Design of high affinity cyclic pentapeptide ligands for kappa-opioid receptors

Peptidomimetics and Their Applications for Opioid Peptide Drug Discovery

Despite various advantages, opioid peptides have been limited in their therapeutic uses due to the main drawbacks in metabolic stability, blood-brain barrier permeability, and bioavailability. Therefore, extensive studies have focused on overcoming the problems and optimizing the therapeutic potential. Currently, numerous peptide-based drugs are being marketed thanks to new synthetic strategies for optimizing metabolism and alternative routes of administration. This tutorial review briefly introduces the history and role of natural opioid peptides and highlights the key findings on their structure-activity relationships for the opioid receptors. It discusses details on opioid peptidomimetics applied to develop therapeutic candidates for the treatment of pain from the pharmacological and structural points of view. The main focus is the current status of various mimetic tools and the successful applications summarized in tables and figures.

Novel Cyclic Biphalin Analogues by Ruthenium-Catalyzed Ring Closing Metathesis: in Vivo and in Vitro Biological Profile

In this work we report the application of the ring-closing metathesis (RCM) to the preparation of two cyclic olefin-bridged analogues of biphalin (Tyr-d-Ala-Gly-Phe-NH-NH ← Phe ← Gly ← d-Ala ← Tyr), using the second generation Grubbs' catalyst. The resulting cis- and trans-cyclic isomers were identified, fully characterized, and tested in vitro at μ (ΜΟR), δ (DOR), and κ (KOR) opioid receptors and in vivo for antinociceptive activity. Both were shown to be full agonists at MOR and potential partial antagonists at DOR, with low potency KOR agonism. They also share a strong antinociceptive effect after intracerebroventricular (i.c.v.) and intravenous (i.v.) administration, higher than that of the cyclic biphalin analogues containing a disulfide bridge between the side chains of two d-Cys or d-Pen residues, previously described by our group.

Cyclic Opioid Peptides

For decades the opioid receptors have been an attractive therapeutic target for the treatment of pain. Since the first discovery of enkephalin, approximately a dozen endogenous opioid peptides have been known to produce opioid activity and analgesia, but their therapeutics have been limited mainly due to low blood brain barrier penetration and poor resistance to proteolytic degradation. One versatile approach to overcome these drawbacks is the cyclization of linear peptides to cyclic peptides with constrained topographical structure. Compared to their linear parents, cyclic analogs exhibit better metabolic stability, lower offtarget toxicity, and improved bioavailability. Extensive structure-activity relationship studies have uncovered promising compounds for the treatment of pain as well as further elucidate structural elements required for selective opioid receptor activity. The benefits that come with employing cyclization can be further enhanced through the generation of polycyclic derivatives. Opioid ligands generally have a short peptide chain and thus the realm of polycyclic peptides has yet to be explored. In this review, a brief history of designing ligands for the opioid receptors, including classic linear and cyclic ligands, is discussed along with recent approaches and successes of cyclic peptide ligands for the receptors. Various scaffolds and approaches to improve bioavailability are elaborated and concluded with a discourse towards polycyclic peptides.

Multitarget opioid ligands in pain relief: New players in an old game

Still nowadays pain is one of the most common disabling conditions and yet it remains too often unsolved. Analgesic opioid drugs, and mainly MOR agonists such as morphine, are broadly employed for pain management. MOR activation, however, has been seen to cause not only analgesia but also undesired side effects. A potential pain treatment option is represented by the simultaneous targeting of different opioid receptors. In fact, ligands possessing multitarget capabilities led to an improved pharmacological fingerprint. This review focuses on the examination of multitarget opioid ligands which have been distinguished in peptide and non-peptide and further listed as bivalent and bifunctional ligands. Moreover, the potential of these compounds, both as analgesic drugs and pharmacological tools to explore heteromer receptors, has been stressed.

Further Optimization and Evaluation of Bioavailable, Mixed-Efficacy μ-Opioid Receptor (MOR) Agonists/δ-Opioid Receptor (DOR) Antagonists: Balancing MOR and DOR Affinities

In a previously described peptidomimetic series, we reported the development of bifunctional μ-opioid receptor (MOR) agonist and δ-opioid receptor (DOR) antagonist ligands with a lead compound that produced antinociception for 1 h after intraperitoneal administration in mice. In this paper, we expand on our original series by presenting two modifications, both of which were designed with the following objectives: (1) probing bioavailability and improving metabolic stability, (2) balancing affinities between MOR and DOR while reducing affinity and efficacy at the κ-opioid receptor (KOR), and (3) improving in vivo efficacy. Here, we establish that, through N-acetylation of our original peptidomimetic series, we are able to improve DOR affinity and increase selectivity relative to KOR while maintaining the desired MOR agonist/DOR antagonist profile. From initial in vivo studies, one compound (14a) was found to produce dose-dependent antinociception after peripheral administration with an improved duration of action of longer than 3 h.

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.

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.

Cyclic enkephalins with a diversely substituted guanidine bridge or a thiourea bridge: synthesis, biological and structural evaluations

Two series of 22 and 15 atom cyclic enkephalins incorporating a diversely substituted guanidine bridge have been prepared to assess the potential effect of the bridge substitutions on their opioid activity profile. The most notable results were obtained with the shortest cyclic analogues, which showed a significant variation of their binding affinity toward μ and δ opioid receptors in relation to bridge substitution. NMR studies were performed to rationalize these data. Some small analogues were found to exist as at least one major and one minor stable forms, which could be separated by chromatography. In particular, the compounds 13 and 14 with a cyclic substituent were separated in three isomers and the basis of this multiplicity was explored by 2D NMR spectroscopy. All compounds were agonists with slight selectivity for the μ opioid receptor. Compounds 7a (thiourea bridge) and 10a (N-Me-guanidine bridge) showed nanomolar affinity toward μ receptor, the latter being the more selective for this receptor (40-fold).

Ligand/kappa-opioid receptor interactions: insights from the X-ray crystal structure

During the past five years, the three-dimensional structures of 14 different G-protein coupled receptors (GPCRs) have been resolved by X-ray crystallography. The most recently published structures, those of the opioid receptors (ORs), are remarkably important in pain modulation, drug addiction, and mood disorders. These structures, confirmed previously proposed key interactions conferring potency and antagonistic properties, including the well-known interaction with Asp138, conserved in all aminergic GPCRs. In addition, crystallization of the opioid receptors highlighted the potential function of the ECL2 and ICL2 loops. We have previously reported a set of potent and selective kappa opioid receptor peptide agonists, of which ff(D-nle)r-NH₂ is among the most potent and selective ones. These peptides were identified from the deconvolution of a 6,250,000 tetrapeptide combinatorial library. A derivative of this set is currently the subject of a phase 2 clinical trial in the United States. In this work, we describe comparative molecular modeling studies of kappa-OR peptide agonists with the co-crystallized antagonist, JDTic, and also report structure-activity relationships of 23 tetrapeptides. The overall binding and contact interactions are sound and interactions known to favor selectivity and potency were observed. Additional modeling studies will reveal conformational changes that the kappa-OR undergoes upon binding to these peptide agonists.

Modulation of opioid receptor ligand affinity and efficacy using active and inactive state receptor models

Mu opioid receptor (MOR) agonists are widely used for the treatment of pain; however, chronic use results in the development of tolerance and dependence. It has been demonstrated that coadministration of a MOR agonist with a delta opioid receptor (DOR) antagonist maintains the analgesia associated with MOR agonists, but with reduced negative side-effects. Using our newly refined opioid receptor models for structure-based ligand design, we have synthesized several pentapeptides with tailored affinity and efficacy profiles. In particular, we have obtained pentapeptides 8, Tyr-c(S-S)[DCys-1Nal-Nle-Cys]NH(2), and 12, Tyr-c(S-S)[DCys-1Nal-Nle-Cys]OH, which demonstrates high affinity and full agonist behavior at MOR, high affinity but very low efficacy for DOR, and minimal affinity for the kappa opioid receptor (KOR). Functional properties of these peptides as MOR agonists/DOR antagonists lacking undesired KOR activity make them promising candidates for future in vivo studies of MOR/DOR interactions. Subtle structural variation of 12, by substituting D-Cys(5) for L-Cys(5), generated analog 13, which maintains low nanomolar MOR and DOR affinity, but which displays no efficacy at either receptor. These results demonstrate the power and utility of accurate receptor models for structure-based ligand design, as well as the profound sensitivity of ligand function on its structure.

Pentapeptides displaying mu opioid receptor agonist and delta opioid receptor partial agonist/antagonist properties

Chronic use of mu-opioid agonists has been shown to cause neurochemical adaptations resulting in tolerance and dependence. While the analgesic effects of these drugs are mediated by mu-opioid receptors (MOR), several studies have shown that antagonism or knockdown of delta-opioid receptors (DOR) can lessen or prevent development of tolerance and dependence. On the basis of computational modeling of putative active and inactive conformations of MOR and DOR, we have synthesized a series of pentapeptides with the goal of developing a MOR agonist/DOR antagonist peptide with similar affinity at both receptors as a tool to probe functional opioid receptor interaction(s). The eight resulting naphthylalanine-substituted cyclic pentapeptides displayed variable mixed-efficacy profiles. The most promising peptide (9; Tyr-c(S-CH(2)-S)[D-Cys-Phe-2-Nal-Cys]NH(2)) displayed a MOR agonist and DOR partial agonist/antagonist profile and bound with equipotent affinity (K(i) approximately 0.5 nM) to both receptors, but also showed kappa opioid receptor (KOR) agonist activity.

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

The opioid peptide H-Tyr-c[D-Cys-Phe-Phe-Cys]NH(2) cyclized via a methylene dithiother is a potent and selective mu opioid agonist (Przydial M.J. et al., J Peptide Res, 66, 2005, 255). Dicarba analogues of this peptide with Tyr, 2',6'-dimethyltyrosine (Dmt), 3-[2,6-dimethyl-4-hydroxyphenyl)propanoic acid (Dhp) or (2S)-2-methyl-3-(2,6-dimethyl-4-hydroxyphenyl)propanoic acid [(2S)-Mdp] in the 1-position were prepared. The peptides were synthesized on solid-phase by substituting d-allylglycine and (2S)-2-amino-5-hexenoic acid in position 2 and 5, respectively, followed by ring-closing metathesis. Mixtures of cis and trans isomers of the resulting olefinic peptides were obtained, and catalytic hydrogenation yielded the saturated -CH(2)-CH(2)- bridged peptides. All six Tyr(1)- and Dmt(1)-dicarba analogues retained high mu and delta opioid agonist potency and showed only slight or no preference for mu over delta receptors. As expected, the six Dhp(1)- and (2S)-Mdp(1)-dicarba analogues turned out to be mu opioid antagonists but, surprisingly, displayed a range of different efficacies (agonism, partial agonism or antagonism) at the delta receptor. The obtained results indicate that the mu versus delta receptor selectivity and the efficacy at the delta receptor of these cyclic peptides depend on distinct conformational characteristics of the 15-membered peptide ring structure, which may affect the spatial positioning of the exocyclic residue and of the Phe(3) and Phe(4) side chains.

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 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, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).