Novel hybrid compounds, opioid agonist+melanocortin 4 receptor antagonist, as efficient analgesics in mouse chronic constriction injury model of neuropathic pain

Research trends on acupuncture for neuropathic pain: A bibliometric analysis from 1979 to 2023

BACKGROUND

Acupuncture has drawn increasing attention as a complementary and alternative therapy for neuropathic pain (NP). The present study aimed to summarize the current status and research trends on acupuncture for NP over the past several decades.

METHODS

The publications on acupuncture for NP in the database of Web of Science Core Collection from 1979 to 2023 were searched. VOSviewer (1.6.15) and CiteSpace software (5.5.R2) were applied to identify active authors, journals, countries and institutions, co-cited references and hot keywords.

RESULTS

A total of 642 publications were finally included, and the quantitative trend of annual publications on acupuncture for NP have shown overall upward from 1979 to 2023. Peoples R China was the most productive and influential country, while Kyung Hee University from South Korea was both the first in publications and citations. Fang JQ ranked the first productive author and Han JS was the first 1 among the co-cited authors. The first productive journal was Evidence-based Complementary and Alternative Medicine, while the first co-cited journal was Pain. The high-frequency keywords were divided into 9 clusters, and the frontier topic focused on "Chronic pain".

CONCLUSION

This present study visually showed the research status and trends of acupuncture for NP from 1979 to 2023 on the basis of bibliometric analysis, which may in some way help researcher discovery and explore some new research directions and ideas in the future.

Melanocortin-4 receptor signaling in the central amygdala mediates chronic inflammatory pain effects on nociception

Chronic inflammatory pain represents one of the largest subsets of chronic pain diagnoses, which affect nearly a quarter of individuals in the United States and cost nearly $600 billion dollars annually. Chronic pain leads to persistent sensory hypersensitivities, as well as emotional and cognitive disturbances. Evidence suggests that melanocortin 4 receptors (MC4Rs) mediate pain-signaling and pain-like behaviors via actions at various nodes in the pain-neural axis, but the field lacks a complete understanding of the potential role of MC4Rs in chronic inflammatory pain in males and females. The central amygdala (CeA) expresses high quantities of MC4R and receives pain-related information from the periphery, and in vivo CeA manipulations alter nociceptive behavior in pain-naïve and in animals with chronic pain. Here, we tested the hypothesis that MC4Rs in the CeA modulate thermal nociception and mechanical sensitivity, as well as pain avoidance, in male and female Wistar rats, using a model of chronic inflammatory pain (Complete Freud's Adjuvant; CFA). First, we report that CFA produces long-lasting hyperalgesia in adult male and female Wistar rats, and long-lasting pain avoidance in male Wistar rats. Second, we report that MC4R antagonism in the CeA reduces thermal nociception and mechanical sensitivity in male and female Wistar rats treated with CFA. Finally, we report that MC4R antagonism in the CeA reduces pain avoidance in male, and that this effect is not due to drug effects on locomotor activity. Our results indicate that a model of chronic inflammatory pain produces long-lasting increases in pain-like behaviors in adult male and female Wistar rats, and that antagonism of MC4Rs in the CeA reverses those 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.

Bifunctional Opioid/Melanocortin Peptidomimetics for Use in Neuropathic Pain: Variation in the Type and Length of the Linker Connecting the Two Pharmacophores

Based on the mechanism of neuropathic pain induction, a new type of bifunctional hybrid peptidomimetics was obtained for potential use in this type of pain. Hybrids consist of two types of pharmacophores that are connected by different types of linkers. The first pharmacophore is an opioid agonist, and the second pharmacophore is an antagonist of the pronociceptive system, i.e., an antagonist of the melanocortin-4 receptor. The results of tests in acute and neuropathic pain models of the obtained compounds have shown that the type of linker used to connect pharmacophores had an effect on antinociceptive activity. Peptidomimetics containing longer flexible linkers were very effective at low doses in the neuropathic pain model. To elucidate the effect of linker lengths, two hybrids showing very high activity and two hybrids with lower activity were further tested for affinity for opioid (mu, delta) and melanocortin-4 receptors. Their complexes with the target receptors were also studied by molecular modelling. Our results do not show a simple relationship between linker length and affinity for particular receptor types but suggest that activity in neuropathic pain is related to a proper balance of receptor affinity rather than maximum binding to any or all of the target receptors.

Translational advances of melanocortin drugs: Integrating biology, chemistry and genetics

Melanocortin receptors have emerged as important targets with a very unusual versatility, as their widespread distribution on multiple tissues (e.g. skin, adrenal glands, brain, immune cells, exocrine glands) together with the variety of physiological processes they control (pigmentation, cortisol release, satiety mechanism, inflammation, secretions), place this family of receptors as genuine therapeutic targets for many disorders. This review focuses in the journey of the development of melanocortin receptors as therapeutic targets from the discovery of their existence in the early 1990 s to the approval of the first few drugs of this class. Two major areas of development characterise the current state of melanocortin drug development: their role in obesity, recently culminated with the approval of setmelanotide, and their potential for the treatment of chronic inflammatory and autoimmune diseases like rheumatoid arthritis, multiple sclerosis or fibrosis. The pro-resolving nature of these drugs offers the advantage of acting by mimicking the way our body naturally resolves inflammation, expecting fewer side effects and a more balanced (i.e. non-immunosuppressive) response from them. Here we also review the approaches followed for the design and development of novel compounds, the importance of the GPCR nature of these receptors in the process of drug development, therapeutic value, current challenges and successes, and the potential for the implementation of precision medicine approaches through the incorporation of genetics advances.

Biphalin-A Potent Opioid Agonist-As a Panacea for Opioid System-Dependent Pathophysiological Diseases?

Biphalin, one of the opioid agonists, is a dimeric analog of enkephalin with a high affinity for opioid receptors. Opioid receptors are widespread in the central nervous system and in peripheral neuronal and non-neuronal tissues. Hence, these receptors and their agonists, which play an important role in pain blocking, may also be involved in the regulation of other physiological functions. Biphalin was designed and synthesized in 1982 by Lipkowski as an analgesic peptide. Extensive further research in various laboratories on the antinociceptive effects of biphalin has shown its excellent properties. It has been demonstrated that biphalin exhibits an analgesic effect in acute, neuropathic, and chronic animal pain models, and is 1000 times more potent than morphine when administered intrathecally. In the course of the broad conducted research devoted primarily to the antinociceptive effect of this compound, it has been found that biphalin may also potentially participate in the regulation of other opioid system-dependent functions. Nearly 40 years of research on the properties of biphalin have shown that it may play a beneficial role as an antiviral, antiproliferative, anti-inflammatory, and neuroprotective agent, and may also affect many physiological functions. This integral review analyzes the literature on the multidirectional biological effects of biphalin and its potential in the treatment of many opioid system-dependent pathophysiological diseases.

Novel bifunctional hybrid compounds designed to enhance the effects of opioids and antagonize the pronociceptive effects of nonopioid peptides as potent analgesics in a rat model of neuropathic pain

ABSTRACT

The purpose of our work was to determine the role of nonopioid peptides derived from opioid prohormones in sensory hypersensitivity characteristics of neuropathic pain and to propose a pharmacological approach to restore the balance of these endogenous opioid systems. Nonopioid peptides may have a pronociceptive effect and therefore contribute to less effective opioid analgesia in neuropathic pain. In our study, we used unilateral chronic constriction injury (CCI) of the sciatic nerve as a neuropathic pain model in rats. We demonstrated the pronociceptive effects of proopiomelanocortin- and proenkephalin-derived nonopioid peptides assessed by von Frey and cold plate tests, 7 to 14 days after injury. The concentration of proenkephalin-derived pronociceptive peptides was increased more robustly than that of Met-enkephalin in the ipsilateral lumbar spinal cord of CCI-exposed rats, as shown by mass spectrometry, and the pronociceptive effect of one of these peptides was blocked by an antagonist of the melanocortin 4 (MC4) receptor. The above results confirm our hypothesis regarding the possibility of creating an analgesic drug for neuropathic pain based on enhancing opioid activity and blocking the pronociceptive effect of nonopioid peptides. We designed and synthesized bifunctional hybrids composed of opioid (OP) receptor agonist and MC4 receptor antagonist (OP-linker-MC4). Moreover, we demonstrated that they have potent and long-lasting antinociceptive effects after a single administration and a delayed development of tolerance compared with morphine after repeated intrathecal administration to rats subjected to CCI. We conclude that the bifunctional hybrids OP-linker-MC4 we propose are important prototypes of drugs for use in neuropathic pain.

Bidirectional Action of Cenicriviroc, a CCR2/CCR5 Antagonist, Results in Alleviation of Pain-Related Behaviors and Potentiation of Opioid Analgesia in Rats With Peripheral Neuropathy

Clinical management of neuropathic pain is unsatisfactory, mainly due to its resistance to the effects of available analgesics, including opioids. Converging evidence indicates the functional interactions between chemokine and opioid receptors and their influence on nociceptive processes. Recent studies highlight that the CC chemokine receptors type 2 (CCR2) and 5 (CCR5) seem to be of particular interest. Therefore, in this study, we investigated the effects of the dual CCR2/CCR5 antagonist, cenicriviroc, on pain-related behaviors, neuroimmune processes, and the efficacy of opioids in rats after chronic constriction injury (CCI) of the sciatic nerve. To define the mechanisms of action of cenicriviroc, we studied changes in the activation/influx of glial and immune cells and, simultaneously, the expression level of CCR2, CCR5, and important pronociceptive cytokines in the spinal cord and dorsal root ganglia (DRG). We demonstrated that repeated intrathecal injections of cenicriviroc, in a dose-dependent manner, alleviated hypersensitivity to mechanical and thermal stimuli in rats after sciatic nerve injury, as measured by von Frey and cold plate tests. Behavioral effects were associated with the beneficial impact of cenicriviroc on the activation/influx level of C1q/IBA-1-positive cells in the spinal cord and/or DRG and GFAP-positive cells in DRG. In parallel, administration of cenicriviroc decreased the expression of CCR2 in the spinal cord and CCR5 in DRG. Concomitantly, we observed that the level of important pronociceptive factors (e.g., IL-1beta, IL-6, IL-18, and CCL3) were increased in the lumbar spinal cord and/or DRG 7 days following injury, and cenicriviroc was able to prevent these changes. Additionally, repeated administration of this dual CCR2/CCR5 antagonist enhanced the analgesic effects of morphine and buprenorphine in neuropathic rats, which can be associated with the ability of cenicriviroc to prevent nerve injury-induced downregulation of all opioid receptors at the DRG level. Overall, our results suggest that pharmacological modulation based on the simultaneous blockade of CCR2 and CCR5 may serve as an innovative strategy for the treatment of neuropathic pain, as well as in combination with opioids.

The Role of Melanocortin Plasticity in Pain-Related Outcomes After Alcohol Exposure

The global COVID-19 pandemic has shone a light on the rates and dangers of alcohol misuse in adults and adolescents in the US and globally. Alcohol exposure during adolescence causes persistent molecular, cellular, and behavioral changes that increase the risk of alcohol use disorder (AUD) into adulthood. It is established that alcohol abuse in adulthood increases the likelihood of pain hypersensitivity and the genesis of chronic pain, and humans report drinking alcohol to relieve pain symptoms. However, the longitudinal effects of alcohol exposure on pain and the underlying CNS signaling that mediates it are understudied. Specific brain regions mediate pain effects, alcohol effects, and pain-alcohol interactions, and neural signaling in those brain regions is modulated by neuropeptides. The CNS melanocortin system is sensitive to alcohol and modulates pain sensitivity, but this system is understudied in the context of pain-alcohol interactions. In this review, we focus on the role of melanocortin signaling in brain regions sensitive to alcohol and pain, in particular the amygdala. We also discuss interactions of melanocortins with other peptide systems, including the opioid system, as potential mediators of pain-alcohol interactions. Therapeutic strategies that target the melanocortin system may mitigate the negative consequences of alcohol misuse during adolescence and/or adulthood, including effects on pain-related outcomes.

Multifunctional Opioid-Derived Hybrids in Neuropathic Pain: Preclinical Evidence, Ideas and Challenges

When the first- and second-line therapeutics used to treat neuropathic pain (NP) fail to induce efficient analgesia—which is estimated to relate to more than half of the patients—opioid drugs are prescribed. Still, the pathological changes following the nerve tissue injury, i.a. pronociceptive neuropeptide systems activation, oppose the analgesic effects of opiates, enforcing the use of relatively high therapeutic doses in order to obtain satisfying pain relief. In parallel, the repeated use of opioid agonists is associated with burdensome adverse effects due to compensatory mechanisms that arise thereafter. Rational design of hybrid drugs, in which opioid ligands are combined with other pharmacophores that block the antiopioid action of pronociceptive systems, delivers the opportunity to ameliorate the NP-oriented opioid treatment via addressing neuropathological mechanisms shared both by NP and repeated exposition to opioids. Therewith, the new dually acting drugs, tailored for the specificity of NP, can gain in efficacy under nerve injury conditions and have an improved safety profile as compared to selective opioid agonists. The current review presents the latest ideas on opioid-comprising hybrid drugs designed to treat painful neuropathy, with focus on their biological action, as well as limitations and challenges related to this therapeutic approach.

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

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