Optimization of a Series of Mu Opioid Receptor (MOR) Agonists with High G Protein Signaling Bias

Synthesis and evaluation of 3,4,5-trisubstituted triazoles as G protein-biased kappa opioid receptor agonists

Kappa opioid receptor (KOR) agonists represent promising therapeutics for pain relief due to their analgesic properties along with lower abuse potential than opioids that act at the mu opioid receptor. However, typical KOR agonists produce sedation and dysphoria. Previous studies have shown that G protein signaling-biased KOR agonists may present a means to untangle the desired analgesic properties from undesired side effects. In this paper, we report a new series of G protein signaling-biased KOR agonists entailing -S- → -CH2- replacement in a previously reported KOR agonist, triazole 1.1. With an optimized carbon linker in hand, further development of the scaffold was undertaken to investigate the appendages of the triazole core. The structure-activity relationship study of this series is described, including several analogues that display enhanced potency while maintaining G protein-signaling bias compared to triazole 1.1.

Discovery of novel pyridone-benzamide derivatives possessing a 1-methyl-2-benzimidazolinone moiety as potent EZH2 inhibitors for the treatment of B-cell lymphomas

Enhancer of zeste homolog 2 (EZH2) is a promising therapeutic target for diffuse large B-cell lymphoma. In this study, based on the binding model of 1 (tazemetostat) with polycomb repressive complex 2 (PRC2), we designed and synthesized a series of tazemetostat analogs bearing a 1-methyl-2-benzimidazolinone moiety to improve the inhibitory activity of EZH2 wild-type (WT) and Y641 mutants and enhance metabolic stability. After the assessment of the structure-activity relationship at enzymatic and cellular levels, compound N40 was identified. Biochemical assays showed that compound N40 (IC50 = 0.32 nM) exhibited superior inhibitory activity against EZH2 WT, compared with 1 (IC50 = 1.20 nM), and high potency against EZH2 Y641 mutants (EZH2 Y641F, IC50 = 0.03 nM; EZH2 Y641N, IC50 = 0.08 nM), which were approximately 10-fold more active than those of 1 (EZH2 Y641F, IC50 = 0.37 nM; EZH2 Y641N, IC50 = 0.85 nM). Furthermore, compound N40 (IC50 = 3.52 ± 1.23 nM) effectively inhibited the proliferation of Karpas-422 cells and was more potent than 1 (IC50 = 35.01 ± 1.28 nM). Further cellular experiments showed that N40 arrested Karpas-422 cells in the G1 phase and induced apoptosis in a dose-dependent manner. Moreover, N40 inhibited the trimethylation of lysine 27 on histone H3 (H3K27Me3) in Karpas-422 cells bearing the EZH2 Y641N mutant. Additionally, N40 (T1/2 = 177.69 min) showed improved metabolic stability in human liver microsomes compared with 1 (T1/2 = 7.97 min). Our findings suggest N40 as a promising EZH2 inhibitor; further investigation remains warranted to confirm our findings and further develop N40.

Plasma drug screening using paper spray mass spectrometry with integrated solid phase extraction

Drug overdoses have risen dramatically in recent years. We developed a simple nontargeted method using a disposable paper spray cartridge with an integrated solid phase extraction column. This method was used to screen for ~160 fentanyl analogs, synthetic cannabinoids, other synthetic drugs, and traditional drugs of abuse in over 300 authentic overdose samples collected at emergency departments in Indianapolis. A solid phase extraction step was implemented on the paper spray cartridge to enable subnanograms per milliliter synthetic drugs screening in plasma. Analysis was performed on a quadrupole orbitrap mass spectrometer using the sequential window acquisition of all theoretical fragment ion spectra approach in which tandem mass spectrometry was performed using 7 m/z isolation windows in the quadrupole. Calibration curves with isotopically labeled internal standards were constructed for 35 of the most frequently encountered synthetic and traditional illicit drugs by US toxicology labs. Additional qualitative-only drugs in a suspect screening list were also included. Limits of detection in plasma for synthetic cannabinoids ranged from 0.1 to 0.5 and 0.1 to 0.3 ng/mL for fentanyl and its analogs and between 1 and 5 ng/mL for most other drugs. Relative matrix effects were evaluated by determining the variation of the calibration slope in 10 different lots of biofluid and found to be between 3% and 20%. The method was validated on authentic overdose samples collected from two emergency departments in Indianapolis, Indiana, from suspected or known overdoses. Commonly detected synthetic drugs included fentanyl related substances, designer benzodiazepines such as flubromazolam, and the synthetic cannabinoid 5F-PB-22.

A Novel Bifunctional μOR Agonist and σ1R Antagonist with Potent Analgesic Responses and Reduced Adverse Effects

Bifunctional ligands possessing both μOR agonism and σ1R antagonism have shown promise in producing strong analgesic effects with reduced opioid-related side effects. However, the μOR agonism activity of most dual ligands diminishes compared with classical opioids, raising concern about their effectiveness in managing nociceptive pain. In this study, a new class of dual μOR agonist/σ1R antagonist was reported. Through structure-activity relationship analyses, we identified the optimal compound, 4x, which displayed picomolar μOR agonism activity (EC50: 0.6 ± 0.2 nM) and good σ1R inhibitory activity (Ki: 363.7 ± 5.6 nM) with excellent selectivity. Compound 4x exhibited robust analgesic effects in various pain models, with significantly reduced side effects. Importantly, compound 4x also possessed good safety profiles and no abnormalities were observed in biological parameters even under a high dosage. Our findings suggest that 4x may be a promising lead compound for developing safer opioids and warrants further in-depth studies.

Comparative neuropharmacology of structurally distinct non-fentanyl opioids that are appearing on recreational drug markets worldwide

BACKGROUND

The emergence of novel synthetic opioids (NSOs) is contributing to the opioid overdose crisis. While fentanyl analogs have historically dominated the NSO market, a shift towards non-fentanyl compounds is now occurring.

METHODS

Here, we examined the neuropharmacology of structurally distinct non-fentanyl NSOs, including U-47700, isotonitazene, brorphine, and N-desethyl isotonitazene, as compared to morphine and fentanyl. Compounds were tested in vitro using opioid receptor binding assays in rat brain tissue and by monitoring forskolin-stimulated cAMP accumulation in cells expressing the human mu-opioid receptor (MOR). Compounds were administered subcutaneously to male Sprague-Dawley rats, and hot plate antinociception, catalepsy score, and body temperature changes were measured.

RESULTS

Receptor binding results revealed high MOR selectivity for all compounds, with MOR affinities comparable to those of morphine and fentanyl (i.e., nM). All drugs acted as full-efficacy MOR agonists in the cyclic AMP assay, but nitazene analogs had greater functional potencies (i.e., pM) compared to the other drugs (i.e., nM). When administered to rats, all compounds induced opioid-like antinociception, catalepsy, and body temperature changes, but nitazenes were the most potent. Similar to fentanyl, the nitazenes had faster onset and decline of in vivo effects when compared to morphine. In vivo potencies to induce antinociception and catalepsy (i.e., ED50s) correlated with in vitro functional potencies (i.e., EC50s) but not binding affinities (i.e., Kis) at MOR.

CONCLUSIONS

Collectively, our findings indicate that non-fentanyl NSOs pose grave danger to those individuals who use opioids. Continued vigilance is needed to identify and characterize synthetic opioids as they emerge in clandestine drug markets.

Detection, chemical analysis, and pharmacological characterization of dipyanone and other new synthetic opioids related to prescription drugs

The emergence of structurally diverse new synthetic opioids (NSOs) has caused the opioid crisis to spiral to new depths. Little information is available about the pharmacology of most novel opioids when they first emerge. Here, using a β-arrestin 2 recruitment assay, we investigated the in vitro μ-opioid receptor (MOR) activation potential of dipyanone, desmethylmoramide, and acetoxymethylketobemidone (O-AMKD) - recent NSOs that are structurally related to the prescription opioids methadone and ketobemidone. Our findings indicate that dipyanone (EC₅₀=39.9 nM; E_max=155% vs. hydromorphone) is about equally active as methadone (EC₅₀=50.3 nM; E_max=152%), whereas desmethylmoramide (EC₅₀=1335 nM; E_max=126%) is considerably less active. A close structural analogue of ketobemidone (EC₅₀=134 nM; E_max=156%) and methylketobemidone (EC₅₀=335 nM; E_max=117%), O-AMKD showed a lower potency (EC₅₀=1262 nM) and efficacy (E_max=109%). Evaluation of the opioid substitution product buprenorphine and its metabolite norbuprenorphine confirmed the increased in vitro efficacy of the latter. In addition to in vitro characterization, this report details the first identification and full chemical analysis of dipyanone in a seized powder, as well as a postmortem toxicology case from the USA involving the drug. Dipyanone was quantified in blood (370 ng/mL), in which it was detected alongside other NSOs (e.g., 2-methyl AP-237) and novel benzodiazepines (e.g., flualprazolam). While dipyanone is currently not commonly encountered in forensic samples worldwide, its emergence is worrisome and representative of the dynamic NSO market. Graphical Abstract.

Biased Agonism: Lessons from Studies of Opioid Receptor Agonists

In ligand bias different agonist drugs are thought to produce distinct signaling outputs when activating the same receptor. If these signaling outputs mediate therapeutic versus adverse drug effects, then agonists that selectively activate the therapeutic signaling pathway would be extremely beneficial. It has long been thought that μ-opioid receptor agonists that selectively activate G protein- over β-arrestin-dependent signaling pathways would produce effective analgesia without the adverse effects such as respiratory depression. However, more recent data indicate that most of the therapeutic and adverse effects of agonist-induced activation of the μ-opioid receptor are actually mediated by the G protein-dependent signaling pathway, and that a number of drugs described as G protein biased in fact may not be biased, but instead may be low-intrinsic-efficacy agonists. In this review we discuss the current state of the field of bias at the μ-opioid receptor and other opioid receptor subtypes.

Noncoding RNAs: Novel Targets for Opioid Tolerance

As a global health problem, chronic pain is one of the leading causes of disability, and it imposes a huge economic and public health burden on families and society. Opioids represent the cornerstone of analgesic drugs. However, opioid tolerance caused by long-term application of opioids is a major factor leading to drug withdrawal, serious side effects caused by dose increases, and even the death of patients, placing an increasing burden on individuals, medicine, and society. Despite efforts to develop methods to prevent and treat opioid tolerance, no effective treatment has yet been found. Therefore, understanding the mechanism underlying opioid tolerance is crucial for finding new prevention and treatment strategies. Noncoding RNAs (ncRNAs) are important parts of mammalian gene transcriptomes, and there are thousands of unique noncoding RNA sequences in cells. With the rapid development of high-throughput genome technology, research on ncRNAs has become a hot topic in biomedical research. In recent years, studies have shown that ncRNAs mediate physiological and pathological processes, including chromatin remodeling, transcription, posttranscriptional modification and signal transduction, which are key regulators of physiological processes in developmental and disease environments and have become biomarkers and potential therapeutic targets for various diseases. An increasing number of studies have found that ncRNAs are closely related to the development of opioid tolerance. In this review, we have summarized the evidence that ncRNAs play an important role in opioid tolerance and that ncRNAs may be novel targets for opioid tolerance.

Proliferation of Novel Synthetic Opioids in Postmortem Investigations After Core-Structure Scheduling for Fentanyl-Related Substances

ABSTRACT

New generations of novel synthetic opioids (NSOs) have emerged to fill a void in the illicit drug markets left by the decline in popularity of fentanyl analogs subsequent to core-structure scheduling of fentanyl-related substances in the United States and China. These new opioids include members of the 2-benzyl benzimidazole (eg, isotonitazene, metonitazene, N -pyrrolidino etonitazene, protonitazene, etodesnitazene), benzimidazolone (eg, brorphine), and cinnamylpiperazine (eg, AP-238, 2-methyl AP-237) subclasses. Novel synthetic opioids continue to be detected in opioid-related fatal overdoses, demonstrating the harms associated with exposure to these drugs. Between January 2020 and December 2021, 384 casework blood samples were reported by our laboratory to contain 1 or more of the prior listed 8 NSOs. Isotonitazene (n = 144), metonitazene (n = 122), and brorphine (n = 91) were the 3 most prevalent substances, with positivity for isotonitazene and brorphine peaking just before the announcement of emergency scheduling. These NSOs have been documented as significant drivers of drug mortality, and this case series described here highlights the challenges medical examiners and coroners face in staying current with emerging drugs. Challenges include regional differences, rapid turnover, short lifecycles, variable toxicology testing, and difficulty in assessing individual drug toxicity in polydrug cases.

Application of Fabric Phase Sorptive Extraction (FPSE) Engaged to Tandem LC-MS/MS for Quantification of Brorphine in Oral Fluid

Brorphine (1-[1-[1-(4-bromophenyl) ethyl]-piperidin-4-yl]-1,3-dihydro-2H-benzo [d]imidazol-2-one) is one of the most recent novel synthetic opioids (NSOs) on the novel psychoactive substances (NPSs) market, involved in over 100 deaths in 2020. Brorphine is a substituted piperidine-benzimidazolone analogue that retains structural similarities to fentanyl, acting as a full agonist at the μ-opioid receptor. Oral Fluid (OF) is an alternative matrix, frequently analyzed for the detection of NPS. Fabric phase sorptive extraction (FPSE) is a superior, green-sample -preparation technology recently applied for drug analysis. This contribution presents the development and validation of a method, based on the application of FPSE and liquid chromatography-tandem mass spectrometry (LC-MS/MS), to determine/quantitate brorphine in OF. The method's linearity ranged between 0.05 and 50 ng/mL (R² = 0.9993), the bias ranged between 12.0 and 16.8%, and inter- and intra-day precisions ranged between 6.4 and 9.9%. Accuracy and extraction efficiency lied between 65 and 75%. LOD/LOQ were 0.015 ng/mL/0.05 ng/mL. Analyte's post-preparative stability was higher than 95%, while no matrix interferences and carryover between runs were observed. This is the first report introducing the application of FPSE for NPS determination, specifically, the quantification of brorphine in OF, thereby presenting a simple, rapid, sensitive, specific, effective, and reliable procedure engaged to LC-MS/MS that is suitable for routine application and the analysis of more NPSs.

A systematic review on the kappa opioid receptor and its ligands: New directions for the treatment of pain, anxiety, depression, and drug abuse

Kappa opioid receptor (KOR) is a member of the opioid receptor system, the G protein-coupled receptors that are expressed throughout the peripheral and central nervous systems and play crucial roles in the modulation of antinociception and a variety of behavioral states like anxiety, depression, and drug abuse. KOR agonists are known to produce potent analgesic effects and have been used clinically for the treatment of pain, while KOR antagonists have shown efficacy in the treatment of anxiety and depression. This review summarizes the history, design strategy, discovery, and development of KOR ligands. KOR agonists are classified as non-biased, G protein-biased, and β-arrestin recruitment-biased, according to their degrees of bias. The mechanisms and associated effects of the G protein signaling pathway and β-arrestin recruitment signaling pathway are also discussed. Meanwhile, KOR antagonists are classified as long-acting and short-acting, based on their half-lives. In addition, we have special sections for mixed KOR agonists and selective peripheral KOR agonists. The mechanisms of action and pharmacokinetic, pharmacodynamic, and behavioral studies for each of these categories are also discussed in this review.

The methodology for preparing domperidone: strategies, routes and reaction processes

Domperidone is a powerful peripheral dopamine receptor antagonist; however, a systematic review of the synthetic methods and processes of this drug has not been reported so far. This review summarizes the synthetic strategies, synthetic routes and reaction processes of domperidone in detail. Domperidone can be synthesized from the coupling reaction of two benzimidazolone derivatives (intermediates 1 and 2). Intermediate 1 can be prepared by two synthetic routes: the cyclization of o-phenylenediamine with carbonyl reagents followed by coupling with 1,3-dihalopropane, and the coupling reaction of o-halo or o-amino substituted nitrobenzene with 1,3-disubstituted propane followed by reduction and cyclization. The latter route avoids the production of di-substituted by-products and has higher reaction selectivity. Intermediate 2 is synthesized by coupling substituted nitrobenzene with 4-aminopiperidine followed by reduction and cyclization, which is similar to the synthetic route of intermediate 1. Understanding the advantages and drawbacks of these synthetic methodologies would provide insights for the development of new strategies to prepare domperidone. Moreover, the methods used to synthesize domperidone can provide alternative approaches in the preparation of drugs or compounds with similar structure.

Reports of Adverse Events Associated with Use of Novel Psychoactive Substances, 2017-2020: A Review

An important role of modern forensic and clinical toxicologists is to monitor the adverse events of novel psychoactive substances (NPS). Following a prior review from 2013 to 2016, this critical literature review analyzes and evaluates published case reports for NPS from January 2017 through December 2020. The primary objective of this study is to assist in the assessment and interpretation of these cases as well as provide references for confirmation methods. Chemistry, pharmacology, adverse events and user profiles (e.g., polypharmacy) for NPS are provided including case history, clinical symptoms, autopsy findings and analytical results. Literature reviews were performed in PubMed and Google Scholar for publications using search terms such as NPS specific names, general terms (e.g., ‘designer drugs’ and ‘novel psychoactive substances’), drug classes (e.g., ‘designer stimulants’) and outcome-based terms (e.g., ‘overdose’ and ‘death’). Government and website drug surveillance databases and abstracts published by professional forensic science organizations were also searched. Toxicological data and detailed case information were extracted, tabulated, analyzed and organized by drug category. Case reports included overdose fatalities (378 cases), clinical treatment and hospitalization (771 cases) and driving under the influence of drugs (170 cases) for a total of 1,319 cases providing details of adverse events associated with NPS. Confirmed adverse events with associated toxidromes of more than 60 NPS were reported including synthetic cannabinoid, NPS stimulant, NPS hallucinogen, NPS benzodiazepine and NPS opioid cases. Fifty of these NPS were reported for the first time in January 2017 through December 2020 as compared to the previous 4 years surveyed. This study provides insight and context of case findings described in the literature and in digital government surveillance databases and websites during a recent 4-year period. This review will increase the awareness of adverse events associated with NPS use to better characterize international emerging drug threats.

LncRNA MRAK159688 facilitates morphine tolerance by promoting REST-mediated inhibition of mu opioid receptor in rats

Morphine tolerance (MT) caused by the long-term use of morphine is a major medical problem. The molecular mechanism of morphine tolerance remains elusive. Here, we established a morphine tolerance model in rats and verified whether the long noncoding RNA (lncRNA) MRAK159688 is involved in morphine tolerance and its specific molecular mechanism. We show the significant upregulation of MRAK159688 expression in the spinal cord of morphine-tolerant rats. Overexpression of MRAK159688 by a lentivirus reduces the analgesic efficacy of morphine and induces pain behavior. Downregulation of MRAK159688 using a small interfering RNA (siRNA) attenuates the formation of morphine tolerance, partially reverses the development of morphine tolerance and alleviates morphine-induced hyperalgesia. MRAK159688 is located in the nucleus and cytoplasm of neurons, and it colocalizes with repressor element-1 silencing transcription factor (REST) in the nucleus. MRAK159688 potentiates the expression and function of REST, thereby inhibiting the expression of mu opioid receptor (MOR) and subsequently inducing morphine tolerance. Moreover, REST overexpression blocks the effects of MRAK159688 siRNA on relieving morphine tolerance. In general, chronic morphine administration-mediated upregulation of MRAK159688 in the spinal cord contributes to morphine tolerance and hyperalgesia by promoting REST-mediated inhibition of MOR. MRAK159688 downregulation may represent a novel RNA-based therapy for morphine tolerance.

Optimization of bifunctional piperidinamide derivatives as σ1R Antagonists/MOR agonists for treating neuropathic pain

Here, we describe the optimization, synthesis, and associated pharmacological analgesic activities of a new series of bifunctional piperidinamide derivatives as sigma-1 receptor (σ1R) antagonists and mu opioid receptor (MOR) agonists. The new compounds were evaluated in vitro in σ1R and MOR binding assays. The most promising compound 114 (also called HKC-126), showed superior affinities for σ1R and MOR and good selectivity to additional receptors related to pain. Compound 114 showed powerful dose-dependent analgesic effects in the acetic acid writhing test, formalin test, hot plate test, and chronic constriction injury (CCI) neuropathic pain model. In contrast to an equianalgesic dose of fentanyl, compound 114 produced fewer opioid-like side effects, such as reward liability, respiratory depression, physical dependence, and sedation. Lastly, the pharmacokinetic properties of this drug were also acceptable, and these results suggest that compound 114, as a mixed σ1R/MOR ligand, has potential for treating neuropathic pain.

Oxidative Metabolism as a Modulator of Kratom's Biological Actions

The leaves of Mitragyna speciosa (kratom), a plant native to Southeast Asia, are increasingly used as a pain reliever and for attenuation of opioid withdrawal symptoms. Using the tools of natural products chemistry, chemical synthesis, and pharmacology, we provide a detailed in vitro and in vivo pharmacological characterization of the alkaloids in kratom. We report that metabolism of kratom's major alkaloid, mitragynine, in mice leads to formation of (a) a potent mu opioid receptor agonist antinociceptive agent, 7-hydroxymitragynine, through a CYP3A-mediated pathway, which exhibits reinforcing properties, inhibition of gastrointestinal (GI) transit and reduced hyperlocomotion, (b) a multifunctional mu agonist/delta-kappa antagonist, mitragynine pseudoindoxyl, through a CYP3A-mediated skeletal rearrangement, displaying reduced hyperlocomotion, inhibition of GI transit and reinforcing properties, and (c) a potentially toxic metabolite, 3-dehydromitragynine, through a non-CYP oxidation pathway. Our results indicate that the oxidative metabolism of the mitragynine template beyond 7-hydroxymitragynine may have implications in its overall pharmacology in vivo.

Encoding mu-opioid receptor biased agonism with interaction fingerprints

Opioids are potent painkillers, however, their therapeutic use requires close medical monitoring to diminish the risk of severe adverse effects. The G-protein biased agonists of the μ-opioid receptor (MOR) have shown safer therapeutic profiles than non-biased ligands. In this work, we performed extensive all-atom molecular dynamics simulations of two markedly biased ligands and a balanced reference molecule. From those simulations, we identified a protein-ligand interaction fingerprint that characterizes biased ligands. Then, we built and virtually screened a database containing 68,740 ligands with proven or potential GPCR agonistic activity. Exemplary molecules that fulfill the interacting pattern for biased agonism are showcased, illustrating the usefulness of this work for the search of biased MOR ligands and how this contributes to the understanding of MOR biased signaling.

Pharmacological and metabolic characterization of the novel synthetic opioid brorphine and its detection in routine casework

After their first emergence in 2009, Novel synthetic opioids (NSO) have become an emerging class of New Psychoactive Substances (NPS) on the market for these new drugs. So far, 67 NSO have been reported to the Early Warning system of the European Monitoring Centre for Drugs and Drug Addiction (EMCDDA). It is presumed that NSO mainly target the four known opioid receptors, i.e. the μ-opioid (MOR), the δ-opioid (DOR), the κ-opioid (KOR) and nociceptin receptors and that their consumption can result in serious adverse effects such as massive respiratory depression or death. In the present study we investigated the in vivo and in vitro metabolism of brorphine, a NSO that was first identified on the NPS market in August 2019 in the United States, using both a pooled human liver microsome assay and real forensic case samples. For the detection of metabolites LC-HR-MS/MS was used and quantification of brorphine was performed using an LC-MS/MS method. Additionally, we pharmacologically characterized brorphine regarding its activation of the MOR and KOR via G protein recruitment using the [³⁵S]-GTPγS assay. In forensic urine samples, 14 distinct metabolites were identified, whereas in blood only four metabolites could be found. The pooled human liver microsome assay generated six distinct in vitro phase I metabolites. The most prominent in vivo metabolite was formed by N-oxydation, whereas the main in vitro metabolite was formed by hydroxylation. The pharmacological characterization at the MOR and KOR revealed brorphine to be a potent MOR agonist and a weak, partial KOR agonist in the [³⁵S]-GTPγS assay.

Morphine attenuates neuroinflammation and blood-brain barrier disruption following traumatic brain injury through the opioidergic system

Endogenous opiates are suggested to have a role in the pathophysiology of traumatic brain injury (TBI). Furthermore, administration of opioidergic agents in TBI injured animals have been shown to affect the brain injury and provide neuroprotection post-TBI. This study aims to investigate the potential neuroprotective effects of morphine through inhibition of neuroinflammatory pathways in acute severe TBI. Male Wistar rats were divided into seven groups (24 rats per group): Sham, Vehicle (TBI + intraperitoneal (i.p) injection of normal saline), TBI + i.p injection of morphine in 1, 5 and 10 mg/kg doses (MOR 1, MOR 5 and MOR 10 groups), TBI + morphine (5 mg/kg i.p) + Naloxone (NAL + MOR), and TBI + morphine (5 mg/kg i.p) + Naltrindole (NALT + MOR). A severe diffuse TBI model (weight dropping Marmarou model) was used to induce TBI in rats. The veterinary coma scale (VCS), beam-walk, and beam-balance tasks were used to assess short-term neurological deficits. Histolopathological changes of brain tissue was evaluated using light microscopy and hematoxilin and eosin staining. Blood-Brain barrier (BBB) disruption was evaluated by the Evans Blue method 6 h post-injury. Brain water content and cerebrospinal fluid (CSF) content of IL-1β and IL-10 were assessed by the wet-dry method and enzyme-linked immunosorbent assay (ELISA), respectively. Morphine (1 and 5 mg/kg doses) attenuated BBB leakage, improved VCS score, pathological changes of brain tissue, and vestibulomotor function compared to the vehicle group (p < 0.0001). Only 5 mg/kg morphine attenuated brain edema (p < 0.0001). Furthermore, 1 and 5 mg/kg morphine significantly changed CSF concentration of IL-1β and IL-10 compared to the vehicle group (p < 0.0001). Inhibition of opioid receptors by naloxone and naltrindole abolished morphine neuroprotective effects (p < 0.0001 vs. MOR 5 group). This study suggests that morphine administration inhibits TBI-mediated neuroinflammation via opioid receptors and improves neurobehavioral function following TBI, which provides a potential therapeutic opportunity in the treatment of traumatic brain injury.

Low Intrinsic Efficacy Alone Cannot Explain the Improved Side Effect Profiles of New Opioid Agonists

In a recent report in Science Signaling (Gillis, A., et al. Low intrinsic efficacy for G protein activation can explain the improved side effect profiles of new opioid agonists. Sci. Signaling 2020, 13, eaaz3140 10.1126/scisignal.aaz3140), it was suggested that low intrinsic agonism, and not biased agonism, leads to an improvement in the separation of potency in opioid-induced respiratory suppression versus antinociception. Although many of the compounds that were tested have been shown to display G protein signaling bias in prior publications, the authors conclude that because they cannot detect biased agonism in their cellular signaling studies the compounds are therefore not biased agonists. Rather, they conclude that it is low intrinsic efficacy that leads to the therapeutic window improvement. Intrinsic efficacy is the extent to which an agonist can stimulate a G protein-coupled receptor response in a system, while biased agonism takes into consideration not only the intrinsic efficacy but also the potency of an agonist in an assay. Herein, we have reanalyzed the data presented in the published work (10.1126/scisignal.aaz3140) [including the recent Erratum (10.1126/scisignal.abf9803)] to derive intrinsic efficacy and bias factors as ΔΔlog(τ/K_A) and ΔΔlog(Emax/EC₅₀), respectively. On the basis of this reanalysis, the data support the conclusion that biased agonism, favoring G protein signaling, was observed. Moreover, a conservation of rank order intrinsic efficacy was not observed upon comparing responses in each assay, further suggesting that multiple active receptor states were present. These observations agree with prior studies in which oliceridine, PZM21, and SR-17018 were first described as biased agonists with improvement in antinociception over respiratory suppression in mice. Therefore, the data in the Science Signaling paper provide strong corroborating evidence that G protein signaling bias may be a means of improving opioid analgesia while avoiding certain undesirable side effects.

Online surveillance of novel psychoactive substances (NPS): Monitoring Reddit discussions as a predictor of increased NPS-related exposures

BACKGROUND

Novel psychoactive substances (NPS) present continuous and growing challenges for the scientific, medical, and interventional communities as emerging substances on recreational drug markets change national and international drug landscapes. NPS account for an increasing proportion of adverse events, hospitalizations, and deaths due to increasing potency and unanticipated biological effects compared to predecessors. This study evaluated the utility of drug use forums as an early indicator or predictor of impending intoxications with potentially harmful or lethal outcomes prior to their occurrences.

METHODS

Eight NPS were selected for evaluation to assess the relationship between online mentions of drugs and their involvement in toxic exposures or overdoses. Mentions on Reddit drug forum discussions were tallied and toxicology testing results from forensic investigations in the US were assessed. The selected NPS covered several subclasses and a predetermined time range (2013-2020). They included carfentanil, U-47700, eutylone, flualprazolam, N-ethylpentylone, 5F-MDMB-PICA, isotonitazene, and brorphine.

RESULTS

Seven NPS (excluding 5F-MDMB-PICA) appeared in discussions on Reddit prior to their implication in poisonings or intoxications. Distinct increases and decreases in number of mentions and number of exposures were observed. For most substances (n = 5, 63%), a rise in Reddit mentions was soon followed by a corresponding rise in toxicology positivity. Peak positivity for carfentanil and flualprazolam, however, preceded peak Reddit mentions.

CONCLUSIONS

This study demonstrated the utility of social media sites, such as Reddit, as a predictor for future trends in NPS-related exposures. These results provide confirmation that activity on drug use forums in the virtual world can help predict changes in exposures associated with new or re-emerging NPS in the real world. The results warrant further evaluation as a strategy for inclusion in early warning systems.

The Rise and Fall of Isotonitazene and Brorphine: Two Recent Stars in the Synthetic Opioid Firmament

Synthetic opioids constitute one of the fastest growing groups of new psychoactive substances (NPS) worldwide. With fentanyl analogues being increasingly controlled via class-wide scheduling, many non-fentanyl related opioids are now emerging on the recreational opioid market, rendering the landscape highly complex and dynamic. While new compounds are entering the supply in rapid and unpredictable manners, some recent patterns have become apparent. Many of these newly emerging opioids are being pirated from early patent literature and/or research papers, synthesized and sold online through various channels. Burdened by the identification of every newly emerging drug, many toxicology labs struggle to keep up. Moreover, by the time a "new" drug is controlled via legislative measures, illicit drug markets will have already adapted and diversified as manufacturers work to avoid the restricted product(s). Hence, the typical life-cycle of an NPS opioid is generally short (less than 6 months to one year), with only a few drugs escalating to significant numbers of detections. In this review, we summarize the key events in the emergence, rise, and subsequent decline of two non-fentanyl opioids - isotonitazene and brorphine. These two opioids sequentially dominated the NPS opioid market in 2019 and 2020. Both isotonitazene and brorphine remained in circulation for over a year, each contributing to hundreds of deaths and adverse events. By detailing the life-cycles of these opioids from their earliest synthesis as described in scientific literature to their subsequent rise and fall on recreational markets, this review illustrates the new characteristic life-cycle of synthetic opioids in the 'post-fentanyl-analogue' era.

Comprehensive overview of biased pharmacology at the opioid receptors: biased ligands and bias factors

One of the main challenges in contemporary medicinal chemistry is the development of safer analgesics, used in the treatment of pain. Currently, moderate to severe pain is still treated with the "gold standard" opioids whose long-term often leads to severe side effects. With the discovery of biased agonism, the importance of this area of pharmacology has grown exponentially over the past decade. Of these side effects, tolerance, opioid misuse, physical dependence and substance use disorder (SUD) stand out, since these have led to many deaths over the past decades in both USA and Europe. New therapeutic molecules that induce a biased response at the opioid receptors (MOR, DOR, KOR and NOP receptor) are able to circumvent these side effects and, consequently, serve as more advantageous therapies with great promise. The concept of biased signaling extends far beyond the already sizeable field of GPCR pharmacology and covering everything would be vastly outside the scope of this review which consequently covers the biased ligands acting at the opioid family of receptors. The limitation of quantifying bias, however, makes this a controversial subject, where it is dependent on the reference ligand, the equation or the assay used for the quantification. Hence, the major issue in the field of biased ligands remains the translation of the in vitro profiles of biased signaling, with corresponding bias factors to in vivo profiles showing the presence or the lack of specific side effects. This review comprises a comprehensive overview of biased ligands in addition to their bias factors at individual members of the opioid family of receptors, as well as bifunctional ligands.

Discovery, Structure-Activity Relationship, and Mechanistic Studies of 1-((3R,4S)-3-((Dimethylamino)methyl)-4-hydroxy-4-(3-methoxyphenyl)piperidin-1-yl)-2-(2,4,5-trifluorophenyl)ethan-1-one as a Novel Potent Analgesic

Management of moderate to severe pain relies heavily on opioid analgesics such as morphine, oxycodone, and fentanyl in clinics. However, their prolonged use was associated with undesirable side effects. Many new strategies to reduce side effects have been proposed, but not without disadvantages. Using a hot plate model as a phenotypic screening method, our studies identified (3R,4S)-9d with a new scaffold as a potent analgesic with ED₅₀ values of 0.54 mg/kg and 0.021 mg/kg in hot plate and antiwrithing models, respectively. Mechanistic studies showed that it elicited its analgesic effect via the active metabolite (3R,4S)-10a. The mechanism of (3R,4S)-10a-induced activation of the μ opioid receptor (MOR) was proposed by means of molecular dynamics (MD) simulation.

Novel Dual-Target μ-Opioid Receptor and Dopamine D3 Receptor Ligands as Potential Nonaddictive Pharmacotherapeutics for Pain Management

The need for safer pain-management therapies with decreased abuse liability inspired a novel drug design that retains μ-opioid receptor (MOR)-mediated analgesia, while minimizing addictive liability. We recently demonstrated that targeting the dopamine D₃ receptor (D₃R) with highly selective antagonists/partial agonists can reduce opioid self-administration and reinstatement to drug seeking in rodent models without diminishing antinociceptive effects. The identification of the D₃R as a target for the treatment of opioid use disorders prompted the idea of generating a class of ligands presenting bitopic or bivalent structures, allowing the dual-target binding of the MOR and D₃R. Structure-activity relationship studies using computationally aided drug design and in vitro binding assays led to the identification of potent dual-target leads (23, 28, and 40), based on different structural templates and scaffolds, with moderate (sub-micromolar) to high (low nanomolar/sub-nanomolar) binding affinities. Bioluminescence resonance energy transfer-based functional studies revealed MOR agonist-D₃R antagonist/partial agonist efficacies that suggest potential for maintaining analgesia with reduced opioid-abuse liability.

Uncovering the analgesic effects of a pH-dependent mu-opioid receptor agonist using a model of nonevoked ongoing pain

Currently, opioids targeting mu-opioid receptors are the most potent drugs for acute and cancer pain. However, opioids produce adverse side effects such as constipation, respiratory depression, or addiction potential. We recently developed (±)-N-(3-fluoro-1-phenethylpiperidine-4-yl)-N-phenyl propionamide (NFEPP), a compound that does not evoke central or intestinal side effects due to its selective activation of mu-opioid receptors at low pH in peripheral injured tissues. Although we demonstrated that NFEPP effectively abolishes injury-induced pain, hyperalgesia, and allodynia in rodents, the efficacy of NFEPP in nonevoked ongoing pain remains to be established. Here, we examined reward, locomotor activity, and defecation in rats with complete Freund's adjuvant-induced paw inflammation to compare fentanyl's and NFEPP's potentials to induce side effects and to inhibit spontaneous pain. We demonstrate that low, but not higher, doses of NFEPP produce conditioned place preference but not constipation or motor disturbance, in contrast to fentanyl. Using a peripherally restricted antagonist, we provide evidence that NFEPP-induced place preference is mediated by peripheral opioid receptors. Our results indicate that a low dose of NFEPP produces reward by abolishing spontaneous inflammatory pain.

Novel Approaches, Drug Candidates, and Targets in Pain Drug Discovery

Because of the problems associated with opioids, drug discovery efforts have been employed to develop opioids with reduced side effects using approaches such as biased opioid agonism, multifunctional opioids, and allosteric modulation of opioid receptors. Receptor targets such as adrenergic, cannabinoid, P2X3 and P2X7, NMDA, serotonin, and sigma, as well as ion channels like the voltage-gated sodium channels Nav1.7 and Nav1.8 have been targeted to develop novel analgesics. Several enzymes, such as soluble epoxide hydrolase, sepiapterin reductase, and MAGL/FAAH, have also been targeted to develop novel analgesics. In this review, old and recent targets involved in pain signaling and compounds acting at these targets are summarized. In addition, strategies employed to reduce side effects, increase potency, and efficacy of opioids are also elaborated. This review should aid in propelling drug discovery efforts to discover novel analgesics.

Addressing opioid tolerance and opioid-induced hypersensitivity: Recent developments and future therapeutic strategies

Opioids are a commonly prescribed and efficacious medication for the treatment of chronic pain but major side effects such as addiction, respiratory depression, analgesic tolerance, and paradoxical pain hypersensitivity make them inadequate and unsafe for patients requiring long-term pain management. This review summarizes recent advances in our understanding of the outcomes of chronic opioid administration to lay the foundation for the development of novel pharmacological strategies that attenuate opioid tolerance and hypersensitivity; the two main physiological mechanisms underlying the inadequacies of current therapeutic strategies. We also explore mechanistic similarities between the development of neuropathic pain states, opioid tolerance, and hypersensitivity which may explain opioids' lack of efficacy in certain patients. The findings challenge the current direction of analgesic research in developing non-opioid alternatives and we suggest that improving opioids, rather than replacing them, will be a fruitful avenue for future research.

DARK Classics in Chemical Neuroscience: Etonitazene and Related Benzimidazoles

Etonitazene and related 2-benzylbenzimidazoles are potent analgetics invented in the research laboratories of the Swiss pharmaceutical giant CIBA in the late 1950s. Though the unprecedented structure distinguishes this class of compounds from poppy-derived and other synthetic analgetics, a range of studies indicate that these drugs are selective μ opioid receptor agonists possessing morphine-like pharmacotoxicological properties in animals as well as humans. Several unscheduled members of this synthetically readily accessible class of opioids that are not controlled under the international and national drug control systems have recently emerged on the illicit drug market. Among them, isotonitazene has been implicated in at least 200 fatalities in Europe and North America. None of the 2-benzylbenzimidazole derivatives have been developed into medicines, but etonitazene and some of its derivatives have been used as receptor probes and in addiction behavior studies in animals. The unique structure has inspired research on such benzimidazoles and related benzimidazolones of which "brorphine" made its debut as one of the newest psychoactive substance to emerge on the illicit opioid drug market in mid-2019. This in-depth review provides a historical introduction, an overview on the chemistry, pharmacological profiles, adverse effects, addiction liability, regulatory status, and the impact on chemical neuroscience of the 2-benzylbenzimidazoles. Structurally related benzimidazoles with opioid and/or analgesic properties are also discussed briefly.

First Report on Brorphine: The Next Opioid on the Deadly New Psychoactive Substance Horizon?

New psychoactive substances continue to appear on the drug market. Until recently, new synthetic opioids, which are among the most dangerous new psychoactive substances, primarily encompassed analogs of the potent analgesic fentanyl. Lately, also other new synthetic opioids have increasingly started to surface. This is the first report on the identification and full chemical characterization of brorphine, a novel potent synthetic opioid with a piperidine benzimidazolone structure. A powder, identified as brorphine, was obtained from a patient seeking medical help for detoxification. Brorphine was also found in a serum sample of the patient. Liquid chromatography-high-resolution mass spectrometry (LC-HRMS) identified an exact mass of m/z 400.1020 and 402.1005 for the compound, corresponding to both bromine isotopes. Further chemical characterization was performed by gas chromatography-mass spectrometry, liquid chromatography-diode array detection and Fourier-transform infrared spectroscopy analyses. Finally, the structure was confirmed by performing 1H-NMR and 13C-NMR spectroscopy. In vitro biological activity of brorphine was determined by a cell-based µ-opioid receptor activation assay, resulting in an EC50 of 30.9 nM (13.5 ng/mL) and an Emax of 209% relative to hydromorphone, confirming the high potency and efficacy of this compound. In a serum sample of the patient, brorphine and a hydroxy-metabolite were found using the LC-HRMS screening method. The presence of opioid activity in the serum was also confirmed via the activity-based opioid screening assay. The occurrence of brorphine is yet another example of how the illicit drug market is continuously evolving in an attempt to escape international legislation. Its high potency poses a serious and imminent health threat for any user.

Brorphine-Investigation and quantitation of a new potent synthetic opioid in forensic toxicology casework using liquid chromatography-mass spectrometry

New synthetic opioids continue to appear as novel psychoactive substances (NPS) on illicit drug markets. Isotonitazene emerged in mid-2019, becoming the most prevalent NPS opioid in the United States within a few months. Notification by the Drug Enforcement Administration of its intent to schedule isotonitazene in mid-2020 led to its decline in popularity and replacement with a new NPS opioid: brorphine. Brorphine is a potent synthetic opioid, but little information was previously available regarding its toxicity or involvement in impairment and death. Our laboratory developed an assay for the identification and quantitative confirmation of brorphine using standard addition. Quantitative analysis was performed using liquid chromatography-tandem mass spectrometry (LC-MS/MS). In vitro and in vivo metabolism studies were performed using pooled human liver microsomes and authentic biological specimens, respectively, with analysis by liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QTOF-MS). Brorphine was confirmed in 20 authentic forensic cases, commonly found in combination with fentanyl (100%) and flualprazolam (80%). The average concentration of brorphine in blood was 2.5 ± 3.1 ng/mL (median: 1.1 ng/mL, range: 0.1-10 ng/mL). The average concentration of brorphine in urine was 4.6 ± 7.6 ng/mL (median: 1.6 ng/mL, range: 0.2-23 ng/mL). The majority of cases originated from Midwestern states. Metabolism was verified to included N-dealkylation and hydroxylation. Detailed case histories and autopsy findings are presented herein. The prevalence of brorphine continues to increase in the United States. Forensic scientists should remain aware of the ongoing emergence of new opioids, especially those outside a standard scope of toxicology testing.

Discovery of potentially biased agonists of mu-opioid receptor (MOR) through molecular docking, pharmacophore modeling, and MD simulation

Opioids are well known for their potent analgesic efficacy and severe side effects. Studies have shown that analgesic effects are mediated by the downstream G-protein-dependent pathway of the μ-opioid receptor (MOR), and another β-arrestin-dependent pathway mediates side effects such as respiratory depression, constipation and tolerance etc. TRV130 is a biased ligand for G-protein-dependent pathway, which has high analgesia and has fewer side effects than morphine. In this study, the structure similarity search was performed on the IBSSC database using Oliceridine (TRV130) and PZM21 as templates. The 3D structure-based pharmacophore model was built and combined molecular docking prediction mode was selected to filter out small molecules, Finally, based on affinity prediction, four candidate molecules were obtained. Molecular dynamics simulations explored the detailed interaction mechanism of proteins with small molecules under dynamics. These results suggest that these candidate molecules are potential MOR agonists.

The G Protein Signal-Biased Compound TRV130; Structures, Its Site of Action and Clinical Studies

Opioid agonists elicit their analgesic action mainly via μ opioid receptors; however, their use is limited because of adverse events including constipation and respiratory depression. It has been shown that analgesic action is transduced by the G protein-mediated pathway whereas adverse events are by the β-arrestin-mediated pathway through μ opioid receptor signaling. The first new-generation opioid TRV130, which preferentially activates G protein- but not β-arrestin-mediated signal, was constructed and developed to reduce adverse events. TRV130 and other G protein-biased compounds tend to elicit desirable analgesic action with less adverse effects. In clinical trials, the intravenous TRV130 (oliceridine) was evaluated in Phase I, II and III clinical studies. Here we review the discovery and synthesis of TRV130, its main action as a novel analgesic having less adverse events, its up-to-date status in clinical trials, and additional concerns about TRV130 as demonstrated in the literature.

Endogenous Opiates and Behavior: 2018

This paper is the forty-first consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2018 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (2), the roles of these opioid peptides and receptors in pain and analgesia in animals (3) and humans (4), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (5), opioid peptide and receptor involvement in tolerance and dependence (6), stress and social status (7), learning and memory (8), eating and drinking (9), drug abuse and alcohol (10), sexual activity and hormones, pregnancy, development and endocrinology (11), mental illness and mood (12), seizures and neurologic disorders (13), electrical-related activity and neurophysiology (14), general activity and locomotion (15), gastrointestinal, renal and hepatic functions (16), cardiovascular responses (17), respiration and thermoregulation (18), and immunological responses (19).

Biased Opioid Ligands

Achieving effective pain management is one of the major challenges associated with modern day medicine. Opioids, such as morphine, have been the reference treatment for moderate to severe acute pain not excluding chronic pain modalities. Opioids act through the opioid receptors, the family of G-protein coupled receptors (GPCRs) that mediate pain relief through both the central and peripheral nervous systems. Four types of opioid receptors have been described, including the μ-opioid receptor (MOR), κ-opioid receptor (KOR), δ-opioid receptor (DOR), and the nociceptin opioid peptide receptor (NOP receptor). Despite the proven success of opioids in treating pain, there are still some inherent limitations. All clinically approved MOR analgesics are associated with adverse effects, which include tolerance, dependence, addiction, constipation, and respiratory depression. On the other hand, KOR selective analgesics have found limited clinical utility because they cause sedation, anxiety, dysphoria, and hallucinations. DOR agonists have also been investigated but they have a tendency to cause convulsions. Ligands targeting NOP receptor have been reported in the preclinical literature to be useful as spinal analgesics and as entities against substance abuse disorders while mixed MOR/NOP receptor agonists are useful as analgesics. Ultimately, the goal of opioid-related drug development has always been to design and synthesize derivatives that are equally or more potent than morphine but most importantly are devoid of the dangerous residual side effects and abuse potential. One proposed strategy is to take advantage of biased agonism, in which distinct downstream pathways can be activated by different molecules working through the exact same receptor. It has been proposed that ligands not recruiting β-arrestin 2 or showing a preference for activating a specific G-protein mediated signal transduction pathway will function as safer analgesic across all opioid subtypes. This review will focus on the design and the pharmacological outcomes of biased ligands at the opioid receptors, aiming at achieving functional selectivity.

DAMGO-induced μ opioid receptor internalization and recycling restore morphine sensitivity in tolerant rat

This study investigated the effect of DAMGO-induced μ opioid receptor (MOR) internalization on morphine tolerance. Male Sprague-Dawley rats (200-250 g) aged 6-8 weeks were administered morphine via intrathecal (i.t.) injection (15 μg/10 μl twice daily for 6 days) to induce antinociceptive tolerance, which was evaluated using the tail-flick and paw-withdrawal tests. Response latency was calculated as the percentage of maximum possible effect (%MPE). A bolus of DAMGO was administered by i.t. injection on day 6, and the tail-flick and paw-withdrawal tests were carried out 24, 48, and 72 h later. Membrane and cytosolic MOR expression was assessed by western blotting. HEK293 cells were transfected with MOR-FLAG plasmid and after 6 days of morphine treatment (10 μM), the cells were treated with 1 μM DAMGO, and MOR localization was examined by immunofluorescence analysis 30 and 60 min later. Repeated morphine treatment induced tolerance after 5 days; however, i.t. DAMGO administration restored morphine sensitivity and enhanced acute morphine-induced antinociception after 24, 48, and 72 h. In HEK293 cells, DAMGO treatment stimulated MOR internalization after 30 min and MOR recycling to the membrane after 1 h. Membrane and cytoplasmic MOR expression in vivo was unchanged 24, 48, and 72 h after i.t. DAMGO injection. Morphine does not cause significant MOR internalization or downregulation, and can readily induce tolerance. DAMGO counters this effect by enhancing receptor endocytosis, thereby reversing morphine-induced antinociceptive tolerance and restoring its analgesic efficacy.

Synthesis, Radiosynthesis and Biological Evaluation of Buprenorphine-Derived Phenylazocarboxamides as Novel μ-Opioid Receptor Ligands

Targeted structural modifications have led to a novel type of buprenorphine-derived opioid receptor ligand displaying an improved selectivity profile for the μ-OR subtype. On this basis, it is shown that phenylazocarboxamides may serve as useful bioisosteric replacements for the widely occurring cinnamide units, without loss of OR binding affinity or subtype selectivity. This study further includes functional experiments pointing to weak partial agonist properties of the novel μ-OR ligands, as well as docking and metabolism experiments. Finally, the unique bifunctional character of phenylazocarboxylates, herein serving as precursors for the azocarboxamide subunit, was exploited to demonstrate the accessibility of an 18 F-fluorinated analogue.

In Vitro Effects of Ligand Bias on Primate Mu Opioid Receptor Downstream Signaling

Interest has emerged in biased agonists at the mu opioid receptor (MOR) as a possible means for maintaining potent analgesis with reduced side effect profiles. While approaches measuring in vitro biased agonism are used in the development of these compounds, their therapeutic utility will ultimately be determined by in vivo functional effects. Nonhuman primates (NHPs) are the most translational model for evaluating the behavioral effects of candidate medications, but biased signaling of these drugs at NHP MOR receptors has been unstudied. The goal of the current work was to characterize MOR ligand bias in rhesus macaques, focusing on agonists that have previously been reported to show different patterns of biased agonism in rodents and humans. Downstream signaling pathways that responded to MOR activation were identified using a luciferase reporter array. Concentration-response curves for specific pathways (cAMP, NF-ĸB, MAPK/JNK) were generated using six agonists previously reported to differ in terms of signaling bias at rodent and human MORs. Using DAMGO as a reference ligand, relative cAMP, NF-ĸB and MAPK/JNK signaling by morphine, endomorphin-1, and TRV130 were found to be comparable between species. Further, the bias patterns of across ligands for NF-ĸB and MAPK/JNK were largely similar between species. There was a high degree of concordance between rhesus macaque and human MOR receptor signaling bias for all agonists tested, further demonstrating their utility for future translational behavioral studies.

[11 C]Carbonyl Difluoride-a New and Highly Efficient [11 C]Carbonyl Group Transfer Agent

Herein, the synthesis and use of [11 C]carbonyl difluoride for labeling heterocycles with [11 C]carbonyl groups in high molar activity is described. A very mild single-pass gas-phase conversion of [11 C]carbon monoxide into [11 C]carbonyl difluoride over silver(II) fluoride provides easy access to this new synthon in robust quantitative yield for labeling a broad range of cyclic substrates, for example, imidazolidin-2-ones, thiazolidin-2-ones, and oxazolidin-2-ones. Labeling reactions may utilize close-to-stoichiometric precursor quantities and short reaction times at room temperature in a wide range of solvents while also showing high water tolerability. The overall radiosynthesis protocol is both simple and reproducible. The required apparatus can be constructed from widely available parts and is therefore well suited to be automated for PET radiotracer production. We foresee that this straightforward method will gain wide acceptance for PET radiotracer syntheses across the radiochemistry community.

Toward Directing Opioid Receptor Signaling to Refine Opioid Therapeutics

The mu opioid receptor (MOR) is a diversely regulated target for the alleviation of pain in the clinical setting. However, untoward side effects such as tolerance, dependence, respiratory suppression, constipation, and abuse liability detract from the general activation of these receptors. Studies in genetically modified rodent models suggest that activating G protein signaling pathways while avoiding phosphorylation of the receptor or recruitment of β-arrestin scaffolding proteins could preserve the analgesic properties of MOR agonists while avoiding certain side effects. With the development of novel MOR "biased" agonists, which lead to preferential activation of G protein pathways over receptor phosphorylation, internalization, or interaction with other effectors, this hypothesis can be tested in a native, physiological setting. Overall, it is clear that the MOR is not a simple on-off switch and that the diverse means by which the receptor can be regulated may present an opportunity to refine therapeutics for the treatment of pain.

Progress in the development of more effective and safer analgesics for pain management

Opioid analgesics have been used for thousands of years in the treatment of pain and related disorders, and have become among the most widely prescribed medications. Among opioid analgesics, mu opioid receptor (MOR) agonists are the most commonly used and are indicated for acute and chronic pain management. However, their use results in a plethora of well-described side-effects. From selective delta opioid receptor (DOR) and kappa opioid receptor (KOR) agonists to multitarget MOR/DOR and MOR/KOR ligands, medicinal chemistry provided different approaches aimed at the development of opioid analgesics with an improved pharmacological and tolerability fingerprint. The emergent medicinal chemistry strategy to develop ameliorated opioid analgesics is based upon the concept that functional selectivity for G-protein signalling is necessary for the therapeutic effect, whether β-arrestin recruitment is mainly responsible for the manifestation of side effects, including the development of tolerance after repeated administrations. This review summarises most relevant biased MOR, DOR, KOR and multitarget MOR/DOR ligands synthesised in the last decade and their pharmacological profile in "in vitro" and "in vivo" studies. Such biased ligands could have a significant impact on modern drug discovery and represent a new strategy for the development of better-tolerated drug candidates.

The Opioid Crisis and the Future of Addiction and Pain Therapeutics

Opioid misuse and addiction are a public health crisis resulting in debilitation, deaths, and significant social and economic impact. Curbing this crisis requires collaboration among academic, government, and industrial partners toward the development of effective nonaddictive pain medications, interventions for opioid overdose, and addiction treatments. A 2-day meeting, The Opioid Crisis and the Future of Addiction and Pain Therapeutics: Opportunities, Tools, and Technologies Symposium, was held at the National Institutes of Health (NIH) to address these concerns and to chart a collaborative path forward. The meeting was supported by the NIH Helping to End Addiction Long-Term^SM (HEAL) Initiative, an aggressive, trans-agency effort to speed scientific solutions to stem the national opioid crisis. The event was unique in bringing together two research disciplines, addiction and pain, in order to create a forum for crosscommunication and collaboration. The output from the symposium will be considered by the HEAL Initiative; this article summarizes the scientific presentations and key takeaways. Improved understanding of the etiology of acute and chronic pain will enable the discovery of novel targets and regulatable pain circuits for safe and effective therapeutics, as well as relevant biomarkers to ensure adequate testing in clinical trials. Applications of improved technologies including reagents, assays, model systems, and validated probe compounds will likely increase the delivery of testable hypotheses and therapeutics to enable better health outcomes for patients. The symposium goals were achieved by increasing interdisciplinary collaboration to accelerate solutions for this pressing public health challenge and provide a framework for focused efforts within the research community. SIGNIFICANCE STATEMENT: This article summarizes key messages and discussions resulting from a 2-day symposium focused on challenges and opportunities in developing addiction- and pain-related medications. Speakers and attendees came from 40 states in the United States and 15 countries, bringing perspectives from academia, industry, government, and healthcare by researchers, clinicians, regulatory experts, and patient advocates.

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).

(2S)-N-2-methoxy-2-phenylethyl-6,7-benzomorphan compound (2S-LP2): Discovery of a biased mu/delta opioid receptor agonist

The pivotal role of the stereocenter at the N-substituent of the 6,7-benzomorphan scaffold was investigated combining synthetic and pharmacological approaches. 2R- and 2S-diastereoisomers of the multitarget MOR/DOR antinociceptive ligand LP2 (1) were synthesized and their pharmacological profile was evaluated in in vitro and vivo assays. From our results, 2S-LP2 (5) showed an improved pharmacological profile in comparison to LP2 (1) and 2R-LP2 (4). 2S-LP2 (5) elicited an antinociceptive effect with a 1.5- and 3-times higher potency than LP2 (1) and R-antipode (4), respectively. In vivo effect of 2S-LP2 (5) was consistent with the improved MOR/DOR efficacy profile assessed by radioligand binding assay, to evaluate the opioid receptor affinity, and BRET assay, to evaluate the capability to promote receptor/G-protein and receptor/β-arrestin 2 interaction. 2S-LP2 (5) was able to activate, with different efficacy, G-protein pathway over β-arrestin 2, behaving as biased agonist at MOR and mainly at DOR. Considering the therapeutic potential of both multitarget MOR/DOR agonism and functional selectivity over G-protein, the 2S-LP2 (5) biased multitarget MOR/DOR agonist could provide a safer treatment opportunity.