ASP147 in the third transmembrane helix of the rat mu opioid receptor forms ion-pairing with morphine and naltrexone

Effects of pH on opioid receptor activation and implications for drug design

G-protein-coupled receptors are integral membrane proteins that transduce chemical signals from the extracellular matrix into the cell. Traditional drug design has considered ligand-receptor interactions only under normal conditions. However, studies on opioids indicate that such interactions are very different in diseased tissues. In such microenvironments, protons play an important role in structural and functional alterations of both ligands and receptors. The pertinent literature strongly suggests that future drug design should take these aspects into account in order to reduce adverse side effects while preserving desired effects of novel compounds.

N-substituted tetrahydro-beta-carboline as mu-opioid receptors ligands: in silico study; molecular docking, ADMET and molecular dynamics approach

Manipulating intracellular signals by interaction with transmembranal G-protein-coupled receptors (GPCRs) is the way of action of more than 30% of available medicines. Designing molecules against GPCRs is most challenging due to their flexible binding orthosteric and allosteric pockets, a property that lead to different mode and extent of activation of intracellular mediators. Here, in the current study we aimed to design N-substituted tetrahydro-beta-carbolines (THβC's) targeting Mu Opioid Receptors (MORs). We performed ligand docking study for reference and designed compounds against active and inactive states of MOR, as well as the active state bound to intracellular mediator of Gi. The reference compounds include 40 known agonists and antagonists, while the designed compounds include 25,227 N-substituted THβC analogues. Out of the designed compounds, 15 compounds were comparatively having better extra precision (XP) Gscore and were analyzed for absorption, distribution, metabolism, and excretion-toxicity (ADMET) properties, drug-likness, and molecular dynamic (MD) simulation. The results showed that N-substituted tetrahydro-beta-carbolines with and without C6-methoxy group substitutions (THBC/6MTHBC) analogues of A1/B1 and A9/B9 have relatively acceptable affinity and within pocket-stability toward MOR compared to the reference compounds of morphine (agonist) and naloxone (antagonist). Moreover, the designed analogues interact with key residue within the binding pocket of Asp 147 that is reported to be involved in receptor activation. In conclusion, the designed THBC analogues represent a good starting point for designing opioid receptor ligands other than morphinan scaffold, that have good synthetic accessibility which promotes feasible structural manipulation to tailor pharmacological effects with minimal side effects.

Applications of biaryl cyclization in the synthesis of cyclic enkephalin analogs with a highly restricted flexibility

A series of 10 cyclic, biaryl analogs of enkephalin, with Tyr or Phe residues at positions 1 and 4, were synthesized according to the Miyaura borylation and Suzuki coupling methodology. Biaryl bridges formed by side chains of the two aromatic amino acid residues are of the meta-meta, meta-para, para-meta, and para-para configuration. Conformational properties of the peptides were studied by CD and NMR. CD studies allowed only to compare conformations of individual peptides while NMR investigations followed by XPLOR calculations provided detailed information on their conformation. Reliability of the XPLOR calculations was confirmed by quantum chemical ones performed for one of the analogs. No intramolecular hydrogen bonds were found in all the peptides. They are folded and adopt the type IV β-turn conformation. Due to a large steric strain, the aromatic carbon atoms forming the biaryl bond are distinctly pyramidalized. Seven of the peptides were tested in vitro for their affinity for the µ-opioid receptor.

Carfentanil is a β-arrestin-biased agonist at the μ opioid receptor

BACKGROUND AND PURPOSE

The illicit use of fentanyl-like drugs (fentanyls), which are μ opioid receptor agonists, and the many overdose deaths that result, has become a major problem. Fentanyls are very potent in vivo, leading to respiratory depression and death. However, the efficacy and possible signalling bias of different fentanyls is not clearly known. Here, we compared the relative efficacy and bias of a series of fentanyls.

EXPERIMENTAL APPROACH

For agonist signalling bias and efficacy measurements, Bioluminescence Resonance Energy Transfer experiments were undertaken in HEK293T cells transiently transfected with μ opioid receptors, to assess Gi protein activation and β-arrestin 2 recruitment. Agonist-induced cell surface receptor loss was assessed using an enzyme-linked immunosorbent assay, whilst agonist-induced G protein-coupled inwardly rectifying potassium channel current activation was measured electrophysiologically from rat locus coeruleus slices. Ligand poses in the μ opioid receptor were determined in silico using molecular dynamics simulations.

KEY RESULTS

Relative to the reference ligand DAMGO, carfentanil was β-arrestin-biased, whereas fentanyl, sufentanil and alfentanil did not display bias. Carfentanil induced potent and extensive cell surface receptor loss, whilst the marked desensitisation of G protein-coupled inwardly rectifying potassium channel currents in the continued presence of carfentanil in neurones was prevented by a GRK2/3 inhibitor. Molecular dynamics simulations suggested unique interactions of carfentanil with the orthosteric site of the receptor that could underlie the bias.

CONCLUSIONS AND IMPLICATIONS

Carfentanil is a β-arrestin-biased opioid drug at the μ receptor. It is uncertain how such bias influences in vivo effects of carfentanil relative to other fentanyls.

Fentanyl and the Fluorinated Fentanyl Derivative NFEPP Elicit Distinct Hydrogen-Bond Dynamics of the Opioid Receptor

The development of safe therapeutics to manage pain is of central interest for biomedical applications. The fluorinated fentanyl derivative N-(3-fluoro-1-phenethylpiperidin-4-yl)-N-phenylpropionamide (NFEPP) is potentially a safer alternative to fentanyl because unlike fentanyl─which binds to the μ-opioid receptor (MOR) at both physiological and acidic pH─NFEPP might bind to the MOR only at acidic pH typical of inflamed tissue. Knowledge of the protonation-coupled dynamics of the receptor-drug interactions is thus required to understand the molecular mechanism by which receptor activation initiates cell signaling to silence pain. To this end, here we have carried out extensive atomistic simulations of the MOR in different protonation states, in the absence of opioid drugs, and in the presence of fentanyl vs NFEPP. We used graph-based analyses to characterize internal hydrogen-bond networks that could contribute to the activation of the MOR. We find that fentanyl and NFEPP prefer distinct binding poses and that, in their binding poses, fentanyl and NFEPP partake in distinct internal hydrogen-bond networks, leading to the cytoplasmic G-protein-binding region. Moreover, the protonation state of functionally important aspartic and histidine side chains impacts hydrogen-bond networks that extend throughout the receptor, such that the ligand-bound MOR presents at its cytoplasmic G-protein-binding side, a hydrogen-bonding environment where dynamics depend on whether fentanyl or NFEPP is bound, and on the protonation state of specific MOR groups. The exquisite sensitivity of the internal protein-water hydrogen-bond network to the protonation state and to details of the drug binding could enable the MOR to elicit distinct pH- and opioid-dependent responses at its cytoplasmic G-protein-binding site.

Modelling altered signalling of G-protein coupled receptors in inflamed environment to advance drug design

We previously reported the successful design, synthesis and testing of the prototype opioid painkiller NFEPP that does not elicit adverse side effects. The design process of NFEPP was based on mathematical modelling of extracellular interactions between G-protein coupled receptors (GPCRs) and ligands, recognizing that GPCRs function differently under pathological versus healthy conditions. We now present an additional and novel stochastic model of GPCR function that includes intracellular dissociation of G-protein subunits and modulation of plasma membrane calcium channels and their dependence on parameters of inflamed and healthy tissue (pH, radicals). The model is validated against in vitro experimental data for the ligands NFEPP and fentanyl at different pH values and radical concentrations. We observe markedly reduced binding affinity and calcium channel inhibition for NFEPP at normal pH compared to lower pH, in contrast to the effect of fentanyl. For increasing radical concentrations, we find enhanced constitutive G-protein activation but reduced ligand binding affinity. Assessing the different effects, the results suggest that, compared to radicals, low pH is a more important determinant of overall GPCR function in an inflamed environment. Future drug design efforts should take this into account.

Molecular Basis of Inhibitory Mechanism of Naltrexone and Its Metabolites through Structural and Energetic Analyses

Naltrexone is a potent opioid antagonist with good blood–brain barrier permeability, targeting different endogenous opioid receptors, particularly the mu-opioid receptor (MOR). Therefore, it represents a promising candidate for drug development against drug addiction. However, the details of the molecular interactions of naltrexone and its derivatives with MOR are not fully understood, hindering ligand-based drug discovery. In the present study, taking advantage of the high-resolution X-ray crystal structure of the murine MOR (mMOR), we constructed a homology model of the human MOR (hMOR). A solvated phospholipid bilayer was built around the hMOR and submitted to microsecond (µs) molecular dynamics (MD) simulations to obtain an optimized hMOR model. Naltrexone and its derivatives were docked into the optimized hMOR model and submitted to µs MD simulations in an aqueous membrane system. The MD simulation results were submitted to the molecular mechanics–generalized Born surface area (MMGBSA) binding free energy calculations and principal component analysis. Our results revealed that naltrexone and its derivatives showed differences in protein–ligand interactions; however, they shared contacts with residues at TM2, TM3, H6, and TM7. The binding free energy and principal component analysis revealed the structural and energetic effects responsible for the higher potency of naltrexone compared to its derivatives.

Modulating β-arrestin 2 recruitment at the δ- and μ-opioid receptors using peptidomimetic ligands

μ-Opioid receptor agonists provide potent and effective acute analgesia; however, their therapeutic window narrows considerably upon repeated administration, such as required for treating chronic pain. In contrast, bifunctional μ/δ opioid agonists, such as the endogenous enkephalins, have potential for treating both acute and chronic pain. However, enkephalins recruit β-arrestins, which correlate with certain adverse effects at μ- and δ-opioid receptors. Herein, we identify the C-terminus of Tyr-ψ[(Z)CF[double bond, length as m-dash]CH]-Gly-Leu-enkephalin, a stable enkephalin derivative, as a key site to regulate bias of both δ- and μ-opioid receptors. Using in vitro assays, substitution of the Leu⁵ carboxylate with amides (NHEt, NMe₂, N^CyPr) reduced β-arrestin recruitment efficacy through both the δ-opioid and μ-opioid, while retaining affinity and cAMP potency. For this series, computational studies suggest key ligand-receptor interactions that might influence bias. These findings should enable the discovery of a range of tool compounds with previously unexplored biased μ/δ opioid agonist pharmacological profiles.

CHARMM Force-Field Parameters for Morphine, Heroin, and Oliceridine, and Conformational Dynamics of Opioid Drugs

Opioid drug binding to specialized G protein-coupled receptors (GPCRs) can lead to analgesia upon activation via downstream G_i protein signaling and to severe side effects via activation of the β-arrestin signaling pathway. Knowledge of how different opioid drugs interact with receptors is essential, as it can inform and guide the design of safer therapeutics. We performed quantum and classical mechanical computations to explore the potential energy landscape of four opioid drugs: morphine and its derivatives heroin and fentanyl and for the unrelated oliceridine. From potential energy profiles for bond twists and from interactions between opioids and water, we derived a set of force-field parameters that allow a good description of structural properties and intermolecular interactions of the opioids. Potential of mean force profiles computed from molecular dynamics simulations indicate that fentanyl and oliceridine have complex energy landscapes with relatively small energy penalties, suggesting that interactions with the receptor could select different binding poses of the drugs.

Structural Assessment of Agonist Efficacy in the μ-Opioid Receptor: Morphine and Fentanyl Elicit Different Activation Patterns

Over the past two decades, the opioid epidemic in the United States and Canada has evidenced the need for a better understanding of the molecular mechanisms of medications used to fight pain. Morphine and fentanyl are widely used in opiate-mediated analgesia for the treatment of chronic pain. These compounds target the μ-opioid receptor (MOR), a class A G protein-coupled receptor (GPCR). In light of described higher efficacy of fentanyl with respect to morphine, we have performed independent μs-length unbiased molecular dynamics (MD) simulations of MOR complexes with each of these ligands, including the MOR antagonist naltrexone as a negative control. Consequently, MD simulations totaling 58 μs have been conducted to elucidate at the atomic level ligand-specific receptor activity and signal transmission in the MOR. In particular, we have identified stable binding poses of morphine and fentanyl, which interact differently with the MOR. Different ligand-receptor interaction landscapes directly induce sidechain conformational changes of orthosteric pocket residues: Asp149^3.32, Tyr150^3.33, Gln126^2.60, and Lys235^5.39. The induced conformations determine Asp149^3.32-Tyr328^7.43 sidechain-sidechain interactions and Trp295^6.48-Ala242^5.46 sidechain-backbone H-bond formations, as well as Met153^3.36 conformational changes. In addition to differences in ligand binding, different intracellular receptor conformational changes are observed as morphine preferentially activates transmembrane (TM) helices: TM3 and TM5, while fentanyl preferentially activates TM6 and TM7. As conformational changes in TM6 and TM7 are widely described as being the most crucial aspect in GPCR activation, this may contribute to the greater efficacy of fentanyl over morphine. These computationally observed functional differences between fentanyl and morphine may provide new avenues for the design of safer but not weaker opioid drugs because it is desirable to increase the safety of medicines without sacrificing their efficacy.

In Vivo, In Vitro and In Silico Studies of the Hybrid Compound AA3266, an Opioid Agonist/NK1R Antagonist with Selective Cytotoxicity

AA3266 is a hybrid compound consisting of opioid receptor agonist and neurokinin-1 receptor (NK1R) antagonist pharmacophores. It was designed with the desire to have an analgesic molecule with improved properties and auxiliary anticancer activity. Previously, the compound was found to exhibit high affinity for μ- and δ-opioid receptors, while moderate binding to NK1R. In the presented contribution, we report on a deeper investigation of this hybrid. In vivo, we have established that AA3266 has potent antinociceptive activity in acute pain model, comparable to that of morphine. Desirably, with prolonged administration, our hybrid induces less tolerance than morphine does. AA3266, contrary to morphine, does not cause development of constipation, which is one of the main undesirable effects of opioid use. In vitro, we have confirmed relatively strong cytotoxic activity on a few selected cancer cell lines, similar to or greater than that of a reference NK1R antagonist, aprepitant. Importantly, our compound affects normal cells to smaller extent what makes our compound more selective against cancer cells. In silico methods, including molecular docking, molecular dynamics simulations and fragment molecular orbital calculations, have been used to investigate the interactions of AA3266 with MOR and NK1R. Insights from these will guide structural optimization of opioid/antitachykinin hybrid compounds.

Pharmacological properties and biochemical mechanisms of μ-opioid receptor ligands might be due to different binding poses: MD studies

Background: Central and peripheral analgesia without adverse effects relies on the identification of μ-opioid agonists that are able to activate 'basal' antinociceptive pathways. Recently developed μ-selective benzomorphan agonists that are not antagonized by naloxone do not activate G-proteins and β-arrestins. Which pathways do μ receptors activate? How can each of them be selectively activated? What role is played by allosteric binding sites? Methodology & results: Molecular modeling studies characterize the amino acid residues involved in the interaction with various classes of endogenous and exogenous ligands and with agonists and antagonists. Conclusions: Critical binding differences between various classes of agonists with different pharmacological profiles have been identified. MML series binding poses may be relevant in the search for an antinociception agent without side effects.

Molecular insights into the interaction of hemorphin and its targets

Hemorphins are atypical endogenous opioid peptides produced by the cleavage of hemoglobin beta chain. Several studies have reported the therapeutic potential of hemorphin in memory enhancement, blood regulation, and analgesia. However, the mode of interaction of hemorphin with its target remains largely elusive. The decapeptide LVV-hemorphin-7 is the most stable form of hemorphin. It binds with high affinity to mu-opioid receptors (MOR), angiotensin-converting enzyme (ACE) and insulin-regulated aminopeptidase (IRAP). In this study, computational methods were used extensively to elucidate the most likely binding pose of mammalian LVV-hemorphin-7 with the aforementioned proteins and to calculate the binding affinity. Additionally, alignment of mammalian hemorphin sequences showed that the hemorphin sequence of the camel harbors a variation - a Q > R substitution at position 8. This study also investigated the binding affinity and the interaction mechanism of camel LVV-hemorphin-7 with these proteins. To gain a better understanding of the dynamics of the molecular interactions between the selected targets and hemorphin peptides, 100 ns molecular dynamics simulations of the best-ranked poses were performed. Simulations highlighted major interactions between the peptides and key residues in the binding site of the proteins. Interestingly, camel hemorphin had a higher binding affinity and showed more interactions with all three proteins when compared to the canonical mammalian LVV-hemorphin-7. Thus, camel LVV-hemorphin-7 could be explored as a potent therapeutic agent for memory loss, hypertension, and analgesia.

Understanding Peptide Binding in Class A G Protein-Coupled Receptors

Many physiologic processes are controlled through the activation of G protein-coupled receptors (GPCRs) by regulatory peptides, making peptide GPCRs particularly useful targets for major human diseases such as diabetes and cancer. Peptide GPCRs are also being evaluated as next-generation targets for the development of novel antiparasite agents and insecticides in veterinary medicine and agriculture. Resolution of crystal structures for several peptide GPCRs has advanced our understanding of peptide-receptor interactions and fueled interest in correlating peptide heterogeneity with receptor-binding properties. In this review, the knowledge of recently crystalized peptide-GPCR complexes, previously accumulated peptide structure-activity relationship studies, receptor mutagenesis, and sequence alignment are integrated to better understand peptide binding to the transmembrane cavity of class A GPCRs. Using SAR data, we show that peptide class A GPCRs can be divided into groups with distinct hydrophilic residues. These characteristic residues help explain the preference of a receptor to bind the C-terminal free carboxyl group, the C-terminal amidated group, or the N-terminal ammonium group of peptides.

[Pain inhibition by opioids-new concepts]

BACKGROUND

Opioids are the oldest and most potent drugs for the treatment of severe pain but they are burdened by detrimental side effects, such as respiratory depression, addiction potential, sedation, nausea and constipation. Their clinical application is undisputed in the treatment of acute (e.g. perioperative) and cancer pain but their long-term use in chronic pain has met increasing criticism and has contributed to the current "opioid crisis".

OBJECTIVES

This article reviews the pharmacological principles and new research strategies aiming at novel opioids with reduced side effects. The basic mechanisms underlying pain and opioid analgesia and other effects of opioids are outlined. To illustrate the clinical situation and medical problems, the plasticity of opioid receptors, intracellular signaling pathways, endogenous and exogenous opioid receptor ligands, central and peripheral sites of analgesic and side effects are discussed.

CONCLUSION

The epidemic of opioid misuse has shown that there is a lack of fundamental knowledge about the characteristics and management of chronic pain, that conflicts of interest and validity of models must be more intensively considered in the context of drug development and that novel analgesics with less addictive potential are urgently needed. Currently, the most promising perspectives appear to be augmenting endogenous opioid actions and the selective activation of peripheral opioid receptors.

Molecular modeling of salsolinol, a full Gi protein agonist of the μ-opioid receptor, within the receptor binding site

(R/S)-Salsolinol is a full agonist of the μ-opioid receptor (μOR) G_i protein pathway via its (S)-enantiomer and is functionally selective as it does not promote β-arrestin recruitment. Compared to (S)-salsolinol, the (R)-enantiomer is a less potent agonist of the G_i protein pathway. We have now studied the interactions of the salsolinol enantiomers docked in the binding pocket of the μOR to determine the molecular interactions that promote enantiomeric specificity and functional selectivity of (R/S)-salsolinol. Molecular dynamics simulations showed that (S)-salsolinol interacted with 8 of the 11 residues of the μOR binding site, enough to stabilize the molecule. (R)-Salsolinol showed higher mobility with fewer prevalent bonds. Hence, the methyl group bound to the (S)-stereogenic center promoted more favorable interactions in the μOR binding site than in the (R)-orientation. Because (S)-salsolinol is a small molecule (179.2 Da), it did not interact with residues implicated in the binding of larger morphinan agonists that are located toward the extracellular portion of the binding pocket: W318^7.35 , I322^7.39 , and Y326^7.43 . Our results suggest that contact with residues which (S)-salsolinol interacts with are enough to elicit G_i protein activation, and possibly define a minimum set required by μOR ligands to promote activation of the G_i protein pathway.

Fentanyl Family at the Mu-Opioid Receptor: Uniform Assessment of Binding and Computational Analysis

Interactions of 21 fentanyl derivatives with μ-opioid receptor (μOR) were studied using experimental and theoretical methods. Their binding to μOR was assessed with radioligand competitive binding assay. A uniform set of binding affinity data contains values for two novel and one previously uncharacterized derivative. The data confirms trends known so far and thanks to their uniformity, they facilitate further comparisons. In order to provide structural hypotheses explaining the experimental affinities, the complexes of the studied derivatives with μOR were modeled and subject to molecular dynamics simulations. Five common General Features (GFs) of fentanyls' binding modes stemmed from these simulations. They include: GF1) the ionic interaction between D147 and the ligands' piperidine NH⁺ moiety; GF2) the N-chain orientation towards the μOR interior; GF3) the other pole of ligands is directed towards the receptor outlet; GF4) the aromatic anilide ring penetrates the subpocket formed by TM3, TM4, ECL1 and ECL2; GF5) the 4-axial substituent (if present) is directed towards W318. Except for the ionic interaction with D147, the majority of fentanyl-μOR contacts is hydrophobic. Interestingly, it was possible to find nonlinear relationships between the binding affinity and the volume of the N-chain and/or anilide's aromatic ring. This kind of relationships is consistent with the apolar character of interactions involved in ligand⁻receptor binding. The affinity reaches the optimum for medium size while it decreases for both large and small substituents. Additionally, a linear correlation between the volumes and the average dihedral angles of W293 and W133 was revealed by the molecular dynamics study. This seems particularly important, as the W293 residue is involved in the activation processes. Further, the Y326 (OH) and D147 (Cγ) distance found in the simulations also depends on the ligands' size. In contrast, neither RMSF measures nor D114/Y336 hydrations show significant structure-based correlations. They also do not differentiate studied fentanyl derivatives. Eventually, none of 14 popular scoring functions yielded a significant correlation between the predicted and observed affinity data (R < 0.30, n = 28).

[Pain inhibition by opioids-new concepts]

BACKGROUND

Opioids are the oldest and most potent drugs for the treatment of severe pain but they are burdened by detrimental side effects, such as respiratory depression, addiction potential, sedation, nausea and constipation. Their clinical application is undisputed in the treatment of acute (e.g. perioperative) and cancer pain but their long-term use in chronic pain has met increasing criticism and has contributed to the current "opioid crisis".

OBJECTIVES

This article reviews the pharmacological principles and new research strategies aiming at novel opioids with reduced side effects. The basic mechanisms underlying pain and opioid analgesia and other effects of opioids are outlined. To illustrate the clinical situation and medical problems, the plasticity of opioid receptors, intracellular signaling pathways, endogenous and exogenous opioid receptor ligands, central and peripheral sites of analgesic and side effects are discussed.

CONCLUSION

The epidemic of opioid misuse has shown that there is a lack of fundamental knowledge about the characteristics and management of chronic pain, that conflicts of interest and validity of models must be more intensively considered in the context of drug development and that novel analgesics with less addictive potential are urgently needed. Currently, the most promising perspectives appear to be augmenting endogenous opioid actions and the selective activation of peripheral opioid receptors.

New concepts in opioid analgesia

INTRODUCTION

Opioids are the oldest and most potent drugs for the treatment of severe pain, but they are burdened by detrimental side effects such as respiratory depression, addiction, sedation, nausea, and constipation. Their clinical application is undisputed in acute (e.g. perioperative) and cancer pain, but their long-term use in chronic pain has met increasing scrutiny and has contributed to the current 'opioid crisis.'

AREAS COVERED

This article reviews pharmacological principles and research strategies aiming at novel opioids with reduced side effects. Basic mechanisms underlying pain, opioid analgesia, and other opioid actions are outlined. To illustrate the clinical situation and medical needs, plasticity of opioid receptors, intracellular signaling pathways, endogenous and exogenous opioid receptor ligands, central and peripheral sites of analgesic, and side effects are discussed.

EXPERT OPINION

The epidemic of opioid misuse has taught us that there is a lack of fundamental knowledge about the characteristics and management of chronic pain, that conflicts of interest and validity of models must be considered in the context of drug development, and that novel analgesics with less abuse liability are badly needed. Currently, the most promising perspectives appear to be augmenting endogenous opioid actions and selectively targeting pathological conformations of peripheral opioid receptors.

A nontoxic pain killer designed by modeling of pathological receptor conformations

Indiscriminate activation of opioid receptors provides pain relief but also severe central and intestinal side effects. We hypothesized that exploiting pathological (rather than physiological) conformation dynamics of opioid receptor-ligand interactions might yield ligands without adverse actions. By computer simulations at low pH, a hallmark of injured tissue, we designed an agonist that, because of its low acid dissociation constant, selectively activates peripheral μ-opioid receptors at the source of pain generation. Unlike the conventional opioid fentanyl, this agonist showed pH-sensitive binding, heterotrimeric guanine nucleotide-binding protein (G protein) subunit dissociation by fluorescence resonance energy transfer, and adenosine 3',5'-monophosphate inhibition in vitro. It produced injury-restricted analgesia in rats with different types of inflammatory pain without exhibiting respiratory depression, sedation, constipation, or addiction potential.

Recent Advances in the Realm of Allosteric Modulators for Opioid Receptors for Future Therapeutics

Opioids, and more specifically μ-opioid receptor (MOR) agonists such as morphine, have long been clinically used as therapeutics for severe pain states but often come with serious side effects such as addiction and tolerance. Many studies have focused on bringing about analgesia from the MOR with attenuated side effects, but its underlying mechanism is not fully understood. Recently, focus has been geared toward the design and elucidation of the orthosteric site with ligands of various biological profiles and mixed subtype opioid activities and selectivities, but targeting the allosteric site is an area of increasing interest. It has been shown that allosteric modulators play key roles in influencing receptor function such as its tolerance to a ligand and affect downstream pathways. There has been a high variance of chemical structures that provide allosteric modulation at a given receptor, but recent studies and reviews tend to focus on the altered cellular mechanisms instead of providing a more rigorous description of the allosteric ligand's structure-function relationship. In this review, we aim to explore recent developments in the structural motifs that potentiate orthosteric binding and their influences on cellular pathways in an effort to present novel approaches to opioid therapeutic design.

Racemic Salsolinol and its Enantiomers Act as Agonists of the μ-Opioid Receptor by Activating the Gi Protein-Adenylate Cyclase Pathway

Background: Several studies have shown that the ethanol-derived metabolite salsolinol (SAL) can activate the mesolimbic system, suggesting that SAL is the active molecule mediating the rewarding effects of ethanol. In vitro and in vivo studies suggest that SAL exerts its action on neuron excitability through a mechanism involving opioid neurotransmission. However, there is no direct pharmacologic evidence showing that SAL activates opioid receptors.

Methods: The ability of racemic (R/S)-SAL, and its stereoisomers (R)-SAL and (S)-SAL, to activate the μ-opioid receptor was tested in cell-based (light-emitting) receptor assays. To further characterizing the interaction of SAL stereoisomers with the μ-opioid receptor, a molecular docking study was performed using the crystal structure of the μ-opioid receptor.

Results: This study shows that SAL activates the μ-opioid receptor by the classical G protein-adenylate cyclase pathway with an half-maximal effective concentration (EC₅₀) of 2 × 10⁻⁵ M. The agonist action of SAL was fully blocked by the μ-opioid antagonist naltrexone. The EC₅₀ for the purified stereoisomers (R)-SAL and (S)-SAL were 6 × 10⁻⁴ M and 9 × 10⁻⁶ M respectively. It was found that the action of racemic SAL on the μ-opioid receptor did not promote the recruitment of β-arrestin. Molecular docking studies showed that the interaction of (R)- and (S)-SAL with the μ-opioid receptor is similar to that predicted for the agonist morphine.

Conclusions: It is shown that (R)-SAL and (S)-SAL are agonists of the μ-opioid receptor. (S)-SAL is a more potent agonist than the (R)-SAL stereoisomer. In silico analysis predicts a morphine-like interaction between (R)- and (S)-SAL with the μ-opioid receptor. These results suggest that an opioid action of SAL or its enantiomers is involved in the rewarding effects of ethanol.

Ignavine: a novel allosteric modulator of the μ opioid receptor

Processed Aconiti tuber (PAT) is used to treat pain associated with various disorders. Although it has been demonstrated that the κ opioid receptor (KOR) signaling pathway is a mediator of the analgesic effect of PAT, active components affecting opioid signaling have not yet been identified. In this study, we explored candidate components of PAT by pharmacokinetic analysis and identified ignavine, which is a different structure from aconitine alkaloids. A receptor binding assay of opioid receptors showed that ignavine specifically binds the μ opioid receptor (MOR), not the KOR. Receptor internalization assay in MOR-expressing cell lines revealed that ignavine augmented the responses produced by D-Ala(2)-N-Me-Phe(4)-Gly-ol(5)-enkephalin (DAMGO), a representative MOR agonist, at a low concentration and inhibited it at a higher concentration. Ignavine also exerted positive modulatory activity for DAMGO, endomorphin-1 and morphine in cAMP assay. Additionally, ignavine alone showed an analgesic effect in vivo. In silico simulation analysis suggested that ignavine would induce a unique structural change distinguished from those induced by a representative MOR agonist and antagonist. These data collectively suggest the possibility that ignavine could be a novel allosteric modulator of the MOR. The present results may open the way for the development of a novel pain management strategy.

μ Opioid receptor: novel antagonists and structural modeling

The μ opioid receptor (MOR) is a prominent member of the G protein-coupled receptor family and the molecular target of morphine and other opioid drugs. Despite the long tradition of MOR-targeting drugs, still little is known about the ligand-receptor interactions and structure-function relationships underlying the distinct biological effects upon receptor activation or inhibition. With the resolved crystal structure of the β-funaltrexamine-MOR complex, we aimed at the discovery of novel agonists and antagonists using virtual screening tools, i.e. docking, pharmacophore- and shape-based modeling. We suggest important molecular interactions, which active molecules share and distinguish agonists and antagonists. These results allowed for the generation of theoretically validated in silico workflows that were employed for prospective virtual screening. Out of 18 virtual hits evaluated in in vitro pharmacological assays, three displayed antagonist activity and the most active compound significantly inhibited morphine-induced antinociception. The new identified chemotypes hold promise for further development into neurochemical tools for studying the MOR or as potential therapeutic lead candidates.

Design, synthesis, pharmacological evaluation and molecular dynamics of β-amino acids morphan-derivatives as novel ligands for opioid receptors

Structure-Activity Relationship (SAR) is a current approach in the design of new pharmacological agents. We previously reported the synthesis of a novel analogue of morphine, a 2-azabicyclo[3.3.1]nonane, which contains a β-amino acid. This bicyclic core exhibits two distinctive chemical handles for further elaboration, which allowed us to create a library of morphan-containing compounds by in silico molecular docking on the μ opioid receptor. Lead candidates were synthesized and biological tests were performed to evaluate their ability to bind to opioid receptors. The four top compounds, three phenyl esters and an N-phenylethyl morphan derivative, were selected for Molecular Dynamics simulations to get topological and thermodynamic information. Aromatic morphan derivatives displayed an interacting domain which fits into a hydrophobic cleft and the effect of the substituents in their affinity was explained by the differences in the calculated binding free energies. Our results indicate that the 3D arrangement of the aromatic ring in the morphine derivatives is not a key issue for a specific ligand - μ receptor interaction. Thus, these morphan derivatives represent a new class of opioid receptor ligands which may be of great use in the clinical practice.

'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.

Beyond standard molecular dynamics: investigating the molecular mechanisms of G protein-coupled receptors with enhanced molecular dynamics methods

The majority of biological processes mediated by G Protein-Coupled Receptors (GPCRs) take place on timescales that are not conveniently accessible to standard molecular dynamics (MD) approaches, notwithstanding the current availability of specialized parallel computer architectures, and efficient simulation algorithms. Enhanced MD-based methods have started to assume an important role in the study of the rugged energy landscape of GPCRs by providing mechanistic details of complex receptor processes such as ligand recognition, activation, and oligomerization. We provide here an overview of these methods in their most recent application to the field.

Mu opioids and their receptors: evolution of a concept

Opiates are among the oldest medications available to manage a number of medical problems. Although pain is the current focus, early use initially focused upon the treatment of dysentery. Opium contains high concentrations of both morphine and codeine, along with thebaine, which is used in the synthesis of a number of semisynthetic opioid analgesics. Thus, it is not surprising that new agents were initially based upon the morphine scaffold. The concept of multiple opioid receptors was first suggested almost 50 years ago (Martin, 1967), opening the possibility of new classes of drugs, but the morphine-like agents have remained the mainstay in the medical management of pain. Termed mu, our understanding of these morphine-like agents and their receptors has undergone an evolution in thinking over the past 35 years. Early pharmacological studies identified three major classes of receptors, helped by the discovery of endogenous opioid peptides and receptor subtypes-primarily through the synthesis of novel agents. These chemical biologic approaches were then eclipsed by the molecular biology revolution, which now reveals a complexity of the morphine-like agents and their receptors that had not been previously appreciated.

Molecular details of the activation of the μ opioid receptor

Molecular details of μ opioid receptor activations were obtained using molecular dynamics simulations of the receptor in the presence of three agonists, three antagonists, and a partial agonist and on the constitutively active T279K mutant. Agonists have a higher probability of direct interactions of their basic nitrogen (N) with Asp147 as compared with antagonists, indicating that direct ligand-Asp147 interactions modulate activation. Medium-size substituents on the basic N of antagonists lead to steric interactions that perturb N-Asp147 interactions, while additional favorable interactions occur with larger basic N substituents, such as in N-phenethylnormorphine, restoring N-Asp147 interactions, leading to agonism. With the orvinols, the increased size of the C19 substituent in buprenorphine over diprenorphine leads to increased interactions with residues adjacent to Asp147, partially overcoming the presence of the cyclopropyl N substituent, such that buprenorphine is a partial agonist. Results also indicate different conformational properties of the intracellular regions of the transmembrane helices in agonists versus antagonists.

Design, synthesis, and pharmacological characterization of novel endomorphin-1 analogues as extremely potent μ-opioid agonists

Recently we reported the synthesis and structure-activity study of endomorphin-1 (EM-1) analogues containing novel, unnatural α-methylene-β-aminopropanoic acids (Map). In the present study, we describe new EM-1 analogues containing Dmt(1), (R/S)-βPro(2), and (ph)Map(4)/(2-furyl)Map(4). All of the analogues showed a high affinity for the μ-opioid receptor (MOR) and increased stability in mouse brain homogenates. Of the new compounds, Dmt(1)-(R)-βPro(2)-Trp(3)-(2-furyl)Map(4) (analogue 12) displayed the highest affinity toward MOR, in the picomolar range (Ki(μ) = 3.72 pM). Forskolin-induced cAMP accumulation assays indicated that this analogue displayed an extremely high agonistic potency, in the subpicomolar range (EC50 = 0.0421 pM, Emax = 99.5%). This compound also displayed stronger in vivo antinociceptive activity after iv administration when compared to morphine in the tail-flick test, which indicates that this analogue was able to cross the blood-brain barrier.

Ligand interaction, binding site and G protein activation of the mu opioid receptor

With the recently solved crystal structure of the murine mu opioid receptor, the elucidation of the structure function relationships of the human mu receptor becomes feasible. In this study, we analyzed the available structural information along with ligand binding and G protein activation of human mu receptor. Affinity determinations were performed in a HEK293 cell line stably transfected with the human mu opioid receptor for 6 different agonists (morphine, DMAGO, and herkinorn) and antagonists (naloxone, beta-Funaltrexamine, and Norbinaltorphimine). G protein activation was investigated in membrane preparations containing human mu receptors treated with the agonist, partial agonist, or antagonist compounds. 4DKL.pdb was utilized for structural analysis and docking calculations for 28 mu receptor ligands. The predicted affinities from docking were compared with those experimentally determined. While all known ligands bind to the receptor through the same binding site that is large enough to accommodate molecules of various sizes, interaction with D147 (D149 in human mu receptor) is essential for binding. No distinguishable interaction pattern in the binding site for agonist, partial agonist, or antagonist to predict pharmacological activities was found. The failure to reconcile the predicted affinities from docking with experimental values indicates that the receptor might undergo significant conformational changes from one state to the other states upon different ligand binding. A simplified model to understand the complicated system is proposed and further study on these multiple conformations using high resolution structural approaches is suggested.

[The estimation method of compounds opiate activity based on universal three-dimensional model of the nonselective opiate pharmacophore]

Created by means alternative strategy of structural similarity search universal three-dimensional model of the nonselective opiate pharmacophore and the estimation method of agonistic and antagonistic properties of opiate receptors ligands based on its were described. The examples of the present method use are given for opiate activity estimation of compounds essentially distinguished on the structure from opiates and traditional opioids.

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.

A new class of highly potent and selective endomorphin-1 analogues containing α-methylene-β-aminopropanoic acids (map)

A new class of endomorphin-1 (EM-1) analogues were synthesized by introduction of novel unnatural α-methylene-β-amino acids (Map) at position 3 or/and position 4. Their binding and functional activity, metabolic stability, and antinociceptive activity were determined and compared. Most of these analogues showed high affinities for the μ-opioid receptor and an increased stability in mouse brain homogenates compared with EM-1. Examination of cAMP accumulation and ERK1/2 phosphorylation in HEK293 cells confirmed the agonist properties of these analogues. Among these new analogues, H-Tyr-Pro-Trp-(2-furyl)Map-NH(2) (analogue 12) exhibited the highest binding potency (K(i)(μ) = 0.221 nM) and efficacy (EC(50) = 0.0334 nM, E(max) = 97.14%). This analogue also displayed enhanced antinociceptive activity in vivo in comparison to EM-1. Molecular modeling approaches were then carried out to demonstrate the interaction pattern of these analogues with the opioid receptors. We found that, compared to EM-1, the incorporation of our synthesized Map at position 4 would bring the analogue to a closer binding mode with the μ-opioid receptor.

Snooker: a structure-based pharmacophore generation tool applied to class A GPCRs

G-protein coupled receptors (GPCRs) are important drug targets for various diseases and of major interest to pharmaceutical companies. The function of individual members of this protein family can be modulated by the binding of small molecules at the extracellular side of the structurally conserved transmembrane (TM) domain. Here, we present Snooker, a structure-based approach to generate pharmacophore hypotheses for compounds binding to this extracellular side of the TM domain. Snooker does not require knowledge of ligands, is therefore suitable for apo-proteins, and can be applied to all receptors of the GPCR protein family. The method comprises the construction of a homology model of the TM domains and prioritization of residues on the probability of being ligand binding. Subsequently, protein properties are converted to ligand space, and pharmacophore features are generated at positions where protein ligand interactions are likely. Using this semiautomated knowledge-driven bioinformatics approach we have created pharmacophore hypotheses for 15 different GPCRs from several different subfamilies. For the beta-2-adrenergic receptor we show that ligand poses predicted by Snooker pharmacophore hypotheses reproduce literature supported binding modes for ∼75% of compounds fulfilling pharmacophore constraints. All 15 pharmacophore hypotheses represent interactions with essential residues for ligand binding as observed in mutagenesis experiments and compound selections based on these hypotheses are shown to be target specific. For 8 out of 15 targets enrichment factors above 10-fold are observed in the top 0.5% ranked compounds in a virtual screen. Additionally, prospectively predicted ligand binding poses in the human dopamine D3 receptor based on Snooker pharmacophores were ranked among the best models in the community wide GPCR dock 2010.

"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.

Exploring molecular mechanisms of ligand recognition by opioid receptors with metadynamics

Opioid receptors are G protein-coupled receptors (GPCRs) of utmost significance in the development of potent analgesic drugs for the treatment of severe pain. An accurate evaluation at the molecular level of the ligand binding pathways into these receptors may play a key role in the design of new molecules with more desirable properties and reduced side effects. The recent characterization of high-resolution X-ray crystal structures of non-rhodopsin GPCRs for diffusible hormones and neurotransmitters presents an unprecedented opportunity to build improved homology models of opioid receptors, and to study in more detail their molecular mechanisms of ligand recognition. In this study, possible pathways for entry of the nonselective antagonist naloxone (NLX) from the water environment into the well-accepted alkaloid binding pocket of a delta opioid receptor (DOR) molecular model based on the beta2-adrenergic receptor crystal structure are explored using microsecond-scale well-tempered metadynamics simulations. Using as collective variables distances that account for the position of NLX and of the receptor extracellular loop 2 in relation to the DOR binding pocket, we were able to distinguish between the different states visited by the ligand (i.e., docked, undocked, and metastable bound intermediates) and to predict a free energy of binding close to experimental values after correcting for possible drawbacks of the sampling approach. The strategy employed herein holds promise for its application to the docking of diverse ligands to the opioid receptors as well as to other GPCRs.

Molecular modeling studies to predict the possible binding modes of endomorphin analogs in mu opioid receptor

The molecular docking of a series of endomorphin analog with the mu opioid receptor was performed. The successive molecular dynamics of several proposed ligand-receptor complexes inserted into the phospholipid bilayer were carried out to optimize the complex and explore the conformational changes. Meaningful differences of their binding modes were detected and the involvement of some essential residues in ligand binding was also identified. Our proposed ligand-receptor model is in good agreement with previous site-directed mutagenesis experiments.

Conformationally constrained opioid ligands: the Dmt-Aba and Dmt-Aia versus Dmt-Tic scaffold

Replacement of the constrained phenylalanine analogue 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid (Tic) in the opioid Dmt-Tic-Gly-NH-Bn scaffold by the 4-amino-1,2,4,5-tetrahydro-indolo[2,3-c]azepin-3-one (Aia) and 4-amino-1,2,4,5-tetrahydro-2-benzazepin-3-one (Aba) scaffolds has led to the discovery of novel potent mu-selective agonists (Structures 5 and 12) as well as potent and selective delta-opioid receptor antagonists (Structures 9 and 15). Both stereochemistry and N-terminal N,N-dimethylation proved to be crucial factors for opioid receptor selectivity and functional bioactivity in the investigated small peptidomimetic templates. In addition to the in vitro pharmacological evaluation, automated docking models of Dmt-Tic and Dmt-Aba analogues were constructed in order to rationalize the observed structure-activity data.

Homology modeling of opioid receptor-ligand complexes using experimental constraints

Opioid receptors interact with a variety of ligands, including endogenous peptides, opiates, and thousands of synthetic compounds with different structural scaffolds. In the absence of experimental structures of opioid receptors, theoretical modeling remains an important tool for structure-function analysis. The combination of experimental studies and modeling approaches allows development of realistic models of ligand-receptor complexes helpful for elucidation of the molecular determinants of ligand affinity and selectivity and for understanding mechanisms of functional agonism or antagonism. In this review we provide a brief critical assessment of the status of such theoretical modeling and describe some common problems and their possible solutions. Currently, there are no reliable theoretical methods to generate the models in a completely automatic fashion. Models of higher accuracy can be produced if homology modeling, based on the rhodopsin X-ray template, is supplemented by experimental structural constraints appropriate for the active or inactive receptor conformations, together with receptor-specific and ligand-specific interactions. The experimental constraints can be derived from mutagenesis and cross-linking studies, correlative replacements of ligand and receptor groups, and incorporation of metal binding sites between residues of receptors or receptors and ligands. This review focuses on the analysis of similarity and differences of the refined homology models of mu, delta, and kappa-opioid receptors in active and inactive states, emphasizing the molecular details of interaction of the receptors with some representative peptide and nonpeptide ligands, underlying the multiple modes of binding of small opiates, and the differences in binding modes of agonists and antagonists, and of peptides and alkaloids.