Cyclic opioid peptide agonists and antagonists obtained via ring-closing metathesis

A Bicyclic Analog of the Linear Peptide Arodyn Is a Potent and Selective Kappa Opioid Receptor Antagonist

Kappa opioid receptor (KOR) antagonists have potential therapeutic applications in the treatment of stress-induced relapse to substance abuse and mood disorders. The dynorphin A analog arodyn (Ac[Phe1,2,3,Arg4,D-Ala8]dynorphin A-(1-11)-NH2) exhibits potent and selective kappa opioid receptor antagonism. Multiple cyclizations in longer peptides, such as dynorphin and its analogs, can extend the conformational constraint to additional regions of the peptide beyond what is typically constrained by a single cyclization. Here, we report the design, synthesis, and pharmacological evaluation of a bicyclic arodyn analog with two constraints in the opioid peptide sequence. The peptide, designed based on structure-activity relationships of monocyclic arodyn analogs, was synthesized by solid-phase peptide synthesis and cyclized by sequential ring-closing metathesis (RCM) in the C- and N-terminal sequences. Molecular modeling studies suggest similar interactions of key aromatic and basic residues in the bicyclic peptide with KOR as found in the cryoEM structure of KOR-bound dynorphin, despite substantial differences in the backbone conformations of the two peptides. The bicyclic peptide's affinities at KOR and mu opioid receptors (MOR) were determined in radioligand binding assays, and its KOR antagonism was determined in the [35S]GTPγS assay in KOR-expressing cells. The bicyclic analog retains KOR affinity and selectivity (Ki = 26 nM, 97-fold selectivity over MOR) similar to arodyn and exhibits potent KOR antagonism in the dynorphin-stimulated [35S]GTPγS assay. This bicyclic peptide represents a promising advance in preparing cyclic opioid peptide ligands and opens avenues for the rational design of additional bicyclic opioid peptide analogs.

Antinociceptive activity of thiazole-containing cyclized DAMGO and Leu-(Met) enkephalin analogs

Numerous studies demonstrate the promise of opioid peptides as analgesics, but poor oral bioavailability has limited their therapeutic development. This study sought to increase the oral bioavailability of opioid peptides by cyclization, using Hantzsch-based macrocyclization strategies to produce two new series of cyclized DAMGO and Leu/Met-enkephalin analogs. Opioid receptor affinity and selectivity for compounds in each series were assessed in vitro with radioligand competition binding assays. Compounds demonstrated modest affinity but high selectivity for the mu, delta, and kappa opioid receptors (MOR, DOR and KOR), while selectivity for mu opioid receptors varied by structure. Antinociceptive activity of each compound was initially screened in vivo following intracerebroventricular (i.c.v.) administration and testing in the mouse 55 °C warm-water tail-withdrawal test. The four most active compounds were then evaluated for dose- and time-dependent antinociception, and opioid receptor selectivity in vivo. Cyclic compounds 1924-10, 1936-1, 1936-7, and 1936-9 produced robust and long- lasting antinociception with ED₅₀ values ranging from 0.32-0.75 nmol following i.c.v. administration mediated primarily by mu- and delta-opioid receptor agonism. Compounds 1924-10, 1936-1 and 1936-9 further displayed significant time-dependent antinociception after oral (10 mg kg^-1, p.o.) administration. A higher oral dose (30 mg kg^-1. p.o.) of all four cyclic peptides also reduced centrally-mediated respiration, suggesting successful penitration into the CNS. Overall, these data suggest cyclized opioid peptides synthesized by a Hantzsch-based macrocyclization strategy can retain opioid agonist activity to produce potent antinociception in vivo while conveying improved bioavailability following oral administration.

On resin click-chemistry-mediated synthesis of novel enkephalin analogues with potent anti-nociceptive activity

Here, we report the chemical synthesis of two DPDPE analogues 7a (NOVA1) and 7b (NOVA2). This entailed the solid-phase synthesis of two enkephalin precursor chains followed by a Cu^I-catalyzed azide-alkyne cycloaddition, with the aim of improving in vivo analgesic efficacy versus DPDPE. NOVA2 showed good affinity and selectivity for the μ-opioid receptor (K_I of 59.2 nM, EC₅₀ of 12.9 nM, E_Max of 87.3%), and long lasting anti-nociceptive effects in mice when compared to DPDPE.

Tryptophan-Containing Non-Cationizable Opioid Peptides - a new chemotype with unusual structure and in vivo activity

Recently, a new family of opioid peptides containing tryptophan came to the spotlight for the absence of the fundamental protonable tyramine 'message' pharmacophore. Structure-activity relationship investigations led to diverse compounds, characterized by different selectivity profiles and agonist or antagonist effects. Substitution at the indole of Trp clearly impacted peripheral/central antinociceptivity. These peculiarities prompted to gather all the compounds in a new class, and to coin the definition 'Tryptophan-Containing Non-Cationizable Opioid Peptides', in short 'TryCoNCOPs'. Molecular docking analysis suggested that the TryCoNCOPs can still interact with the receptors in an agonist-like fashion. However, most TryCoNCOPs showed significant differences between the in vitro and in vivo activities, suggesting that opioid activity may be elicited also via alternative mechanisms.

Stereoselective synthesis and structural elucidation of dicarba peptides

A facile stereoselective synthesis of cis and trans unsaturated dicarba peptides has been established using preformed diaminosuberic acid derivatives as bridging units. In addition, characteristic spectral differences in the (13)C-NMR spectra of the cis- and trans-isomers show that the chemical shift of carbons in the Δ4,5-diaminosuberic acid residue can be used to assign stereochemistry in unsaturated dicarba peptides formed from ring closing metathesis of linear peptide sequences.

Macrocyclic Inhibitors of GPCR's, Integrins and Protein–Protein Interactions

This chapter summarizes some highlights of macrocyclic drug discovery in the area of GPCRs, integrins, and protein–protein interactions spanning roughly the last 30 years. Several examples demonstrate that incorporation of pharmacophores derived from natural peptide ligands into the context of a constrained macrocycle (“lock of the bioactive conformation”) has proven a powerful approach for the discovery of potent and selective macrocyclic drugs. In addition, it will be shown that macrocycles, due to their semi-rigid nature, can exhibit unique properties that can be beneficially exploited by medicinal chemists. Macrocycles can adapt their conformation during binding to a flexible protein target surface (“induced fit”), and due to their size, can interact with larger protein interfaces (“hot spots”). Also, macrocycles can display favorable ADME properties well beyond the rule of 5 in particular exhibiting favorable cell penetrating properties and oral bioavailability.

'Carba'-carfentanil (trans isomer): a μ opioid receptor (MOR) partial agonist with a distinct binding mode

There is strong evidence to indicate that a positively charged nitrogen of endogenous and exogenous opioid ligands forms a salt bridge with the Asp residue in the third transmembrane helix of opioid receptors. To further examine the role of this electrostatic interaction in opioid receptor binding and activation, we synthesized 'carba'-analogues of the highly potent μ opioid analgesic carfentanil (3), in which the piperidine nitrogen was replaced with a carbon. The resulting trans isomer (8b) showed reduced, but still significant MOR binding affinity (Ki(μ)=95.2nM) with no MOR versus DOR binding selectivity and was a MOR partial agonist. The cis isomer (8a) was essentially inactive. A MOR docking study indicated that 8b bound to the same binding pocket as parent 3, but its binding mode was somewhat different. A re-evaluation of the uncharged morphine derivative N-formylnormorphine (9) indicated that it was a weak MOR antagonist showing no preference for MOR over KOR. Taken together, the results indicate that deletion of the positively charged nitrogen in μ opioid analgesics reduces MOR binding affinity by 2-3 orders of magnitude and may have pronounced effects on the intrinsic efficacy and on the opioid receptor selectivity profile.

Investigational peptide and peptidomimetic μ and δ opioid receptor agonists in the relief of pain

INTRODUCTION

Current methods for treating prolonged and neuropathic pain are inadequate and lead to toxicities that greatly diminish quality of life. Therefore, new approaches to the treatment of pain states are needed to address these problems.

AREAS COVERED

The review primarily reviews approaches that have been taken in the peer-reviewed literature of multivalent ligands that interact with both μ and δ opioid receptors as agonists, and in some cases, also with pharmacophores for antagonist ligands that interact with other receptors as antagonists to block pain.

EXPERT OPINION

Although there are a number of drugs currently on the market for the treatment of pain; none of them are 100% successful. In the authors' opinion, it is clear that new directions and modalities are needed to better address the treatment of prolonged and neuropathic pain; one drug or class clearly is not the answer for all pain therapy. Undoubtedly, there are many different phenotypes of prolonged and neuropathic pain and this should be one avenue to further develop appropriate therapies.

N-terminal guanidinylation of TIPP (Tyr-Tic-Phe-Phe) peptides results in major changes of the opioid activity profile

Derivatives of peptides of the TIPP (Tyr-Tic-Phe-Phe; Tic=1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) family containing a guanidino (Guan) function in place of the N-terminal amino group were synthesized in an effort to improve their blood-brain barrier permeability. Unexpectedly, N-terminal amidination significantly altered the in vitro opioid activity profiles. Guan-analogues of TIPP-related δ opioid antagonists showed δ partial agonist or mixed δ partial agonist/μ partial agonist activity. Guanidinylation of the mixed μ agonist/δ antagonists H-Dmt-Tic-Phe-Phe-NH2 (DIPP-NH2) and H-Dmt-TicΨ[CH2NH]Phe-Phe-NH2 (DIPP-NH2[Ψ]) converted them to mixed μ agonist/δ agonists. A docking study revealed distinct positioning of DIPP-NH2 and Guan-DIPP-NH2 in the δ receptor binding site. Lys(3)-analogues of DIPP-NH2 and DIPP-NH2[Ψ] (guanidinylated or non-guanidinylated) turned out to be mixed μ/κ agonists with δ antagonist-, δ partial agonist- or δ full agonist activity. Compounds with some of the observed mixed opioid activity profiles have therapeutic potential as analgesics with reduced side effects or for treatment of cocaine addiction.

Hantzsch based macrocyclization approach for the synthesis of thiazole containing cyclopeptides

An innovative macrocyclization approach via high-yielding solid-phase intramolecular thioalkylation reaction is described. The reaction of S-nucleophiles with newly generated N-terminal 4-chloromethyl thiazoles leads to the desired cyclic products in high purities and good yields.

Opioid activity profiles of oversimplified peptides lacking in the protonable N-terminus

Recently, we described cyclopeptide opioid agonists containing the d-Trp-Phe sequence. To expand the scope of this atypical pharmacophore, we tested the activity profiles of the linear peptides Ac-Xaa-Phe-Yaa (Xaa = l/d-Trp, d-His/Lys/Arg; Yaa = H, GlyNH(2)). Ac-d-Trp-PheNH(2) appeared to be the minimal binding sequence, while Ac-d-Trp-Phe-GlyNH(2) emerged as the first noncationizable short peptide (partial) agonist with high μ-opioid receptor affinity and selectivity. Conformational analysis suggested that 5 adopts in solution a β-turn conformation.

Novel TIPP (H-Tyr-Tic-Phe-Phe-OH) analogues displaying a wide range of efficacies at the δ opioid receptor. Discovery of two highly potent and selective δ opioid agonists

Analogues of the δ opioid antagonist peptide TIPP (H-Tyr-Tic-Phe-Phe-OH; Tic=1,2,3,4-tetrahydroisoquinoline3-carboxylic acid) containing various 4'-[N-(alkyl or aralkyl)carboxamido]phenylalanine analogues in place of Tyr(1) were synthesized. The compounds showed subnanomolar or low nanomolar δ opioid receptor binding affinity and various efficacy at the δ receptor (antagonism, partial agonism, full agonism) in the [(35)S]GTPγS binding assay. Two analogues, [1-Ncp(1)]TIPP (1-Ncp=4'-[N-(2-(naphthalene-1-yl)ethyl)carboxamido]phenylalanine) and [2-Ncp(1)]TIPP (2-Ncp=4'-[N-(2-(naphthalene-2-yl)ethyl)carboxamido]phenylalanine), were identified as potent and selective δ opioid agonists.

Endogenous opiates and behavior: 2009

This paper is the 32nd consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2009 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Two-Steps Hantzsch Based Macrocyclization Approach for the Synthesis of Thiazole Containing Cyclopeptides

Macrocyclization via an efficient high-yielding solid-phase intramolecular thioalkylation reaction is described. The reaction of S-nucleophiles with newly generated N-terminal 4-chloromethyl thiazoles led to the desired macrocyclization products 5 in high purities and good overall yields.

Two-step Hantzsch based macrocyclization approach for the synthesis of thiazole-containing cyclopeptides

Macrocyclization via an efficient high-yielding solid-phase intramolecular thioalkylation reaction is described. The reaction of S-nucleophiles with newly generated N-terminal 4-chloromethyl thiazoles led to the desired macrocyclization products 5 in high purities and good overall yields.

Structural and pharmacological effects of ring-closing metathesis in peptides

Applications of ring-closing alkene metathesis (RCM) in acyclic α- and β-peptides and closely related systems are reviewed, with a special emphasis on the structural and pharmacological effects of cyclization by RCM.

"Carba"-analogues of fentanyl are opioid receptor agonists

There is evidence to indicate that the Asp residue in the third transmembrane helix (TMH) of opioid receptors forms a salt bridge with the positively charged nitrogen of endogenous and exogenous opioid ligands. To further examine the role of this electrostatic interaction in receptor binding and activation, we synthesized "carba"-analogues of a published fentanyl analogue containing a 3-(guanidinomethyl)-benzyl group in place of the phenyl moiety attached to the ethylamido group (C. Dardonville et al., Bioorg. Med. Chem. 2006, 14, 6570-6580 (1)), in which the piperidine ring nitrogen was replaced with a carbon. As expected, the resulting cis and trans isomers (8a and 8b) showed reduced mu and kappa opioid receptor binding affinities as compared to 1 but, surprisingly, retained opioid full agonist activity with about half the potency of leucine-enkephalin in the guinea pig ileum assay. In conjunction with performed receptor docking studies, these results indicate that the electrostatic interaction of the protonated nitrogen in the piperidine ring of fentanyl analogues with the Asp residue in the third TMH is not a conditio sine qua non for opioid receptor activation.