Structural Optimization and Characterization of Potent Analgesic Macrocyclic Analogues of Neurotensin (8–13)

Development of Macrocyclic Neurotensin Receptor Type 2 (NTS2) Opioid-Free Analgesics

The opioid crisis has highlighted the urgent need to develop non-opioid alternatives for managing pain, with an effective, safe, and non-addictive pharmacotherapeutic profile. Using an extensive structure-activity relationship approach, here we have identified a new series of highly selective neurotensin receptor type 2 (NTS2) macrocyclic compounds that exert potent, opioid-independent analgesia in various experimental pain models. To our knowledge, the constrained macrocycle in which the Ile12 residue of NT(7-12) was substituted by cyclopentylalanine, Pro7 and Pro10 were replaced by allyl-glycine followed by side-chain to side-chain cyclization is the most selective analog targeting NTS2 identified to date (Ki 2.9 nM), showing 30,000-fold selectivity over NTS1. Of particular importance, this macrocyclic analog is also able to potentiate the analgesic effects of morphine in a dose- and time-dependent manner. Exerting complementary analgesic actions via distinct mechanisms of nociceptive transmission, NTS2-selective macrocycles can therefore be exploited as opioid-free analgesics or as opioid-sparing therapeutics, offering superior pain relief with reduced adverse effects to pain patients.

Chemoselective, regioselective, and positionally selective fluorogenic stapling of unprotected peptides for cellular uptake and direct cell imaging

Peptide stapling reactions represent powerful methods for structuring native α-helices to improve their bioactivity in targeting protein-protein interactions (PPIs). In light of a growing need for regio- and positionally selective stapling methods involving natural amino acid residues in their unprotected states, we report a rapid, mild, and highly chemoselective three-component stapling reation using a class of molecular linchpins based on 2-arylketobenzaldehydes (ArKBCHOs) that create a fluorescent staple, hereafter referred to as a Fluorescent Isoindole Crosslink (FlICk). This methodology offers positional selectivity favouring i, i + 4 helical staples comprising a lysine and cysteine, in the presence of competing nucleophiles on unprotected peptides. In our efforts to further validate this chemistry, we have successfully shown in vitro cytotoxicity of a FlICk-ed peptide (IC50 = 5.10 ± 1.27 μM), equipotent to an olefin-stapled congener. In harnessing the innate fluorescence of the thiol-isoindole, we report new blue-green fluorophores, which arise as a consequence of stapling, with appreciable quantum yields that enable direct cellular imaging in the assessment of cell permeability, thus bridging therapeutic potential with cytological probe development.

Insights on Structure-Passive Permeability Relationship in Pyrrole and Furan-Containing Macrocycles

Macrocycles have recognized therapeutic potential, but their limited cellular permeability can hinder their development as oral drugs. To better understand the structure-permeability relationship of heterocycle-containing, semipeptidic macrocycles, a library was synthesized. These compounds were created by developing two novel reactions described herein: the reduction of activated oximes by LiBH4 and the aqueous reductive mono-N-alkylation of aldehydes using catalytic SmI2 and stoichiometric Zn. The permeability of the macrocycles was evaluated through a parallel artificial membrane permeability assay (PAMPA), and the results indicated that macrocycles with a furan incorporated into the structure have better passive permeability than those with a pyrrole moiety. Compounds bearing a 2,5-disubstituted pyrrole (endo orientation) were shown to be implicated in intramolecular H-bonds, enhancing their permeability. This study highlighted the impact of heterocycles moieties in semipeptides, creating highly permeable macrocycles, thus showing promising avenues for passive diffusion of drugs beyond the rule-of-five chemical space.

Encounter Complexes Between the N-terminal of Neurotensin with the Extracellular Loop 2 of the Neurotensin Receptor 1 Steer Neurotensin to the Orthosteric Binding Pocket

Neurotensin (NT) is a linear disordered peptide that activates two different class A GPCRs, neurotensin receptor 1 (NTS1) and NTS2. Resolved structures of the complex of the C-terminal fragment of NT, NT8-13, with NTS1 shows the peptide takes a well-defined structure in the bound state. However, the mechanisms underlying NT recognition of NTS1, and the conformational transition of NT upon binding NTS1 is an open question that if answered may aid discovery of highly selective drugs and reveal potential secondary binding sites on the surface of the receptor. Herein we investigated the interactions guiding NT to the orthosteric binding pocket of NTS1 by combining NMR experiments with kinetic analysis of the binding pathway using stopped-flow fluorescence and mutagenesis on both NT and NTS1. We show the presence of transient structures in the middle part of NT that kinetically regulate the binding of NT to NTS1. Moreover, our results indicate that the binding pathway of NT onto NTS1 is mediated via electrostatic interactions between the N-terminal region of NT with the extracellular loop 2 of NTS1. These interactions induce backbone conformational changes in neurotensin similar to the bound-state neurotensin, suggesting that the N-terminal region of NT and these interactions should be considered for development of selective drugs against NTS1.

Neurotensin(8-13) analogs as dual NTS1 and NTS2 receptor ligands with enhanced effects on a mouse model of Parkinson's disease

The modulatory interactions between neurotensin (NT) and the dopaminergic neurotransmitter system in the brain suggest that NT may be associated with the progression of Parkinson's disease (PD). NT exerts its neurophysiological effects by interactions with the human NT receptors type 1 (hNTS1) and 2 (hNTS2). Therefore, both receptor subtypes are promising targets for the development of novel NT-based analogs for the treatment of PD. In this study, we used a virtually guided molecular modeling approach to predict the activity of NT(8-13) analogs by investigating the docking models of ligands designed for binding to the human NTS1 and NTS2 receptors. The importance of the residues at positions 8 and/or 9 for hNTS1 and hNTS2 receptor binding affinity was experimentally confirmed by radioligand binding assays. Further in vitro ADME profiling and in vivo studies revealed that, compared to the parent peptide NT(8-13), compound 10 exhibited improved stability and BBB permeability combined with a significant enhancement of the motor function and memory in a mouse model of PD. The herein reported NTS1/NTS2 dual-specific NT(8-13) analogs represent an attractive tool for the development of therapeutic strategies against PD and potentially other CNS disorders.

Peptidic Inhibitors and a Fluorescent Probe for the Selective Inhibition and Labelling of Factor XIIIa Transglutaminase

Factor XIIIa (FXIIIa) is a transglutaminase of major therapeutic interest for the development of anticoagulants due to its essential role in the blood coagulation cascade. While numerous FXIIIa inhibitors have been reported, they failed to reach clinical evaluation due to their lack of metabolic stability and low selectivity over transglutaminase 2 (TG2). Furthermore, the chemical tools available for the study of FXIIIa activity and localization are extremely limited. To combat these shortcomings, we designed, synthesised, and evaluated a library of 21 novel FXIIIa inhibitors. Electrophilic warheads, linker lengths, and hydrophobic units were varied on small molecule and peptidic scaffolds to optimize isozyme selectivity and potency. A previously reported FXIIIa inhibitor was then adapted for the design of a probe bearing a rhodamine B moiety, producing the innovative KM93 as the first known fluorescent probe designed to selectively label active FXIIIa with high efficiency (k_inact/K_I = 127,300 M^-1 min^-1) and 6.5-fold selectivity over TG2. The probe KM93 facilitated fluorescent microscopy studies within bone marrow macrophages, labelling FXIIIa with high efficiency and selectivity in cell culture. The structure-activity trends with these novel inhibitors and probes will help in the future study of the activity, inhibition, and localization of FXIIIa.

Opening the amino acid toolbox for peptide-based NTS2-selective ligands as promising lead compounds for pain management

Chronic pain is one of the most critical health issues worldwide. Despite considerable efforts to find therapeutic alternatives, opioid drugs remain the gold standard for pain management. The administration of μ-opioid receptor (MOR) agonists is associated with detrimental and limiting adverse effects. Overall, these adverse effects strongly overshadow the effectiveness of opioid therapy. In this context, the development of neurotensin (NT) ligands has shown to be a promising approach for the management of chronic and acute pain. NT exerts its opioid-independent analgesic effects through the binding of two G protein-coupled receptors (GPCRs), NTS1 and NTS2. In the last decades, modified NT analogues have been proven to provide potent analgesia in vivo. However, selective NTS1 and nonselective NTS1/NTS2 ligands cause antinociception associated with hypothermia and hypotension, whereas selective NTS2 ligands induce analgesia without altering the body temperature and blood pressure. In light of this, various structure-activity relationship (SAR) studies provided findings addressing the binding affinity of ligands towards NTS2. Herein, we comprehensively review peptide-based NTS2-selective ligands as a robust alternative for future pain management. Particular emphasis is placed on SAR studies governing the desired selectivity and associated in vivo results.

Recent Advances in Macrocyclic Drugs and Microwave-Assisted and/or Solid-Supported Synthesis of Macrocycles

Macrocycles represent attractive candidates in organic synthesis and drug discovery. Since 2014, nineteen macrocyclic drugs, including three radiopharmaceuticals, have been approved by FDA for the treatment of bacterial and viral infections, cancer, obesity, immunosuppression, etc. As such, new synthetic methodologies and high throughput chemistry (e.g., microwave-assisted and/or solid-phase synthesis) to access various macrocycle entities have attracted great interest in this chemical space. This article serves as an update on our previous review related to macrocyclic drugs and new synthetic strategies toward macrocycles (Molecules, 2013, 18, 6230). In this work, I first reviewed recent FDA-approved macrocyclic drugs since 2014, followed by new advances in macrocycle synthesis using high throughput chemistry, including microwave-assisted and/or solid-supported macrocyclization strategies. Examples and highlights of macrocyclization include macrolactonization and macrolactamization, transition-metal catalyzed olefin ring-closure metathesis, intramolecular C-C and C-heteroatom cross-coupling, copper- or ruthenium-catalyzed azide-alkyne cycloaddition, intramolecular SNAr or SN2 nucleophilic substitution, condensation reaction, and multi-component reaction-mediated macrocyclization, and covering the literature since 2010.

Pharmacodynamic and pharmacokinetic profiles of a neurotensin receptor type 2 (NTS2) analgesic macrocyclic analog

The current opioid crisis highlights the urgent need to develop safe and effective pain medications. Thus, neurotensin (NT) compounds represent a promising approach, as the antinociceptive effects of NT are mediated by activation of the two G protein-coupled receptor subtypes (i.e., NTS1 and NTS2) and produce potent opioid-independent analgesia. Here, we describe the synthesis and pharmacodynamic and pharmacokinetic properties of the first constrained NTS2 macrocyclic NT(8-13) analog. The Tyr¹¹ residue of NT(8-13) was replaced with a Trp residue to achieve NTS2 selectivity, and a rationally designed side-chain to side-chain macrocyclization reaction was applied between Lys⁸ and Trp¹¹ to constrain the peptide in an active binding conformation and limit its recognition by proteolytic enzymes. The resulting macrocyclic peptide, CR-01-64, exhibited high-affinity for NTS2 (K_i 7.0 nM), with a more than 125-fold selectivity over NTS1, as well as an improved plasma stability profile (t_1/2 > 24 h) compared with NT (t_1/2 ~ 2 min). Following intrathecal administration, CR-01-64 exerted dose-dependent and long-lasting analgesic effects in acute (ED₅₀ = 4.6 µg/kg) and tonic (ED₅₀ = 7.1 µg/kg) pain models as well as strong mechanical anti-allodynic effects in the CFA-induced chronic inflammatory pain model. Of particular importance, this constrained NTS2 analog exerted potent nonopioid antinociceptive effects and potentiated opioid-induced analgesia when combined with morphine. At high doses, CR-01-64 did not cause hypothermia or ileum relaxation, although it did induce mild and short-term hypotension, all of which are physiological effects associated with NTS1 activation. Overall, these results demonstrate the strong therapeutic potential of NTS2-selective analogs for the management of pain.

Design, Structural Optimization, and Characterization of the First Selective Macrocyclic Neurotensin Receptor Type 2 Non-opioid Analgesic

Neurotensin (NT) receptor type 2 (NTS2) represents an attractive target for the development of new NT-based analgesics. Here, we report the synthesis and functional in vivo characterization of the first constrained NTS2-selective macrocyclic NT analog. While most chemical optimization studies rely on the NT(8-13) fragment, we focused on NT(7-12) as a scaffold to design NTS2-selective macrocyclic peptides. Replacement of Ile12 by Leu, and Pro7/Pro10 by allylglycine residues followed by cyclization via ring-closing metathesis led to macrocycle 4, which exhibits good affinity for NTS2 (50 nM), high selectivity over NTS1 (>100 μM), and improved stability compared to NT(8-13). In vivo profiling in rats reveals that macrocycle 4 produces potent analgesia in three distinct rodent pain models, without causing the undesired effects associated with NTS1 activation. We further provide evidence of its non-opioid antinociceptive activity, therefore highlighting the strong therapeutic potential of NTS2-selective analogs for the management of acute and chronic pain.

Metabolically stable neurotensin analogs exert potent and long-acting analgesia without hypothermia

The endogenous tridecapeptide neurotensin (NT) has emerged as an important inhibitory modulator of pain transmission, exerting its analgesic action through the activation of the G protein-coupled receptors, NTS1 and NTS2. Whereas both NT receptors mediate the analgesic effects of NT, NTS1 activation also produces hypotension and hypothermia, which may represent obstacles for the development of new pain medications. In the present study, we implemented various chemical strategies to improve the metabolic stability of the biologically active fragment NT(8-13) and assessed their NTS1/NTS2 relative binding affinities. We then determined their ability to reduce the nociceptive behaviors in acute, tonic, and chronic pain models and to modulate blood pressure and body temperature. To this end, we synthesized a series of NT(8-13) analogs carrying a reduced amide bond at Lys⁸-Lys⁹ and harboring site-selective modifications with unnatural amino acids, such as silaproline (Sip) and trimethylsilylalanine (TMSAla). Incorporation of Sip and TMSAla respectively in positions 10 and 13 of NT(8-13) combined with the Lys⁸-Lys⁹ reduced amine bond (JMV5296) greatly prolonged the plasma half-life time over 20 h. These modifications also led to a 25-fold peptide selectivity toward NTS2. More importantly, central delivery of JMV5296 was able to induce a strong antinociceptive effect in acute (tail-flick), tonic (formalin), and chronic inflammatory (CFA) pain models without inducing hypothermia. Altogether, these results demonstrate that the chemically-modified NT(8-13) analog JMV5296 exhibits a better therapeutic profile and may thus represent a promising avenue to guide the development of new stable NT agonists and improve pain management.

Therapeutic approaches targeting the neurotensin receptors

Introduction: Neurotensin is a gut-brain peptide hormone, a 13 amino acid neuropeptide found in the central nervous system and in the GI tract. The neurotensinergic system is implicated in various physiological and pathological processes related to neuropsychiatric and metabolic machineries, cancer growth, food, and drug intake. NT mediates its functions through its two G protein-coupled receptors: neurotensin receptor 1 (NTS1/NTSR1) and neurotensin receptor 2 (NTS2/NTSR2). Over the past decade, the role of NTS3/NTSR3/sortilin has also gained importance in human pathologies. Several approaches have appeared dealing with the discovery of compounds able to modulate the functions of this neuropeptide through its receptors for therapeutic gain.Areas covered: The article provides an overview of over four decades of research and details the drug discovery approaches and patented strategies targeting NTSR in the past decade.Expert opinion: Neurotensin is an important neurotransmitter that enables crosstalk with various neurotransmitter and neuroendocrine systems. While significant efforts have been made that have led to selective agonists and antagonists with promising in vitro and in vivo activities, the therapeutic potential of compounds targeting the neurotensinergic system is still to be fully harnessed for successful clinical translation of compounds for the treatment of several pathologies.

Bioactive Cyclization Optimizes the Affinity of a Proprotein Convertase Subtilisin/Kexin Type 9 (PCSK9) Peptide Inhibitor

Peptides are regarded as promising next-generation therapeutics. However, an analysis of over 1000 bioactive peptide candidates suggests that many have underdeveloped affinities and could benefit from cyclization using a bridging linker sequence. Until now, the primary focus has been on the use of inert peptide linkers. Here, we show that affinity can be significantly improved by enriching the linker with functional amino acids. We engineered a peptide inhibitor of PCSK9, a target for clinical management of hypercholesterolemia, to demonstrate this concept. Cyclization linker optimization from library screening produced a cyclic peptide with ∼100-fold improved activity over the parent peptide and efficiently restored low-density lipoprotein (LDL) receptor levels and cleared extracellular LDL. The linker forms favorable interactions with PCSK9 as evidenced by thermodynamics, structure-activity relationship (SAR), NMR, and molecular dynamics (MD) studies. This PCSK9 inhibitor is one of many peptides that could benefit from bioactive cyclization, a strategy that is amenable to broad application in pharmaceutical design.

Optimized Opioid-Neurotensin Multitarget Peptides: From Design to Structure-Activity Relationship Studies

Fusion of nonopioid pharmacophores, such as neurotensin, with opioid ligands represents an attractive approach for pain treatment. Herein, the μ-/δ-opioid agonist tetrapeptide H-Dmt-d-Arg-Aba-β-Ala-NH2 (KGOP01) was fused to NT(8-13) analogues. Since the NTS1 receptor has been linked to adverse effects, selective MOR-NTS2 ligands are preferred. Modifications were introduced within the native NT sequence, particularly a β3-homo amino acid in position 8 and Tyr11 substitutions. Combination of β3hArg and Dmt led to peptide 7, a MOR agonist, showing the highest NTS2 affinity described to date (Ki = 3 pM) and good NTS1 affinity (Ki = 4 nM), providing a >1300-fold NTS2 selectivity. The (6-OH)Tic-containing analogue 9 also exhibited high NTS2 affinity (Ki = 1.7 nM), with low NTS1 affinity (Ki = 4.7 μM), resulting in an excellent NTS2 selectivity (>2700). In mice, hybrid 7 produced significant and prolonged antinociception (up to 8 h), as compared to the KGOP01 opioid parent compound.

Neurotensins and their therapeutic potential: research field study

The natural tridecapeptide neurotensin has been emerged as a promising therapeutic scaffold for the treatment of neurological diseases and cancer. In this work, we aimed to identify the top 100 most cited original research papers as well as recent key studies related to neurotensins. The Web of Science Core Collection database was searched and the retrieved research articles were analyzed by using the VOSviewer software. The most cited original articles were published between 1973 and 2013. The top-cited article was by Carraway and Leeman reporting the discovery of neurotensin in 1973. The highly cited terms were associated with hypotension and angiotensin-converting-enzyme. The conducted analysis reveals the therapeutic potentials of neurotensin, and further impactful research toward its clinical development is warrantied.

Preclinical PET Imaging of NTSR-1-Positive Tumors with 64Cu- and 68Ga-DOTA-Neurotensin Analogs and Therapy with an 225Ac-DOTA-Neurotensin Analog

Background: The aim of the study was to perform PET imaging and radiotherapy with a novel neurotensin derivative for neurotensin receptor 1 (NTSR-1)-positive tumors in an animal model. Materials and Methods: A di-DOTA analog of NT(6-13) with three unnatural amino acids was synthesized and radiolabeled with either ⁶⁴Cu or ⁶⁸Ga and tested for serum stability and tumor imaging in mice bearing NTSR-1-positive PC3, and HT29 xenografts. A dose-response therapy study was performed with 18.5, 37, and 74 kBq of ²²⁵Ac-di-DOTA-α,ɛ-Lys-NT(6-13). Results: ⁶⁸Ga-di-DOTA-α,ɛ-Lys-NT(6-13) was >99% stable in serum for 48 h, had an IC₅₀ of 5 nM using ¹²⁵I labeled NT(8-13) for binding to HT-29 cells, and high uptake in tumor models expressing NTSR-1. ⁶⁸Ga-di-DOTA-α,ɛ-Lys-NT(6-13) had an average %ID/g (n = 4) at 2 h of 4.0 for tumor, 0.5 for blood, 12.0 for kidney, and <1 for other tissues, resulting in a favorable T/B of 8. Mean survivals of tumor-bearing mice treated with 18.5 or 37 kBq of ²²⁵Ac-di-DOTA-α,ɛ-Lys-NT(6-13) were 81 and 93 d, respectively, versus 53 d for controls. Whole-body toxicity was seen for the 74 kBq dose. Conclusions: Based on the results of the animal model, di-DOTA-α,ɛ-Lys-NT(6-13) is a useful imaging agent for NTSR-1-positive tumors when radiolabeled with ⁶⁸Ga, and when radiolabeled with ²²⁵Ac, a potent therapeutic agent.

Cell-penetrating pepducins targeting the neurotensin receptor type 1 relieve pain

Pepducins are cell-penetrating, membrane-tethered lipopeptides designed to target the intracellular region of a G protein-coupled receptor (GPCR) in order to allosterically modulate the receptor's signaling output. In this proof-of-concept study, we explored the pain-relief potential of a pepducin series derived from the first intracellular loop of neurotensin receptor type 1 (NTS1), a class A GPCR that mediates many of the effects of the neurotensin (NT) tridecapeptide, including hypothermia, hypotension and analgesia. We used BRET-based biosensors to determine the pepducins' ability to engage G protein signaling pathways associated with NTS1 activation. We observed partial Gα_q and Gα₁₃ activation at a 10 μM concentration, indicating that these pepducins may act as allosteric agonists of NTS1. Additionally, we used surface plasmon resonance (SPR) as a label-free assay to monitor pepducin-induced responses in CHO-K1 cells stably expressing hNTS1. This whole-cell integrated assay enabled us to subdivide our pepducin series into three profile response groups. In order to determine the pepducins' antinociceptive potential, we then screened the series in an acute pain model (tail-flick test) by measuring tail withdrawal latencies to a thermal nociceptive stimulus, following intrathecal (i.t.) pepducin administration (275 nmol/kg). We further evaluated promising pepducins in a tonic pain model (formalin test), as well as in neuropathic (Chronic Constriction Injury) and inflammatory (Complete Freund's Adjuvant) chronic pain models. We report one pepducin, PP-001, that consistently reduced rat nociceptive behaviors, even in chronic pain paradigms. Finally, we designed a TAMRA-tagged version of PP-001 and found by confocal microscopy that the pepducin reached the rat dorsal root ganglia post i.t. injection, thus potentially modulating the activity of NTS1 at this location to produce its analgesic effect. Altogether, these results suggest that NTS1-derived pepducins may represent a promising strategy in pain-relief.

Fluorescent Isoindole Crosslink (FlICk) Chemistry: A Rapid, User-friendly Stapling Reaction

The stabilization of peptide secondary structure via stapling is a ubiquitous goal for creating new probes, imaging agents, and drugs. Inspired by indole-derived crosslinks found in natural peptide toxins, we employed ortho-phthalaldehydes to create isoindole staples, thus transforming inactive linear and monocyclic precursors into bioactive monocyclic and bicyclic products. Mild, metal-free conditions give an array of macrocyclic α-melanocyte-stimulating hormone (α-MSH) derivatives, of which several isoindole-stapled α-MSH analogues (K_i ≈1 nm) are found to be as potent as α-MSH. Analogously, late-stage intra-annular isoindole stapling furnished a bicyclic peptide mimic of α-amanitin that is cytotoxic to CHO cells (IC₅₀ =70 μm). Given its user-friendliness, we have termed this approach FlICk (fluorescent isoindole crosslink) chemistry.