression of these thermoregulators in prostate cancer and in other life-threatening tumors, along with their contribution to an increasing number of pathological conditions, opens a plethora of medicinal chemistry opportunities to develop receptor modulators. This Perspective seeks to describe current known modulators for this ion channel because both agonists and antagonists may be useful for the treatment of most TRPM8-mediated pathologies. We primarily focus on SAR data for the different families of compounds and the pharmacological properties of the most promising ligands. Furthermore, we also address the knowledge about the channel structure, although still in its infancy, and the role of the TRPM8 protein signalplex to channel function and dysfunction. We finally outline the potential future prospects of the challenging TRPM8 drug discovery field.

From 1-acyl-beta-lactam human cytomegalovirus protease inhibitors to 1-benzyloxycarbonylazetidines with improved antiviral activity. A straightforward approach to convert covalent to noncovalent inhibitors.

Starting from the structure of known beta-lactam covalent human cytomegalovirus (HCMV) protease inhibitors and from the knowledge of the residues implicated in the active site of this enzyme, we designed a series of phenylalanine-derived 2-azetidinones bearing a 4-carboxylate moiety that could be apt for additional interactions with the guanidine group of the Arg165/Arg166 residues of the viral protease. Some compounds within this series showed anti-HCMV activity at 10-50 muM, but rather high toxicity. The presence of aromatic 1-acyl groups and a certain hydrophobic character in the region of the 4-carboxylate were stringent requirements for anti-HCMV activity. To go a step ahead into the search for effective HCMV medicines, we then envisaged a series of noncovalent inhibitors by simple deletion of the carbonyl group in the beta-lactam derivatives to provide the corresponding azetidines. This led to low micromolar inhibitors of HCMV replication, with 17 and 27 being particularly promising lead compounds for further investigation, although their toxicity still needs to be lowered.

Azetidine-derived amino acids versus proline derivatives. alternative trends in reverse turn induction.

The influence of 2-alkyl-2-carboxyazetidines (Aze) on the 3D structure of model tetrapeptides R2CO-2-R1Aze-l-Ala-NHMe has been analyzed by molecular modeling, 1H NMR, and FT-IR studies. The conformational constraints introduced by the four-membered ring resulted in an effective way to stabilize gamma-turn-like conformations in these short peptides. The conformational preferences of these Aze-containing peptides have been compared to those of the corresponding peptide analogues containing Pro or alpha-MePro in the place of 2-alkyl-Aze residue. In the model studied, both Pro and Aze derivatives are able to induce reverse turns, but the nature of the turn is different as a function of the ring size. While the five-membered ring of Pro tends to induce beta-turns, as previously suggested by different authors, the four-membered ring of Aze residues forces the peptide to preferentially adopt gamma-turn conformations. In both cases, the presence of an alkyl group at the alpha-position of Pro or the azetidine-2-carboxylate ring enhances significantly the turn-inducing ability. These results might open the opportunity of using 2-alkyl-Aze residues as versatile tools in defining the role of gamma-turn structures within the bioactive conformation of selected peptides, and represent an alternative to Pro derivatives as turn inducers.

Old molecules for new receptors: Trp(Nps) dipeptide derivatives as vanilloid TRPV1 channel blockers.

The transient receptor potential vanilloid member 1 (TRPV1), an integrator of multiple pain-producing stimuli, is regarded nowadays as an important biological target for the discovery of novel analgesics. Here, we describe the first experimental evidence for the behavior of an old family of analgesic dipeptides, namely Xaa-Trp(Nps) and Trp(Nps)-Xaa (Xaa=Lys, Arg) derivatives, as potent TRPV1 channel blockers. We also report the synthesis and biological investigation of a series of new conformationally restricted Trp(Nps)-dipeptide derivatives with improved TRPV1/NMDA selectivity. Compound 15 b, which incorporates an N-terminal 2S-azetidine-derived Arg residue, was the most selective compound in this series. Collectively, a new family of TRPV1 channel blockers emerged from our results, although further modifications are required to fine-tune the potency/selectivity/toxicity balance.

New Gly-Pro-Glu (GPE) analogues: expedite solid-phase synthesis and biological activity.

A suitable solid-phase approach, based on Fmoc/(t)Bu methodology and on the use of 2-chlorotrityl resin, allowed a rapid and efficient preparation of new GPE analogues. Most of the synthesized tripeptides displayed glutamate receptor binding affinity comparable to that of GPE, but only a few derivatives showed significant neuroprotective activity.

The neuroprotective activity of GPE tripeptide analogues does not correlate with glutamate receptor binding affinity.

The influence of several modifications on the GPE tripeptide structure upon the binding to GluRs and on their neuroprotective effects has been studied. The results indicated that the prevention of neuronal death showed by GPE and some analogues is not directly related to their affinity at glutamate receptors.

Synthesis and anti-HCMV activity of 1-acyl-beta-lactams and 1-acylazetidines derived from phenylalanine.

Different Phe-derived 1-acyl-beta-lactams, analogous to a series of 2-azetidinones acting as HCMV serine protease inhibitors, were synthesized. Some of these compounds were modest inhibitors of the HCMV replication. Interestingly, removal of the carbonyl group of the beta-lactam ring, most likely acting as the serine trap, resulted in an azetidine derivative with anti-HCMV activity comparable to that of the reference compound ganciclovir.