Toward a Universal μ-Agonist Template for Template-Based Alignment Modeling of Opioid Ligands

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

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

Fentanyl Structure as a Scaffold for Opioid/Non-Opioid Multitarget Analgesics

One of the strategies in the search for safe and effective analgesic drugs is the design of multitarget analgesics. Such compounds are intended to have high affinity and activity at more than one molecular target involved in pain modulation. In the present contribution we summarize the attempts in which fentanyl or its substructures were used as a μ-opioid receptor pharmacophoric fragment and a scaffold to which fragments related to non-opioid receptors were attached. The non-opioid ‘second’ targets included proteins as diverse as imidazoline I₂ binding sites, CB₁ cannabinoid receptor, NK₁ tachykinin receptor, D₂ dopamine receptor, cyclooxygenases, fatty acid amide hydrolase and monoacylglycerol lipase and σ₁ receptor. Reviewing the individual attempts, we outline the chemistry, the obtained pharmacological properties and structure-activity relationships. Finally, we discuss the possible directions for future work.

Endogenous opiates and behavior: 2019

This paper is the forty-second consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2019 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 (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

Multifunctional Enkephalin Analogs with a New Biological Profile: MOR/DOR Agonism and KOR Antagonism

In our previous studies, we developed a series of mixed MOR/DOR agonists that are enkephalin-like tetrapeptide analogs with an N-phenyl-N-piperidin-4-ylpropionamide (Ppp) moiety at the C-terminus. Further SAR study on the analogs, initiated by the findings from off-target screening, resulted in the discovery of LYS744 (6, Dmt-DNle-Gly-Phe(p-Cl)-Ppp), a multifunctional ligand with MOR/DOR agonist and KOR antagonist activity (GTPγS assay: IC₅₀ = 52 nM, I_max = 122% cf. IC₅₀ = 59 nM, I_max = 100% for naloxone) with nanomolar range of binding affinity (K_i = 1.3 nM cf. K_i = 2.4 nM for salvinorin A). Based on its unique biological profile, 6 is considered to possess high therapeutic potential for the treatment of chronic pain by modulating pathological KOR activation while retaining analgesic efficacy attributed to its MOR/DOR agonist activity.

Influence of G protein-biased agonists of μ-opioid receptor on addiction-related behaviors

Opioid analgesics remain a gold standard for the treatment of moderate to severe pain. However, their clinical utility is seriously limited by a range of adverse effects. Among them, their high-addictive potential appears as very important, especially in the context of the opioid epidemic. Therefore, the development of safer opioid analgesics with low abuse potential appears as a challenging problem for opioid research. Among the last few decades, different approaches to the discovery of novel opioid drugs have been assessed. One of the most promising is the development of G protein-biased opioid agonists, which can activate only selected intracellular signaling pathways. To date, discoveries of several biased agonists acting via μ-opioid receptor were reported. According to the experimental data, such ligands may be devoid of at least some of the opioid side effects, such as respiratory depression or constipation. Nevertheless, most data regarding the addictive properties of biased μ-opioid receptor agonists are inconsistent. A global problem connected with opioid abuse also requires the search for effective pharmacotherapy for opioid addiction, which is another potential application of biased compounds. This review discusses the state-of-the-art on addictive properties of G protein-biased μ-opioid receptor agonists as well as we analyze whether these compounds can diminish any symptoms of opioid addiction. Finally, we provide a critical view on recent data connected with biased signaling and its implications to in vivo manifestations of addiction.

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.

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.