The proof of concept of 2-{3-[N-(1-benzylpiperidin-4-yl)propyl]amino}-6-[N-methyl-N-(prop-2-yn-1-yl)amino]-4-phenylpyridine-3,5-dicarbonitrile for the therapy of neuropathic pain.

In the search for new small molecules for the therapy of neuropathic pain, we found that 2-{3-[N-(1-benzylpiperidin-4-yl)propyl]amino}-6-[N-methyl-N-(prop-2-yn-1-yl)amino]-4-phenylpyridine-3,5-dicarbonitrile (12) induced a robust antiallodynic effect in capsaicin-induced mechanical allodynia, a behavioural model of central sensitization, through σ1R antagonism. Furthermore, administration of compound 12 to neuropathic animals, fully reversed mechanical allodynia, increasing its mechanical threshold to levels that were not significantly different from those found in paclitaxel-vehicle treated mice or from basal levels before neuropathy was induced. Ligand 12 is thus a promising hit-compound for the therapy of neuropathic pain.

Sigma-1 receptors control neuropathic pain and macrophage infiltration into the dorsal root ganglion after peripheral nerve injury.

Neuron-immune interaction in the dorsal root ganglia (DRG) plays a pivotal role in the neuropathic pain development after nerve injury. Sigma-1 receptor (Sig-1R) is expressed by DRG neurons but its role in neuropathic pain is not fully understood. We investigated the effect of peripheral Sig-1R on neuroinflammation in the DRG after spared (sciatic) nerve injury (SNI) in mice. Nerve injury induced a decrease in NeuN staining along with the nuclear eccentricity and ATF3 expression in the injured DRG. Sig-1R was present in all DRG neurons examined, and after SNI this receptor translocated to the periphery of the soma and the vicinity of the nucleus, especially in injured ATF3 + neurons. In WT mice, injured DRG produced the chemokine CCL2, and this was followed by massive infiltration of macrophages/monocytes, which clustered mainly around sensory neurons with translocated Sig-1R, accompanied by robust IL-6 increase and mechanical allodynia. In contrast, Sig-1R knockout (Sig-1R-KO) mice showed reduced levels of CCL2, decreased macrophage/monocyte infiltration into DRG, and less IL-6 and neuropathic mechanical allodynia after SNI. Our findings point to an important role of peripheral Sig-1R in sensory neuron-macrophage/monocyte communication in the DRG after peripheral nerve injury; thus, these receptors may contribute to the neuropathic pain phenotype.

Sigma-1 receptor: A drug target for the modulation of neuroimmune and neuroglial interactions during chronic pain.

Immune and glial cells play a pivotal role in chronic pain. Therefore, it is possible that the pharmacological modulation of neurotransmission from an exclusively neuronal perspective may not be enough for adequate pain management, and the modulation of complex interactions between neurons and other cell types might be needed for successful pain relief. In this article, we review the current scientific evidence for the modulatory effects of sigma-1 receptors on communication between the immune and nervous systems during inflammation, as well as the influence of this receptor on peripheral and central neuroinflammation. Several experimental models of pathological pain are considered, including peripheral and central neuropathic pain, osteoarthritic, and cancer pain. Sigma-1 receptor inhibition prevents peripheral (macrophage infiltration into the dorsal root ganglion) and central (activation of microglia and astrocytes) neuroinflammation in several pain models, and enhances immune-driven peripheral opioid analgesia during painful inflammation, maximizing the analgesic potential of peripheral immune cells. Therefore, sigma-1 antagonists may constitute a new class of analgesics with an unprecedented mechanism of action and potential utility in several painful disorders.

Sigma-1 receptors control immune-driven peripheral opioid analgesia during inflammation in mice.

Sigma-1 antagonism potentiates the antinociceptive effects of opioid drugs, so sigma-1 receptors constitute a biological brake to opioid drug-induced analgesia. The pathophysiological role of this process is unknown. We aimed to investigate whether sigma-1 antagonism reduces inflammatory pain through the disinhibition of the endogenous opioidergic system in mice. The selective sigma-1 antagonists BD-1063 and S1RA abolished mechanical and thermal hyperalgesia in mice with carrageenan-induced acute (3 h) inflammation. Sigma-1-mediated antihyperalgesia was reversed by the opioid antagonists naloxone and naloxone methiodide (a peripherally restricted naloxone analog) and by local administration at the inflamed site of monoclonal antibody 3-E7, which recognizes the pan-opioid sequence Tyr-Gly-Gly-Phe at the N terminus of most endogenous opioid peptides (EOPs). Neutrophils expressed pro-opiomelanocortin, the precursor of ß-endorphin (a known EOP), and constituted the majority of the acute immune infiltrate. ß-endorphin levels increased in the inflamed paw, and this increase and the antihyperalgesic effects of sigma-1 antagonism were abolished by reducing the neutrophil load with in vivo administration of an anti-Ly6G antibody. The opioid-dependent sigma-1 antihyperalgesic effects were preserved 5 d after carrageenan administration, where macrophages/monocytes were found to express pro-opiomelanocortin and to now constitute the majority of the immune infiltrate. These results suggest that immune cells harboring EOPs are needed for the antihyperalgesic effects of sigma-1 antagonism during inflammation. In conclusion, sigma-1 receptors curtail immune-driven peripheral opioid analgesia, and sigma-1 antagonism produces local opioid analgesia by enhancing the action of EOPs of immune origin, maximizing the analgesic potential of immune cells that naturally accumulate in painful inflamed areas.

Targeting immune-driven opioid analgesia by sigma-1 receptors: Opening the door to novel perspectives for the analgesic use of sigma-1 antagonists.

Immune cells have a known role in pronociception, since they release a myriad of inflammatory algogens which interact with neurons to facilitate pain signaling. However, these cells also produce endogenous opioid peptides with analgesic potential. The sigma-1 receptor is a ligand-operated chaperone that modulates neurotransmission by interacting with multiple protein partners, including the µ-opioid receptor. We recently found that sigma-1 antagonists are able to induce opioid analgesia by enhancing the action of endogenous opioid peptides of immune origin during inflammation. This opioid analgesia is seen only at the inflamed site, where immune cells naturally accumulate. In this article we review the difficulties of targeting the opioid system for selective pain relief, and discuss the dual role of immune cells in pain and analgesia. Our discussion creates perspectives for possible novel therapeutic uses of sigma-1 antagonists as agents able to maximize the analgesic potential of the immune system.

Phenotyping the function of TRPV1-expressing sensory neurons by targeted axonal silencing.

Specific somatosensations may be processed by different subsets of primary afferents. C-fibers expressing heat-sensitive TRPV1 channels are proposed, for example, to be heat but not mechanical pain detectors. To phenotype in rats the sensory function of TRPV1(+) afferents, we rapidly and selectively silenced only their activity, by introducing the membrane-impermeant sodium channel blocker QX-314 into these axons via the TRPV1 channel pore. Using tandem mass spectrometry we show that upon activation with capsaicin, QX-314 selectively accumulates in the cytosol only of TRPV1-expressing cells, and not in control cells. Exposure to QX-314 and capsaicin induces in small DRG neurons a robust sodium current block within 30 s. In sciatic nerves, application of extracellular QX-314 with capsaicin persistently reduces C-fiber but not A-fiber compound action potentials and this effect does not occur in TRPV1(-/-) mice. Behavioral phenotyping after selectively silencing TRPV1(+) sciatic nerve axons by perineural injections of QX-314 and capsaicin reveals deficits in heat and mechanical pressure but not pinprick or light touch perception. The response to intraplantar capsaicin is substantially reduced, as expected. During inflammation, silencing TRPV1(+) axons abolishes heat, mechanical, and cold hyperalgesia but tactile and cold allodynia remain following peripheral nerve injury. These results indicate that TRPV1-expressing sensory neurons process particular thermal and mechanical somatosensations, and that the sensory channels activated by mechanical and cold stimuli to produce pain in naive/inflamed rats differ from those in animals after peripheral nerve injury.

Adults with excess weight or obesity, but not with overweight, report greater pain intensities than individuals with normal weight: a systematic review and meta-analysis.

Context: Over 1.9 billion adult people have overweight or obesity. Considered as a chronic disease itself, obesity is associated with several comorbidities. Chronic pain affects approximately 60 million people and its connection with obesity has been displayed in several studies. However, controversial results showing both lower and higher pain thresholds in subjects with obesity compared to individuals with normal weight and the different parameters used to define such association (e.g., pain severity, frequency or duration) make it hard to draw straight forward conclusions in the matter. The objective of this article is to examine the relationship between overweight and obesity (classified with BMI as recommended by WHO) and self-perceived pain intensity in adults. Methods: A literature search was conducted following PRISMA guidelines using the databases CINAHL, Cochrane Library, EMBASE, PEDro, PubMed, Scopus and Web of Science to identify original studies that provide BMI values and their associated pain intensity assessed by self-report scales. Self-report pain scores were normalized and pooled within meta-analyses. The Cochrane's Q test and I2 index were used to clarify the amount of heterogeneity; meta-regression was performed to explore the relationship between each outcome and the risk of bias. Results: Of 2194 studies, 31 eligible studies were identified and appraised, 22 of which provided data for a quantitative analysis. The results herein suggested that adults with excess weight (BMI ≥ 25.0) or obesity (BMI ≥ 30.0) but not with overweight (pre-obesity) alone (BMI 25.0-29.9), are more likely to report greater intensities of pain than individuals of normal weight (BMI 18.5-24.9). Subgroup analyses regarding the pathology of the patients showed no statistically significant differences between groups. Also, influence of age in the effect size, evaluated by meta-regression, was only observed in one of the four analyses. Furthermore, the robustness of the findings was supported by two different sensitivity analyses. Conclusion: Subjects with obesity and excess weight, but not overweight, reported greater pain intensities than individuals with normal weight. This finding encourages treatment of obesity as a component of pain management. More research is required to better understand the mechanisms of these differences and the clinical utility of the findings. Systematic Review Registration: https://doi.org/10.17605/OSF.IO/RF2G3, identifier OSF.IO/RF2G3.

5,6-EET is released upon neuronal activity and induces mechanical pain hypersensitivity via TRPA1 on central afferent terminals.

Epoxyeicosatrienoic acids (EETs) are cytochrome P450-epoxygenase-derived metabolites of arachidonic acid that act as endogenous signaling molecules in multiple biological systems. Here we have investigated the specific contribution of 5,6-EET to transient receptor potential (TRP) channel activation in nociceptor neurons and its consequence for nociceptive processing. We found that, during capsaicin-induced nociception, 5,6-EET levels increased in dorsal root ganglia (DRGs) and the dorsal spinal cord, and 5,6-EET is released from activated sensory neurons in vitro. 5,6-EET potently induced a calcium flux (100 nm) in cultured DRG neurons that was completely abolished when TRPA1 was deleted or inhibited. In spinal cord slices, 5,6-EET dose dependently enhanced the frequency, but not the amplitude, of spontaneous EPSCs (sEPSCs) in lamina II neurons that also responded to mustard oil (allyl isothiocyanate), indicating a presynaptic action. Furthermore, 5,6-EET-induced enhancement of sEPSC frequency was abolished in TRPA1-null mice, suggesting that 5,6-EET presynaptically facilitated spinal cord synaptic transmission by TRPA1. Finally, in vivo intrathecal injection of 5,6-EET caused mechanical allodynia in wild-type but not TRPA1-null mice. We conclude that 5,6-EET is synthesized on the acute activation of nociceptors and can produce mechanical hypersensitivity via TRPA1 at central afferent terminals in the spinal cord.

Antiallodynic and analgesic effects of maslinic acid, a pentacyclic triterpenoid from Olea europaea.

The effects of maslinic acid (1), a pentacyclic triterpenoid obtained from Olea europaea, were studied in several tests for nociception in mice. Systemic administration of 1 reduced acetic acid-induced writhing, the inflammatory phase of formalin-induced pain, and capsaicin-induced mechanical allodynia. However, it did not induce motor incoordination in the rotarod test. The topical administration of 1 also reduced the inflammatory phase of the formalin test, indicating that at least some of its effects are mediated peripherally. The present results demonstrate for the first time that maslinic acid induces antinociceptive and antiallodynic effects.

"Bedside-to-Bench" Behavioral Outcomes in Animal Models of Pain: Beyond the Evaluation of Reflexes.

Despite the myriad promising new targets and candidate analgesics recently identified in preclinical pain studies, little translation to novel pain medications has been generated. The pain phenotype in humans involves complex behavioral alterations, including changes in daily living activities and psychological disturbances. These behavioral changes are not reflected by the outcome measures traditionally used in rodents for preclinical pain testing, which are based on reflexes evoked by sensory stimuli of different types (mechanical, thermal or chemical). These measures do not evaluate the impact of the pain experience on the global behavior or disability of the animals, and therefore only consider a limited aspect of the pain phenotype. The development of relevant new outcomes indicative of pain to increase the validity of animal models of pain has been increasingly pursued over the past few years. The aim has been to translate "bedside-to-bench" outcomes from the human pain phenotype to rodents, in order to complement traditional pain outcomes by providing a closer and more realistic measure of clinical pain in rodents. This review summarizes and discusses the most important nonstandard outcomes for pain assessment in preclinical studies. The advantages and drawbacks of these techniques are considered, and their potential impact on the validation of potential analgesics is evaluated.

Small Synthetic Hyaluronan Disaccharide BIS014 Mitigates Neuropathic Pain in Mice.

Neuropathic pain (NP) is a challenging condition to treat, as the need for new drugs to treat NP is an unmet goal. We investigated the analgesic potential of a new sulfated disaccharide compound, named BIS014. Oral administration (p.o.) of this compound induced ameliorative effects in formalin-induced nociception and capsaicin-induced secondary mechanical hypersensitivity in mice, but also after partial sciatic nerve transection (spared nerve injury), chemotherapy (paclitaxel)-induced NP, and diabetic neuropathy induced by streptozotocin. Importantly, BIS014, at doses active on neuropathic hypersensitivity (60 mg/kg/p.o.), did not alter exploratory activity or motor coordination (in the rotarod test), unlike a standard dose of gabapentin (40 mg/kg/p.o.) which although inducing antiallodynic effects on the NP models, it also markedly decreased exploration and motor coordination. In docking and molecular dynamic simulation studies, BIS014 interacted with TRPV1, a receptor involved in pain transmission where it behaved as a partial agonist. Additionally, similar to capsaicin, BIS014 increased cytosolic Ca2+ concentration ([Ca2+]c) in neuroblastoma cells expressing TRPV1 receptors; these elevations were blocked by ruthenium red. BIS014 did not block capsaicin-elicited [Ca2+]c transients, but inhibited the increase in the firing rate of action potentials in bradykinin-sensitized dorsal root ganglion neurons stimulated with capsaicin. Perspective: We report that the oral administration of a new sulfated disaccharide compound, named BIS014, decreases neuropathic pain from diverse etiology in mice. Unlike the comparator gabapentin, BIS014 does not induce sedation. Thus, BIS014 has the potential to become a new efficacious non-sedative oral medication for the treatment of neuropathic pain.

Minocycline Prevents the Development of Key Features of Inflammation and Pain in DSS-induced Colitis in Mice.

Abdominal pain is a common feature in inflammatory bowel disease (IBD) patients, and greatly compromises their quality of life. Therefore, the identification of new therapeutic tools to reduce visceral pain is one of the main goals for IBD therapy. Minocycline, a broad-spectrum tetracycline antibiotic, has gained attention in the scientific community because of its immunomodulatory and anti-inflammatory properties. The aim of this study was to evaluate the potential of this antibiotic as a therapy for the management of visceral pain in dextran sodium sulfate (DSS)-induced colitis in mice. Preemptive treatment with minocycline markedly reduced histological features of intestinal inflammation and the expression of inflammatory markers (Tlr4, Tnfα, Il1ß, Ptgs2, Inos, Cxcl2, and Icam1), and attenuated the decrease of markers of epithelial integrity (Tjp1, Ocln, Muc2, and Muc3). In fact, minocycline restored normal epithelial permeability in colitic mice. Treatment with the antibiotic also reversed the changes in the gut microbiota profile induced by colitis. All these ameliorative effects of minocycline on both inflammation and dysbiosis correlated with a decrease in ongoing pain and referred hyperalgesia, and with the improvement of physical activity induced by the antibiotic in colitic mice. Minocycline might constitute a new therapeutic approach for the treatment of IBD-induced pain. PERSPECTIVE: This study found that the intestinal anti-inflammatory effects of minocycline ameliorate DSS-associated pain in mice. Therefore, minocycline might constitute a novel therapeutic strategy for the treatment of IBD-induced pain.

The sigma-1 receptor curtails endogenous opioid analgesia during sensitization of TRPV1 nociceptors.

BACKGROUND AND PURPOSE: Peripheral sensitization contributes to pathological pain. While prostaglandin E2 (PGE2) and nerve growth factor (NGF) sensitize peptidergic C-nociceptors (TRPV1+), glial cell line-derived neurotrophic factor (GDNF) sensitizes non-peptidergic C-neurons (IB4+). The sigma-1 receptor (sigma-1R) is a Ca2+ -sensing chaperone known to modulate opoid analgesia. This receptor binds both to TRPV1 and the µ opioid receptor, although the functional repercussions of these physical interactions in peripheral sensitization are unknown. EXPERIMENTAL APPROACH: We tested the effects of sigma-1 antagonism on PGE2-, NGF-, and GDNF-induced mechanical and heat hyperalgesia in mice. We used immunohistochemistry to determine the presence of endomorphin-2, an endogenous µ receptor agonist, on dorsal root ganglion (DRG) neurons. Recombinant proteins were used to study the interactions between sigma-1R, µ- receptor, and TRPV1. We used calcium imaging to study the effects of sigma-1 antagonism on PGE2-induced sensitization of TRPV1+ nociceptors. KEY RESULTS: Sigma1 antagonists reversed PGE2- and NGF-induced hyperalgesia but not GDNF-induced hyperalgesia. Endomorphin-2 was detected on TRPV1+ but not on IB4+ neurons. Peripheral opioid receptor antagonism by naloxone methiodide or administration of an anti-endomorphin-2 antibody to a sensitized paw reversed the antihyperalgesia induced by sigma-1 antagonists. Sigma-1 antagonism transfers sigma-1R from TRPV1 to µ receptors, suggesting that sigma-1R participate in TRPV1-µ receptor crosstalk. Moreover, sigma-1 antagonism reversed, in a naloxone-sensitive manner, PGE2-induced sensitization of DRG neurons to the calcium flux elicited by capsaicin, the prototypic TRPV1 agonist. CONCLUSION AND IMPLICATIONS: Sigma-1 antagonism harnesses endogenous opioids produced by TRPV1+ neurons to reduce hyperalgesia by increasing µ receptor activity.

Synthesis, Computational Insights, and Evaluation of Novel Sigma Receptors Ligands.

The development of diazabicyclo[4.3.0]nonane and 2,7-diazaspiro[3.5]nonane derivatives as sigma receptors (SRs) ligands is reported. The compounds were evaluated in S1R and S2R binding assays, and modeling studies were carried out to analyze the binding mode. The most notable compounds, 4b (AD186, KiS1R = 2.7 nM, KiS2R = 27 nM), 5b (AB21, KiS1R = 13 nM, KiS2R = 102 nM), and 8f (AB10, KiS1R = 10 nM, KiS2R = 165 nM), have been screened for analgesic effects in vivo, and their functional profile was determined through in vivo and in vitro models. Compounds 5b and 8f reached the maximum antiallodynic effect at 20 mg/kg. The selective S1R agonist PRE-084 completely reversed their action, indicating that the effects are entirely dependent on the S1R antagonism. Conversely, compound 4b sharing the 2,7-diazaspiro[3.5]nonane core as 5b was completely devoid of antiallodynic effect. Interestingly, compound 4b fully reversed the antiallodynic effect of BD-1063, indicating that 4b induces an S1R agonistic in vivo effect. The functional profiles were confirmed by the phenytoin assay. Our study might establish the importance of 2,7-diazaspiro[3.5]nonane core for the development of S1R compounds with specific agonist or antagonist profile and the role of the diazabicyclo[4.3.0]nonane in the development of novel SR ligands.

Sigma-1 receptor agonism exacerbates immune-driven nociception: Role of TRPV1 + nociceptors.

The analgesic effects of sigma-1 antagonists are undisputed, but the effects of sigma-1 agonists on pain are not well studied. Here, we used a mouse model to show that the administration of the sigma-1 agonists dextromethorphan (a widely used antitussive drug), PRE-084 (a standard sigma-1 ligand), and pridopidine (a selective drug being investigated in clinical trials for the treatment of neurodegenerative diseases) enhances PGE2-induced mechanical hyperalgesia. Superficial plantar incision induced transient weight-bearing asymmetry at early time points, but the mice appeared to recover at 24 h, despite noticeable edema and infiltration of neutrophils (a well-known cellular source of PGE2) at the injured site. Sigma-1 agonists induced a relapse of weight bearing asymmetry in a manner dependent on the presence of neutrophils. The effects of sigma-1 agonists were all reversed by administration of the sigma-1 antagonist BD-1063 in wild-type mice, and were absent in sigma-1 knockout mice, supporting the selectivity of the effects observed. The proalgesic effects of sigma-1 agonism were also abolished by the TRP antagonist ruthenium red and by in vivo resiniferatoxin ablation of TRPV1 + peripheral sensory neurons. Therefore, sigma-1 agonism exacerbates pain-like responses in mice with a mild inflammatory state through the action of TRPV1 + nociceptors. We also show that sigma-1 receptors are present in most (if not all) mouse and human DRG neurons. If our findings translate to humans, further studies will be needed to investigate potential proalgesic effects induced by sigma-1 agonism in patients treated with sigma-1 agonists.

Discovery of AD258 as a Sigma Receptor Ligand with Potent Antiallodynic Activity.

The design and synthesis of a series of 2,7-diazaspiro[4.4]nonane derivatives as potent sigma receptor (SR) ligands, associated with analgesic activity, are the focus of this work. In this study, affinities at S1R and S2R were measured, and molecular modeling studies were performed to investigate the binding pose characteristics. The most promising compounds were subjected to in vitro toxicity testing and subsequently screened for in vivo analgesic properties. Compound 9d (AD258) exhibited negligible in vitro cellular toxicity and a high binding affinity to both SRs (KiS1R = 3.5 nM, KiS2R = 2.6 nM), but not for other pain-related targets, and exerted high potency in a model of capsaicin-induced allodynia, reaching the maximum antiallodynic effect at very low doses (0.6-1.25 mg/kg). Functional activity experiments showed that S1R antagonism is needed for the effects of 9d and that it did not induce motor impairment. In addition, 9d exhibited a favorable pharmacokinetic profile.

Dual Piperidine-Based Histamine H3 and Sigma-1 Receptor Ligands in the Treatment of Nociceptive and Neuropathic Pain.

In search of new dual-acting histamine H3/sigma-1 receptor ligands, we designed a series of compounds structurally based on highly active in vivo ligands previously studied and described by our team. However, we kept in mind that within the previous series, a pair of closely related compounds, KSK67 and KSK68, differing only in the piperazine/piperidine moiety in the structural core showed a significantly different affinity at sigma-1 receptors (σ1Rs). Therefore, we first focused on an in-depth analysis of the protonation states of piperazine and piperidine derivatives in the studied compounds. In a series of 16 new ligands, mainly based on the piperidine core, we selected three lead structures (3, 7, and 12) for further biological evaluation. Compound 12 showed a broad spectrum of analgesic activity in both nociceptive and neuropathic pain models based on the novel molecular mechanism.

Dual Sigma-1 receptor antagonists and hydrogen sulfide-releasing compounds for pain treatment: Design, synthesis, and pharmacological evaluation.

The development of σ1 receptor antagonists hybridized with a H2S-donor is here reported. We aimed to obtain improved analgesic effects when compared to σ1 receptor antagonists or H2S-donors alone. In an in vivo model of sensory hypersensitivity, thioamide 1a induced analgesia which was synergistically enhanced when associated with the σ1 receptor antagonist BD-1063. The selective σ1 receptor agonist PRE-084 completely reversed this effect. Four thioamide H2S-σ1 receptor hybrids (5a-8a) and their amide derivatives (5b-8b) were synthesized. Compound 7a (AD164) robustly released H2S and showed selectivity for σ1 receptor over σ2 and opioid receptors. This compound induced marked analgesia that was reversed by PRE-084. The amide analogue 7b (AD163) showed only minimal analgesia. Further studies showed that 7a exhibited negligible acute toxicity, together with a favorable pharmacokinetic profile. To the best of our knowledge, compound 7a is the first dual-acting ligand with simultaneous H2S-release and σ1 antagonistic activities.

Structural and Molecular Insight into Piperazine and Piperidine Derivatives as Histamine H3 and Sigma-1 Receptor Antagonists with Promising Antinociceptive Properties.

In an attempt to extend recent studies showing that some clinically evaluated histamine H3 receptor (H3R) antagonists possess nanomolar affinity at sigma-1 receptors (σ1R), we selected 20 representative structures among our previously reported H3R ligands to investigate their affinity at σRs. Most of the tested compounds interact with both sigma receptors to different degrees. However, only six of them showed higher affinity toward σ1R than σ2R with the highest binding preference to σ1R for compounds 5, 11, and 12. Moreover, all these ligands share a common structural feature: the piperidine moiety as the fundamental part of the molecule. It is most likely a critical structural element for dual H3/σ1 receptor activity as can be seen by comparing the data for compounds 4 and 5 (hH3R Ki = 3.17 and 7.70 nM, σ1R Ki = 1531 and 3.64 nM, respectively), where piperidine is replaced by piperazine. We identified the putative protein-ligand interactions responsible for their high affinity using molecular modeling techniques and selected compounds 5 and 11 as lead structures for further evaluation. Interestingly, both ligands turned out to be high-affinity histamine H3 and σ1 receptor antagonists with negligible affinity at the other histamine receptor subtypes and promising antinociceptive activity in vivo. Considering that many literature data clearly indicate high preclinical efficacy of individual selective σ1 or H3R ligands in various pain models, our research might be a breakthrough in the search for novel, dual-acting compounds that can improve existing pain therapies. Determining whether such ligands are more effective than single-selective drugs will be the subject of our future studies.

Synthesis, In Vitro Profiling, and In Vivo Evaluation of Benzohomoadamantane-Based Ureas for Visceral Pain: A New Indication for Soluble Epoxide Hydrolase Inhibitors.

The soluble epoxide hydrolase (sEH) has been suggested as a pharmacological target for the treatment of several diseases, including pain-related disorders. Herein, we report further medicinal chemistry around new benzohomoadamantane-based sEH inhibitors (sEHI) in order to improve the drug metabolism and pharmacokinetics properties of a previous hit. After an extensive in vitro screening cascade, molecular modeling, and in vivo pharmacokinetics studies, two candidates were evaluated in vivo in a murine model of capsaicin-induced allodynia. The two compounds showed an anti-allodynic effect in a dose-dependent manner. Moreover, the most potent compound presented robust analgesic efficacy in the cyclophosphamide-induced murine model of cystitis, a well-established model of visceral pain. Overall, these results suggest painful bladder syndrome as a new possible indication for sEHI, opening a new range of applications for them in the visceral pain field.