The role of the hydrophilic Asn230 residue of the mu-opioid receptor in the potency of various opioid agonists

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.

DARK Classics in Chemical Neuroscience: Fentanyl

Fentanyl rose to prominence as an alternative analgesic to morphine nearly 50 years ago; today, fentanyl has re-emerged as a dangerous recreational substance. The increased potency and analgesic effect of fentanyl are advantageous in the treatment of pain but are also responsible for the rise in unintentional opioid overdose deaths. In response to this crisis, fentanyl, its analogues, and even precursors are under heightened regulatory scrutiny. Despite this controversial history, derivatization of fentanyl has resulted in numerous synthetic analogues that provide valuable insights into opioid receptor binding and signaling events. In this review, the impact of fentanyl on chemical neuroscience is shown through its synthesis and properties, manufacturing, metabolism, pharmacology, approved and off-label indications, adverse effects, and the responsibility it has in the opioid epidemic.

Clinical Pharmacology and Pharmacotherapy of Opioid Switching in Cancer Patients

Pain is one of the most common and often most feared symptoms in patients with cancer. Ongoing or progressive pain is physically debilitating and has a marked impact on quality of life. Since a third of the population will die from cancer, and of these, 80% will experience severe pain in their final year of life, effective treatment of cancer-related pain remains both a high priority and an ongoing challenge in clinical practice. Individuals with moderate to severe cancer-related pain require treatment with strong analgesics, namely opioids. There is evidence to support the therapeutic maneuver of opioid switching in clinical practice, but further evidence is needed to elucidate the underlying mechanisms for interindividual differences in response to different opioids. Large, robust clinical trials will be needed if clinical differences among side-effect profiles of different opioids are to be clearly demonstrated. This review discusses candidate genes, which contribute to opioid response; many other genes have also been implicated in "pain" from animal or human studies. In order to continue to evaluate the genetic contributions to both pain susceptibility and analgesic response, further candidate genes need to be considered. Good pain control remains a high priority for clinicians and patients, and there is much work to be done to further individualize analgesic therapy for patients with cancer.

Steric interactions and the activity of fentanyl analogs at the μ-opioid receptor

Fentanyl is a highly potent and clinically widely used narcotic analgesic. The synthesis of its analogs remains a challenge in the attempt to develop highly selective mu-opioid receptor agonists with specific pharmacological properties. In this paper, the use of flexible molecular docking in a study of the formation of complexes between a series of active fentanyl analogs and the mu-opioid receptor is described. The optimal position and orientation of fourteen fentanyl analogs in the binding pocket of the mu-receptor were determined. The major receptor amino acids and the ligand functional groups participating in the complex formation were identified. Stereochemical effects on the potency and binding are explained. The proposed model of ligand-receptor binding is in agreement with point mutation experiments explaining the role of the amino acids: Asp147, Tyr148, Asn230, His297, Trp318, His319, Cys321, and Tyr326 in the complex formation. In addition, the following amino acids were identified as being important for ligand binding or receptor activation: Ile322, Gly325, Val300, Met203, Leu200, Val143, and Ile144.

Molecular recognition of opioid receptor ligands

The cloning of the opioid receptors and subsequent use of recombinant DNA technology have led to many new insights into ligand binding. Instead of focusing on the structural features that lead to increased affinity and selectivity, researchers are now able to focus on why these features are important. Site-directed mutagenesis and chimeric data have often been at the forefront in answering these questions. Herein, we survey pharmacophores of several opioid ligands in an effort to understand the structural requirements for ligand binding and selectivity. Models are presented and compared to illustrate key sites of recognition for both opiate and nonopiate ligands. The results indicate that different ligand classes may recognize different sites within the receptor, suggesting that multiple epitopes may exist for ligand binding and selectivity.

Clinical response to morphine in cancer patients and genetic variation in candidate genes

Morphine is the analgesic of choice for moderate to severe cancer pain; however, 10-30% of patients do not tolerate morphine. This study evaluated genetic variation in the mu-opioid receptor, betaarrestin2, stat6 and uridine diphosphate-glucuronysltransferase 2B7 (UGT2B7) genes, in patients who responded to morphine vs those who were switched to alternative opioids. We prospectively recruited and genotyped 162 Caucasian patients (117 controls, 39 switchers). Switchers, were more likely to carry the common allele at 1182 G/A, 5864 G/A, 8622T/C and 11143 G/A in the betaarrestin2 gene (P = 0.021, 0.043, 0.013, 0.043, respectively). Switchers had increased carriage of the T allele (-1714 C/T) and a significant difference in the allelic frequency at 9065 C/T (chi(2) = 3.86, P = 0.049) in the stat6 gene. No differences were seen in genotype or allele frequencies of SNPs in the mu-opioid receptor gene or UGT2B7 gene. This study presents novel data suggesting that variation in genes involved in mu-opioid receptor signalling influence clinical response to morphine.

Morphine-6-glucuronide: Actions and mechanisms

Morphine-6-glucuronide (M6G) appears to show equivalent analgesia to morphine but to have a superior side-effect profile in terms of reduced liability to induce nausea and vomiting and respiratory depression. The purpose of this review is to examine the evidence behind this statement and to identify the possible reasons that may contribute to the profile of M6G. The vast majority of available data supports the notion that both M6G and morphine mediate their effects by activating the micro-opioid receptor. The differences for which there is a reasonable consensus in the literature can be summarized as: (1) Morphine has a slightly higher affinity for the micro-opioid receptor than M6G, (2) M6G shows a slightly higher efficacy at the micro-opioid receptor, (3) M6G has a lower affinity for the kappa-opioid receptor than morphine, and (4) M6G has a very different absorption, distribution, metabolism, and excretion (ADME) profile from morphine. However, none of these are adequate alone to explain the clinical differences between M6G and morphine. The ADME differences are perhaps most likely to explain some of the differences but seem unlikely to be the whole story. Further work is required to examine further the profile of M6G, notably whether M6G penetrates differentially to areas of the brain involved in pain and those involved in nausea, vomiting, and respiratory control or whether micro-opioid receptors in these brain areas differ in either their regulation or pharmacology.

Synthesis and electrophysiological characterization of cyclic morphiceptin analogues

A challenge in opioid peptide chemistry and pharmacology is the possibility to develop novel peptides with peripheral selectivity. An enzymatically stable opioid peptide could involve an antidiarrheal effect. For this reason, we constrained the highly selective and potent tetrapeptide morphiceptin with a 6-atom bridge, resulting in a cyclic amide and an ester analogue, 2 and 3, respectively. Taking advantage of the functional coupling of the opioid receptor with the heteromultimeric G-protein-coupled inwardly rectifying K+ (GIRK1/GIRK2) channel, either the wild-type mu-, kappa-, delta- or a mutated mu-opioid receptor (hMORS329A) was functionally co-expressed with GIRK1/GIRK2 channels and a regulator of G-protein signaling (RGS4) in Xenopus laevis oocytes. The two-microelectrode voltage clamp technique was used to measure the opioid receptor activated GIRK1/GIRK2 channel responses. Both cyclic analogues were equally potent via the wild-type mu-opioid receptor hMORwt (EC(50) value 976.5 +/- 41.7 for 2 and 1017.7 +/- 60.7 for 3), while the EC(50) value for Tyr-Pro-Phe-D-Pro-NH(2) measured 59.3 +/- 4.8 nM. These three agonists displayed a four to five times decreased potency via hMORS329A as compared to the wild type. Interestingly, no effect on kappa- and delta-opioid receptors was observed. The intramolecular bridge created by cyclization of morphiceptin prevents dipeptidyl peptidase IV from interacting with these analogues. We conclude that constraining morphiceptin with a 6-atom bridge resulted in enzymatically stable peptidomimetics that are exclusively active on mu-opioid receptors. These analogues provide an interesting template in the promising approach for the design of potential antidiarrheal agents.

Decreased immunodensities of micro-opioid receptors, receptor kinases GRK 2/6 and beta-arrestin-2 in postmortem brains of opiate addicts

The homologous regulation of opioid receptors, through G protein-coupled receptor kinases (GRKs) and beta-arrestins, is an initial step in the complex molecular mechanisms leading to opiate tolerance and dependence. This study was designed to evaluate in parallel the contents of immunolabeled micro-opioid receptors (glycosylated proteins), two representative GRKs (GRK 2 and GRK 6) and beta-arrestin-2 in brains of opiate addicts who had died of an opiate overdose (heroin or methadone). The immunodensities of micro-opioid receptors were decreased (66 kDa protein: 24%, n=24, P<0.0001; 85 kDa protein: 16%, n=24, P<0.05) in the prefrontal cortex of opiate addicts compared with sex-, age-, and PMD-matched controls. This down-regulation of brain micro-opioid receptors was more pronounced in opiate addicts dying of a heroin overdose (27-30%, n=13) than in those who died of a methadone overdose (5-16%, n=11). In the same brains, significant decreases in the immunodensities of GRK 2 (19%, n=24, P<0.05), GRK 6 (25%, n=24, P<0.002) and beta-arrestin-2 (22%, n=24, P< 0.0005) were also quantitated. In contrast, the content of alpha-internexin (a neuronal marker used as a negative control) was not changed in brains of opiate addicts. In these subjects, there was a significant correlation between the densities of GRK 6 and beta-arrestin-2 (r=0.63, n=24, P=0.001), suggesting that both proteins are regulated in a coordinated manner by opiate drugs in the brain. The results indicate that opiate addiction in humans (tolerant state) is associated with down-regulation of brain micro-opioid receptors and regulatory GRK 2/6 and beta-arrestin-2 proteins. These molecular adaptations may be relevant mechanisms for the induction of opiate tolerance in brains of opiate addicts.

Serine 329 of the mu-opioid receptor interacts differently with agonists

To investigate the effect of the hydrophilic Ser amino acid in position 329 of the human mu-opioid receptor (hMORwt) on the potency of various agonists, we mutated this residue to Ala (hMORS329A). Taking advantage of the functional coupling of the opioid receptor with the heteromultimeric G-protein-coupled inwardly rectifying potassium channel (GIRK1/GIRK2), either the wild-type hMOR or the mutated receptor (hMORS329A) was functionally coexpressed with GIRK1 and GIRK2 channels together with a regulator of G-protein signaling (RGS4) in Xenopus laevis oocytes. The two-microelectrode voltage-clamp technique was used to measure the opioid receptor activated GIRK1/GIRK2 channel responses. The potency of the peptide agonist [D-Ala(2),N-MePhe(4),Gly(5)-ol]-enkephalin (DAMGO) decreased as measured via hMORS329A, whereas the potency of nonpeptide agonists like morphine, fentanyl, and beta-hydroxyfentanyl (R004333) increased via the mutated receptor. Our results are indicative for the existence of hydrophilic interactions between Ser(329) and DAMGO, thereby decreasing the potency of DAMGO via the mutated receptor, whereas hydrophobic interactions between the mutated receptor and the N-phenylethyl of morphine and fentanyl can explain the increased potency. We conclude that the hydroxyl group of Ser(329) is not involved in the formation of a hydrogen bond with the beta-hydroxy group of fentanyl and that mutation of this residue to alanine caused dual effects depending on the nature of the ligand.