Synthesis and the 5-HT6 receptor antagonistic effect of 3-arylsulfonylamino-5,6-dihydro-6-substituted pyrazolo[3,4]pyridinones for neuropathic pain treatment

Neuropathic pain-alleviating activity of novel 5-HT6 receptor inverse agonists derived from 2-aryl-1H-pyrrole-3-carboxamide

The diverse signaling pathways engaged by serotonin type 6 receptor (5-HT₆R) together with its high constitutive activity suggests different types of pharmacological interventions for the treatment of CNS disorders. Non-physiological activation of mTOR kinase by constitutively active 5-HT₆R under neuropathic pain conditions focused our attention on the possible repurposing of 5-HT₆R inverse agonists as a strategy to treat painful symptoms associated with neuropathies of different etiologies. Herein, we report the identification of compound 33 derived from the library of 2-aryl-1H-pyrrole-3-carboxamides as a potential analgesic agent. Compound 33 behaves as a potent 5-HT₆R inverse agonist at Gs, Cdk5, and mTOR signaling. Preliminary ADME/Tox studies revealed preferential distribution of 33 to the CNS and placed it in the low-risk safety space. Finally, compound 33 dose-dependently reduced tactile allodynia in spinal nerve ligation (SNL)-induced neuropathic rats.

Identification of a Potent and Selective 5-HT1A Receptor Agonist with In Vitro and In Vivo Antinociceptive Activity

Opioids are the gold standard drugs for the treatment of acute and chronic severe pain, although their serious side effects constitute a big limitation. In the search for new and safer drugs, 5-HT_1AR agonists are emerging as potential candidates in pain relief therapy. In this work, we evaluated the affinity and activity of enantiomers of the two newly synthesized, potent 5-HT_1AR agonists N-[(2,2-diphenyl-1,3-dioxolan-4-yl)methyl]-2-[2-(pyridin-4-yl)phenoxy]ethan-1-ammonium hydrogenoxalate (rac-1) and N-((2,2-diphenyl-1,3-dioxolan-4-yl)methyl)-2-(2-(1-methyl-1H-imidazol-5-yl)phenoxy)ethan-1-ammonium hydrogenoxalate (rac-2) in vitro and in vivo. The role of chirality in the interaction with 5-HT_1AR was evaluated by molecular docking. The activity of the rac-1 was tested in mouse models of acute pain (hot plate) and severe tonic nociceptive stimulation (intraplantar formalin test). Rac-1 was active in the formalin test with a reduction in paw licking in both phases at 10 mg/kg, and its effect was abolished by the selective 5-HT_1AR antagonist, WAY-100635. The eutomer (S)-1, but not the racemate, was active during the hot plate test at 10 and 20 mg/kg, and this effect was abolished by 30 min treatment with WAY-100635 at 30 min. Similarly to 8-OH-DPAT, (S)-1 evoked a slow outward current and depressed spontaneous glutamatergic transmission in superficial dorsal horn neurons, more effectively than rac-1. The eutomer (S)-1 showed promising developability properties, such as high selectivity over 5-HT subtypes, no interaction with the μ receptors, and low hepato- and cardiotoxicity. Therefore, (S)-1 may represent a potential candidate for the treatment of acute and chronic pain without having the adverse effects that are commonly associated with the classic opioid drugs.

Role of 5-HT receptors in neuropathic pain: potential therapeutic implications

5-HT plays a crucial role in the progress and adjustment of pain both centrally and peripherally. The therapeutic action of the 5-HT receptors` agonist and antagonist in neuropathic pain have been widely reported in many studies. However, the specific roles of 5-HT subtype receptors have not been reviewed comprehensively. Therefore, we summarized the recent findings on multiple subtypes of 5-HT receptors in both central and peripheral nervous system in neuropathic pain, particularly, 5-HT1, 5-HT2, 5-HT3 and 5-HT7 receptors. In addition, 5-HT4, 5-HT5 and 5-HT6 receptors were also reviewed. Most of studies focused on the function of 5-HT subtype receptors in spinal level compared to brain areas. Based on these evidences, the pain process can be facilitated or inhibited that depending on the specific subtypes and the distribution of 5-HT receptors. Therefore, this review may provide potential therapeutic implications in treatment of neuropathic pain.

5-HT6 receptor antagonist attenuates the memory deficits associated with neuropathic pain and improves the efficacy of gabapentinoids

BACKGROUND

Memory deficit is a co-morbid disorder in patients suffering from neuropathic pain. Gabapentin and pregabalin (gabapentinoids) are among the widely prescribed medications for the treatment of neuropathic pain. Memory loss and sedation are the commonly reported side effects with gabapentinoids. Improving the cognitive functions and attenuating drug-induced side effects may play a crucial role in the management of pain.

METHODS

We evaluated the effects of 5-HT6 receptor antagonists on the memory deficits associated with neuropathy. We also studied the effects of 5-HT6 receptor antagonists on the side effects, and the analgesic effects of gabapentinoids.

RESULTS

5-HT6 receptor antagonists attenuated the cognitive deficits in neuropathic rats. Neuropathic rats co-treated with 5-HT6 receptor antagonist and gabapentinoids showed improvement in memory. 5-HT6 receptor antagonists enhanced the analgesic effects of gabapentinoids but had no effect on the motor side effects. The observed effects may not be due to pharmacokinetic interactions.

CONCLUSIONS

5-HT6 receptor antagonist attenuate the cognitive deficits associated with neuropathy, and this effect is also seen when co-treated with gabapentinoids. Since, 5-HT6 antagonists improved the effectiveness of gabapentinoids, reduction in the dosage and frequency of gabapentinoids treatment may reduce the side effects. Combining 5-HT6 receptor antagonist with gabapentinoids may offer a novel treatment strategy for neuropathic pain.

Computational approaches to the design of novel 5-HT6 R ligands

5-Hydroxytryptamine (5-HT, serotonin) subtype 6 receptor (5-HT6 receptor, 5-HT6 R) belongs to a 5-HT subclass of a relatively wide G protein-coupled receptor (GPCR) family. Accumulated biological data indicate that 5-HT6 R antagonists and agonists have a great potential for the treatment of neuropathological disorders, such as Parkinson's disease, Alzheimer's disease, and schizophrenia. A number of painstaking efforts have been made toward the design of novel 5-HT6 R ligands; however, there are still no drugs that successfully passed all the clinical trials and entered the market, except for several multimodal ligands. Novel active molecules are strongly needed to progress this development forward. The in silico drug design has some benefits compared with the other rough approaches in terms of thoroughness and predictive accuracy; therefore, it can be effectively used as a solid foundation for the design of novel 5-HT6 R ligands with high potency and selectivity. Here, we provide an overview of the reported computational approaches to the design of novel 5-HT6 R ligands.