Binding mode characterization of 6α- and 6β-N-heterocyclic substituted naltrexamine derivatives via docking in opioid receptor crystal structures and site-directed mutagenesis studies: application of the 'message-address' concept in development of mu opioid receptor selective antagonists

Blocking potential metabolic sites on NAT to improve its safety profile while retaining the pharmacological profile

The number of opioid-related overdose deaths and individuals that have suffered from opioid use disorders have significantly increased over the last 30 years. FDA approved maintenance therapies to treat opioid use disorder may successfully curb drug craving and prevent relapse but harbor adverse effects that reduce patient compliance. This has created a need for new chemical entities with improved patient experience. Previously our group reported a novel lead compound, NAT, a mu-opioid receptor antagonist that potently antagonized the antinociception of morphine and showed significant blood-brain barrier permeability. However, NAT belongs to thiophene containing compounds which are known structural alerts for potential oxidative metabolism. To overcome this, 15 NAT derivatives with various substituents at the 5'-position of the thiophene ring were designed and their structure-activity relationships were studied. These derivatives were characterized for their binding affinity, selectivity, and functional activity at the mu opioid receptor and assessed for their ability to antagonize the antinociceptive effects of morphine in vivo. Compound 12 showed retention of the basic pharmacological attributes of NAT while improving the withdrawal effects that were experienced in opioid-dependent mice. Further studies will be conducted to fully characterize compound 12 to examine whether it would serve as a new lead for opioid use disorder treatment and management.

Novel 1-(1-Arylimiazolin-2-Yl)-3-Arylalkilurea Derivatives with Modulatory Activity on Opioid MOP Receptors

μ-opioid receptor ligands such as morphine and fentanyl are the most known and potent painkillers. However, the severe side effects seen with their use significantly limit their widespread use. The continuous broadening of knowledge about the properties of the interactions of the MOP receptor (human mu opioid receptor, OP3) with ligands and specific intracellular signaling pathways allows for the designation of new directions of research with respect to compounds with analgesic effects in a mechanism different from classical ligands. Allosteric modulation is an extremely promising line of research. Compounds with modulator properties may provide a safer alternative to the currently used opioids. The aim of our research was to obtain a series of urea derivatives of 1-aryl-2-aminoimidazoline and to determine their activity, mechanism of biological action and selectivity toward the MOP receptor. The obtained compounds were subjected to functional tests (cAMP accumulation and β-arrestin recruitment) in vitro. One of the obtained compounds, when administered alone, did not show any biological activity, while when co-administered with DAMGO, it inhibited β-arrestin recruitment. These results indicate that this compound is a negative allosteric modulator (NAM) of the human MOP receptor.

IUPHAR review: Recent progress in the development of Mu opioid receptor modulators to treat opioid use disorders

Opioid Use Disorder (OUD) can be described as intense preoccupation with using or obtaining opioids despite the negative consequences associated with their use. As the number of OUD cases in the U.S. increase, so do the number of opioid-related overdose deaths. In 2022, opioid-related overdose became the No. 1 cause of death for individuals in the U.S. between the ages of 25 and 64 years of age. Because of the introduction of highly potent synthetic opioids (e.g. fentanyl) to the illicit drug market, there is an urgent need for therapeutics that successfully reduce the number of overdoses and can help OUD patients maintain sobriety. Most abused opioids stimulate the mu-opioid receptor (MOR) and activation of this receptor can lead to positive (e.g., euphoria) consequences. However, the negative side effects of MOR stimulation can be fatal (e.g., sedation, respiratory depression). Therefore, the MOR is an attractive target for developing medications to treat OUD. Current FDA drugs include MOR agonists that aid in detoxification and relapse prevention, and MOR antagonists that also serve as maintenance therapies or reverse overdose. These medications are limited by their abuse potential, adverse effects, or pharmacological profiles which leaves ample room for research into designing new chemical entities with optimal physiological effects. These includes, orthosteric ligands that target the primary binding site of the MOR, allosteric ligands that positively, negatively, or "silently" modulate receptor function, and lastly, bitopic ligands target both the orthosteric and allosteric sites simultaneously.

Historical perspectives and recent advances in small molecule ligands of selective/biased/multi-targeted μ/δ/κ opioid receptor (2019-2022)

The opioids have been used for more than a thousand years and are not only the most widely prescribed drugs for moderate to severe pain and acute pain, but also the preferred drugs. However, their non-analgesic effects, especially respiratory depression and potential addiction, are important factors that plague the safety of clinical use and are an urgent problem for pharmacological researchers to address. Current research on analgesic drugs has evolved into different directions: de-opioidization; application of pharmacogenomics to individualize the use of opioids; development of new opioids with less adverse effects. The development of new opioid drugs remains a hot research topic, and with the in-depth study of opioid receptors and intracellular signal transduction mechanisms, new research ideas have been provided for the development of new opioid analgesics with less side effects and stronger analgesic effects. The development of novel opioid drugs in turn includes selective opioid receptor ligands, biased opioid receptor ligands, and multi-target opioid receptor ligands and positive allosteric modulators (PAMs) or antagonists and the single compound as multi-targeted agnoists/antagonists for different receptors. PAMs strategies are also getting newer and are the current research hotspots, including the BMS series of compounds and others, which are extensive and beyond the scope of this review. This review mainly focuses on the selective/biased/multi-targeted MOR/DOR/KOR (mu opioid receptor/delta opioid receptor/kappa opioid receptor) small molecule ligands and involves some cryo-electron microscopy (cryoEM) and structure-based approaches as well as the single compound as multi-targeted agnoists/antagonists for different receptors from 2019 to 2022, including discovery history, activities in vitro and vivo, and clinical studies, in an attempt to provide ideas for the development of novel opioid analgesics with fewer side effects.

Design, Synthesis, and Biological Evaluation of NAP Isosteres: A Switch from Peripheral to Central Nervous System Acting Mu-Opioid Receptor Antagonists

The μ opioid receptor (MOR) has been an intrinsic target to develop treatment of opioid use disorders (OUD). Herein, we report our efforts on developing centrally acting MOR antagonists by structural modifications of 17-cyclopropylmethyl-3,14-dihydroxy-4,5α-epoxy-6β-[(4'-pyridyl) carboxamido] morphinan (NAP), a peripherally acting MOR-selective antagonist. An isosteric replacement concept was applied and incorporated with physiochemical property predictions in the molecular design. Three analogs, namely, 25, 26, and 31, were identified as potent MOR antagonists in vivo with significantly fewer withdrawal symptoms than naloxone observed at similar doses. Furthermore, brain and plasma drug distribution studies supported the outcomes of our design strategy on these compounds. Taken together, our isosteric replacement of pyridine with pyrrole, furan, and thiophene provided insights into the structure-activity relationships of NAP and aided the understanding of physicochemical requirements of potential CNS acting opioids. These efforts resulted in potent, centrally efficacious MOR antagonists that may be pursued as leads to treat OUD.

Exploring naltrexamine derivatives featuring azaindole moiety via nitrogen-walk approach to investigate their in vitro pharmacological profiles at the mu opioid receptor

In the present work, we reported the application of a nitrogen-walk approach on developing a series of novel opioid ligands containing an azaindole moiety at the C6-position of the epoxymorphinan skeleton. In vitro study results showed that introducing a nitrogen atom around the indole moiety not only retained excellent binding affinity, but also led to significant functional switch at the mu opioid receptor (MOR). Further computational investigations provided corroborative evidence and plausible explanations of the results of the in vitro studies. Overall, our current work implemented a series of novel MOR ligands with high binding affinity and considerably low efficacy, which may shed light on rational design of low efficacy MOR ligands for opioid use disorder therapeutics.

Design, Synthesis, and Structure-Activity Relationship Exploration of Alkyl/Phenylalkyl Piperidine Analogues as Novel Highly Potent and Selective μ Opioid Receptor Agonists

Pain was implicated in many diseases. Despite effectiveness to treat moderate to severe pain, opioid analgesics elicited many side effects, greatly limiting their prescription in clinics. Based on M1, an active metabolite of tramadol, 3-((dimethylamino)methyl)-4-(3-hydroxyphenyl)piperidin-4-ol analogues were designed, synthesized, and evaluated in vitro. Among all the compounds tested, compound 23 was found to be a novel, highly selective, and potent MOR agonist (K_i MOR = 0.0034 nM, EC_50 MOR = 0.68 nM, E_max = 206.5%; K_i DOR = 41.67 nM; K_i KOR = 7.9 nM). Structure-activity relationship exploration showed that the linker between the piperidine ring and the phenyl ring as well as substituent pattern of the phenyl ring played a pivotal role in binding affinity and selectivity. (3R, 4S)-23 (K_i MOR = 0.0021 ± 0.0001 nM, EC_50 MOR = 0.0013 ± 0.0001 nM, E_max = 209.1 ± 1.4%; K_i DOR = 18.4 ± 0.7 nM, EC_50 DOR = 74.5 ± 2.8 nM, E_max = 267.1 ± 1.4%; K_i KOR = 25.8 ± 0.2 nM, EC_50 DOR = 116.2 ± 4.4 nM, E_max = 209.5 ± 1.4%) had more potent activity for opioid receptors than its enantiomer (3S, 4R)-23 and was found to be a potent, highly selective MOR agonist with novel scaffold. High binding affinity and selectivity of (3R, 4S)-23 for MOR over KOR and DOR and its mechanism of activating MOR were proposed by docking and molecular dynamics simulations, respectively.

Tuning Down the Pain - An Overview of Allosteric Modulation of Opioid Receptors: Mechanisms of Modulation, Allosteric Sites, Modulator Syntheses

Opioid signaling plays a central role in pain perception. As such, it remains the main target in the development of antinociceptive agents, despite serious side effects involved. In recent years, hopes for improved opioid painkillers are rising, together with our understanding of allosterism and biased signaling mechanisms. In this review, we focus on recently discovered allosteric modulators of opioid receptors, insights into phenomena underlying their action, as well as on how they extend our understanding of mechanisms of previously known compounds. A brief overlook of their synthesis is also presented.

Computational insights into the molecular mechanisms of differentiated allosteric modulation at the mu opioid receptor by structurally similar bitopic modulators

Targeting the mu opioid receptor (MOR) by applying orthosteric ligands is the most frequently employed method to treat opioid use disorder (OUD). Unfortunately, most of MOR orthosteric ligands produce severe side effects, mainly due to their low selectivity over other opioid receptors. In contrast, some G protein-coupled receptor allosteric modulators have been reported to exhibit high subtype selectivity and can effectively modulate the potency and/or efficacy of orthosteric ligands. Recently, NAQ and its analog NCQ were identified as novel MOR bitopic modulators. Interestingly, NAQ and NCQ were similar in structure but exhibited different efficacy profiles to the MOR. NAQ exhibited an antagonism activity to the MOR while NCQ showed a partial agonism activity to the MOR. In the present study, molecular modeling methods were applied to explore the putative molecular mechanisms of their different functional profiles to the MOR. When NAQ binding with the inactive MOR, the 'address' portion of NAQ interacted with the MOR allosteric site but showed no significant allosteric modulation of the efficacy of the 'message' portion of NAQ. However, when NCQ binding with the inactive and active MOR, the 'address' portion of NCQ seemed to be able to positively modulate the efficacy of the 'message' portion of NCQ at varying levels. Evidentially, the substituents at the 1'- and 4'-positions of the isoquinoline ring of NCQ seemed to play a critical role in the modulatory function of the 'address' portion of NCQ. These findings will be invaluable to develop our next generation of MOR bitopic modulators with high affinity and subtype selectivity to potentially treat OUD.

Characterization of 17-Cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6α-(indole-7-carboxamido)morphinan (NAN) as a Novel Opioid Receptor Modulator for Opioid Use Disorder Treatment

The opioid crisis is a significant public health issue with more than 115 people dying from opioid overdose per day in the United States. The aim of the present study was to characterize the in vitro and in vivo pharmacological effects of 17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6α-(indole-7-carboxamido)morphinan (NAN), a μ opioid receptor (MOR) ligand that may be a potential candidate for opioid use disorder treatment that produces less withdrawal signs than naltrexone. The efficacy of NAN was compared to varying efficacy ligands at the MOR, and determined at the δ opioid receptor (DOR) and κ opioid receptor (KOR). NAN was identified as a low efficacy partial agonist for G-protein activation at the MOR and DOR, but had relatively high efficacy at the KOR. In contrast to high efficacy MOR agonists, NAN did not induce MOR internalization, downregulation, or desensitization, but it antagonized agonist-induced MOR internalization and stimulation of intracellular Ca2+ release. Opioid withdrawal studies conducted using morphine-pelleted mice demonstrated that NAN precipitated significantly less withdrawal signs than naltrexone at similar doses. Furthermore, NAN failed to produce fentanyl-like discriminative stimulus effects in rats up to doses that produced dose- and time-dependent antagonism of fentanyl. Overall, these results provide converging lines of evidence that NAN functions mainly as a MOR antagonist and support further consideration of NAN as a candidate medication for opioid use disorder treatment.

Structure-Activity Relationship Studies of 6α- and 6β-Indolylacetamidonaltrexamine Derivatives as Bitopic Mu Opioid Receptor Modulators and Elaboration of the "Message-Address Concept" To Comprehend Their Functional Conversion

Structure-activity relationship (SAR) studies of numerous opioid ligands have shown that introduction of a methyl or ethyl group on the tertiary amino group at position 17 of the epoxymorphinan skeleton generally results in a mu opioid receptor (MOR) agonist while introduction of a cyclopropylmethyl group typically leads to an antagonist. Furthermore, it has been shown that introduction of heterocyclic ring systems at position 6 can favor antagonism. However, it was reported that 17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6β-[(2'-indolyl)acetamido]morphinan (INTA), which bears a cyclopropylmethyl group at position 17 and an indole ring at position 6, acted as a MOR agonist. We herein report a SAR study on INTA with a series of its complementary derivatives to understand how introduction of an indole moiety with α or β linkage at position 6 of the epoxymorphinan skeleton may influence ligand function. Interestingly, one of INTA derivatives, compound 15 (NAN) was identified as a MOR antagonist both in vitro and in vivo. Molecular modeling studies revealed that INTA and NAN may interact with different domains of the MOR allosteric binding site. In addition, INTA may interact with W293 and N150 residues found in the orthosteric site to stabilize MOR activation conformation while NAN does not. These results suggest that INTA and NAN may be bitopic ligands and the type of allosteric interactions with the MOR influence their functional activity. These insights along with our enriched comprehension of the "message-address" concept will to benefit future ligand design.

Bifunctional opioid receptor ligands as novel analgesics

Prolonged treatment of chronic severe pain with opioid analgesics is frought with problematic adverse effects including tolerance, dependence, and life-threatening respiratory depression. Though these effects are mediated predominately through preferential activation of μ opioid peptide (μOP) receptors, there is an emerging appreciation that actions at κOP and δOP receptors contribute to the observed pharmacologic and behavioral profile of μOP receptor agonists and may be targeted simultaneously to afford improved analgesic effects. Recent developments have also identified the related nociceptin opioid peptide (NOP) receptor as a key modulator of the effects of μOP receptor signaling. We review here the available literature describing OP neurotransmitter systems and highlight recent drug and probe design strategies.

Methylation Products of 6β- N-Heterocyclic Substituted Naltrexamine Derivatives as Potential Peripheral Opioid Receptor Modulators

Two 6β- N-heterocyclic naltrexamine derivatives, NAP and NMP, have been identified as peripherally selective mu opioid receptor (MOR) antagonists. To further enhance the peripheral selectivity of both compounds, the 17-amino group and the nitrogen atom of the pyridine ring in both NAP and NMP were methylated to obtain dMNAP and dMNMP, respectively. Compared with NAP and NMP, the binding affinities of dMNAP and dMNMP shifted to MOR and KOR (kappa opioid receptor) dual selective and they acted as moderate efficacy partial agonists. The results from radioligand binding studies were further confirmed by molecular docking studies. In vivo studies demonstrated that dMNAP and dMNMP did not produce antinociception nor did they antagonize morphine's antinociceptive activity, indicating that these compounds did not act on the central nervous system. Meanwhile, both dMNAP and dMNMP significantly slowed down fecal excretion, which indicated that they were peripherally acting opioid receptor agonists. All together, these results suggested that dMNAP and dMNMP acted as peripheral mu/kappa opioid receptor modulators and may be applicable in the treatment of diarrhea in patients with bowel dysfunction.

Role of mu-opioid agonist efficacy on antinociceptive interactions between mu agonists and the nociceptin opioid peptide agonist Ro 64-6198 in rhesus monkeys

Mu-opioid receptor agonists are clinically effective analgesics, but also produce undesirable effects that limit their clinical utility. The nociceptin opioid peptide (NOP) receptor system also modulates nociception, and NOP agonists might be useful adjuncts to enhance the analgesic effects or attenuate the undesirable effects of mu-opioid agonists. The present study determined behavioral interactions between the NOP agonist (-)-Ro 64-6198 and mu-opioid ligands that vary in mu-opioid receptor efficacy (17-cyclopropylmethyl-3,14β-dihyroxy-4,5α-epoxy-6α-[(3 ́-isoquinolyl)acetamindo]morphinan (NAQ) < buprenorphine < nalbuphine < morphine = oxycodone < methadone) in male rhesus monkeys. For comparison, Ro 64-6198 interactions were also examined with the kappa-opioid receptor agonist nalfurafine. Each opioid ligand was examined alone and following fixed-dose Ro 64-6198 pretreatments in assays of thermal nociception (n = 3-4) and schedule-controlled responding (n = 3). Ro 64-6198 alone failed to produce significant antinociception up to doses (0.32 mg/kg, IM) that significantly decreased rates of responding. All opioid ligands, except NAQ and nalfurafine, produced dose- and thermal intensity-dependent antinociception. Ro 64-6198 enhanced the antinociceptive potency of buprenorphine, nalbuphine, methadone, and nalfurafine. Ro 64-6198 enhancement of nalbuphine antinociception was NOP antagonist SB-612111 reversible and occurred under a narrow range of dose and time conditions. All opioid ligands, except NAQ and buprenorphine, produced dose-dependent decreases in rates of responding. Ro 64-6198 did not significantly alter mu-opioid ligand rate-decreasing effects. Although these results suggest that NOP agonists may selectively enhance the antinociceptive vs. rate-suppressant effects of some mu-opioid agonists, this small enhancement occurred under a narrow range of conditions dampening enthusiasm for NOP agonists as candidate "opioid-sparing" adjuncts.

Binding mode analyses of NAP derivatives as mu opioid receptor selective ligands through docking studies and molecular dynamics simulation

Mu opioid receptor selective antagonists are highly desirable because of their utility as pharmacological probes for receptor characterization and functional studies. Furthermore, the mu opioid receptors act as an important target in drug abuse and addiction treatment. Previously, we reported NAP as a novel lead compound with high selectivity and affinity towards the mu opioid receptor. Based on NAP, we have synthesized its derivatives and further characterized their binding affinities and selectivity towards the receptor. NMP and NGP were identified as the two most selective MOR ligands among NAP derivatives. In the present study, molecular modeling methods were applied to assess the dual binding modes of NAP derivatives, particularly on NMP and NGP, in three opioid receptors, in order to analyze the effects of structural modifications on the pyridyl ring of NAP on the binding affinity and selectivity. The results indicated that the steric hindrance, electrostatic, and hydrophobic effects caused by the substituents on the pyridyl ring of NAP contributed complimentarily on the binding affinity and selectivity of NAP derivatives to three opioid receptors. Analyses of these contributions provided insights on future design of more potent and selective mu opioid receptor ligands.

Design, synthesis, and characterization of rhein analogs as novel inhibitors of scavenger receptor A

Scavenger receptor A (SRA) has been known as an immunosuppressive factor and therefore therapeutic inhibition of SRA may be potentially exploited for cancer immunotherapy. Our previously work suggested that rhein may act as an inhibitor of SRA in reversing immunosuppression of SRA during T cells activation. Herein, three deconstruction analogs of rhein, compound 1, 2, and 3, were further studied as inhibitors of SRA. These three compounds, particularly compound 1, also known as a natural product danthron, enhanced T cells activation, indicated by increased transcriptional activation of interleukin 2 (Il2) gene, production of IL-2 protein, and proliferation of T cells. Additionally, the interaction between these compounds and SRA was studied by molecular modeling. Compound 1 showed a favorable binding mode with the cysteine rich domain of SRA protein compared to compound 2 and 3. Collectively, those results would provide insight for future design and development of next generation rhein derivatives as SRA inhibitors.

17-Cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6β-(4'-pyridylcarboxamido)morphinan (NAP) Modulating the Mu Opioid Receptor in a Biased Fashion

Mounting evidence has suggested that G protein-coupled receptors can be stabilized in multiple conformations in response to distinct ligands, which exert discrete functions through selective activation of various downstream signaling events. In accordance with this concept, we report biased signaling of one C6-heterocyclic substituted naltrexamine derivative, namely, 17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6β-(4'-pyridylcarboxamido)morphinan (NAP) at the mu opioid receptor (MOR). NAP acted as a low efficacy MOR partial agonist in the G protein-mediated [(35)S]GTPγS binding assay, whereas it did not significantly induce calcium flux or β-arrestin2 recruitment. In contrast, it potently blocked MOR full agonist-induced β-arrestin2 recruitment and translocation. Additionally, NAP dose-dependently antagonized MOR full agonist-induced intracellular calcium flux and β-arrestin2 recruitment. Further results in an isolated organ bath preparation confirmed that NAP reversed the morphine-induced reduction in colon motility. Ligand docking and dynamics simulation studies of NAP at the MOR provided more supporting evidence for biased signaling of NAP at an atomic level. Due to the fact that NAP is MOR selective and preferentially distributed peripherally upon systemic administration while β-arrestin2 is reportedly required for impairment of intestinal motility by morphine, biased antagonism of β-arrestin2 recruitment by NAP further supports its utility as a treatment for opioid-induced constipation.

Design, syntheses, and pharmacological characterization of 17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6α-(isoquinoline-3'-carboxamido)morphinan analogues as opioid receptor ligands

A series of 17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6α-(isoquinoline-3'-carboxamido)morphinan (NAQ) analogues were synthesized and pharmacologically characterized to study their structure-activity relationship at the mu opioid receptor (MOR). The competition binding assay showed two-atom spacer and aromatic side chain were optimal for MOR selectivity. Meanwhile, substitutions at the 1'- and/or 4'-position of the isoquinoline ring retained or improved MOR selectivity over the kappa opioid receptor while still possessing above 20-fold MOR selectivity over the delta opioid receptor. In contrast, substitutions at the 6'- and/or 7'-position of the isoquinoline ring reduced MOR selectivity as well as MOR efficacy. Among this series of ligands, compound 11 acted as an antagonist when challenged with morphine in warm-water tail immersion assay and produced less significant withdrawal symptoms compared to naltrexone in morphine-pelleted mice. Compound 11 also antagonized the intracellular Ca(2+) increase induced by DAMGO. Molecular dynamics simulation studies of 11 in three opioid receptors indicated orientation of the 6'-nitro group varied significantly in the different 'address' domains of the receptors and played a crucial role in the observed binding affinities and selectivity. Collectively, the current findings provide valuable insights for future development of NAQ-based MOR selective ligands.

Endogenous opiates and behavior: 2013

This paper is the thirty-sixth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2013 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, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.