WO2025244120A1 - Analgesic agent - Google Patents

Abstract

The present invention provides a novel analgesic agent. The analgesic agent contains a heterocyclic compound represented by formula (I) or a salt thereof as an active ingredient (in the formula, ring A is a 5- to 7-membered heterocycle containing one or two heteroatoms selected from among a nitrogen atom, an optionally oxidized sulfur atom, and an oxygen atom; R¹, R², R³, and R⁴ are each independently a hydrogen atom, a C_1-6 alkyl group, a C_1-6 alkoxy group, a halogen atom, an amino group, -SH, a C_1-6 alkylthio group, a C_2-6 alkenylthio group, a C_1-6 alkyl-carbonyl group, a formyl group, a C_6-10 aryl group, a C_1-6 alkoxycarbonyl group, a 5- or 6-membered heteroaryl group, or an oxo group; R¹ and R² may be bonded to each other to form an optionally substituted 5- or 6-membered ring; and n is 0, 1, or 2).

Description

painkillers

The present invention relates to a novel analgesic drug.

Although many analgesics have been developed to date, many conditions, such as pain after spinal cord injury, trigeminal autonomic headache, chemotherapy-induced peripheral neuropathy, fibromyalgia, treatment-resistant pain associated with terminal cancer, and chronic postoperative pain, are classified as neuropathic pain or centralized pain, and involve pathological mechanisms such as sensitization, abnormal neuroplasticity, and neurogenesis. For these painful conditions, existing analgesics such as NSAIDs, opioids, and gabapentinoids are insufficiently effective, and the lack of effective biomarkers makes diagnosis and objective evaluation of treatment efficacy difficult. Therefore, there is a strong demand for the development of new analgesics with different mechanisms of action to treat these intractable pain conditions.

Analgesics currently used in medical practice are classified into several categories based on their mechanism of action and target. Among these, anti-inflammatory analgesics, such as nonsteroidal anti-inflammatory drugs (NSAIDs), steroids, and cyclooxygenase (COX) inhibitors, are representative. These drugs suppress the production of inflammatory cytokines and prostaglandins, thereby relieving swelling, redness, and pain associated with inflammation. They are highly effective against inflammatory pain, including arthritis, sprains, muscle pain, rheumatoid arthritis, and bacterial infections. NSAIDs, in particular, inhibit COX-1 and COX-2, thereby suppressing the production of prostaglandin _E2 ( _PGE2 ) and suppressing the sensitization of pain receptors in peripheral nerves. Analgesics based on anti-inflammatory effects have limited effectiveness against non-inflammatory conditions, such as neuropathic pain and cancer pain.

On the other hand, many analgesics exert their analgesic effects not through anti-inflammatory effects, but through interventions related to pain perception, transmission to the brain, or mechanisms involved in pain perception in the brain. Opioid analgesics exert potent central pain relief via μ-opioid receptors, but they have serious side effects such as respiratory depression, constipation, tolerance, dependence, and even the risk of abuse, which have become a social issue in Europe and the United States known as the "opioid crisis." Pregabalin and gabapentin are effective in treating neuropathic pain through their Ca ²⁺ channel blocking properties, but they often have side effects such as drowsiness, dizziness, and memory impairment, and have been noted to increase the risk of falls in elderly people. In recent years, there have been increasing reports of dependence and abuse. Cannabinoid drugs (e.g., THC, CBD) are used medically in some settings, but their application as general-purpose analgesics is limited due to their psychoactive effects, legal restrictions, and unclear mechanisms of action. Furthermore, although antidepressants such as SNRIs and TCAs activate the descending pain inhibitory system and show some effectiveness in treating chronic pain, their tolerability is limited by side effects such as dry mouth, constipation, sexual dysfunction, weight gain, and insomnia. The NMDA receptor antagonist ketamine also suppresses central sensitization and is used for intractable pain, but caution is required in its use due to concerns about psychiatric symptoms such as hallucinations and dissociation, as well as dependency.

As such, while existing analgesics each demonstrate a certain degree of effectiveness, they have limitations such as safety, tolerability, dependency, and a narrow range of indications. Therefore, there is a strong demand for the development of safer, more effective analgesics with new mechanisms of action that differ from conventional ones.

Transient Receptor Potential Ankyrin 1 (TRPA1) is known to be stimulated by invasive substances both inside and outside the body, inducing pain sensation and inflammation. The molecular mechanism has been elucidated in detail, as outlined below. Invasive substances that bind to TRPA1 include exogenous substances such as allylic isothiocyanate (AITC) and formaldehyde, as well as endogenous substances such as 4-hydroxynonenal (4-HNE), bradykinin, and prostaglandins, which are released from cells damaged by trauma or inflammation. TRPA1 is a nonselective cation channel, and when activated by an agonist, calcium ions enter the cell. This activates calcium-calmodulin-dependent protein kinase (CaMKII) and protein kinase C (PKC). Increased intracellular calcium ions and activation of calcium-dependent kinases promote the synthesis of calcitonin gene-related peptide (CGRP) and substance P in neurons and their secretion from synapses. As a result, pain sensation, promotion of inflammatory responses, vasodilation, etc. occur. Therefore, antagonists that suppress the activation of TRPA1 in response to noxious stimuli are thought to be candidates for drugs with anti-inflammatory and analgesic effects (Non-Patent Document 1).

International Publication No. 2019/177142 International Publication No. 2021/193835 International Publication No. 2022/030436

Koivisto, A et al., Basic Clin Pharmacol Toxico. 114(1), 50-55 (2014) Matsuo, T et al., bioRxiv. doi:https://doi.org/10.1101/2020.05.17.100933 (2020) Matsuo, T et al., Commun Biol. 4(1), 101 (2021) Nishi, M et al., ESC Heart Fail. 9(1), 428-441 (2022) Onoe, A et al., Shock. 58(4), 341-347 (2022) Matsuo et al., Nat Commun. 12(1), 2074 (2021)

Contrary to the conventional theory that TRPA1 agonists are stimuli that induce inflammation and pain, and that TRPA1 antagonists are potential therapeutic agents that induce analgesic and anti-inflammatory effects, the inventors have discovered that several types of thiazoline-related fear odors (TFOs), which act as agonists in the sense that they can bind to TRPA1 and generate sensory information, transmit sensory information to the solitary nucleus-parabrachial nucleus pathway in the brainstem, thereby inducing integrated protective effects such as hypothermia, hypometabolism, hypoxia resistance, resistance to ischemia-reperfusion injury, and improvement of organ ischemia, in addition to anti-inflammatory effects (Patent Documents 1-3, Non-Patent Documents 2-5). The inventors have clarified that this protective effect is not induced by the so-called agonist properties of binding to TRPA1 and generating ionic currents, but is induced by its ability to generate sensory information, which increases the expression of appropriate genes such as c-fos, a known neural activity marker, in TRPA1-expressing neurons in the trigeminal and vagus ganglia and in the brainstem pathways to which these neurons project (Non-Patent Document 6).

For example, AITC and cinnamaldehyde (CNA), which are known as conventional TRPA1 agonists, generate ionic currents via TRPA1 but are unable to induce c-fos expression in TRPA1-positive neurons in the trigeminal and vagus nerves. In contrast, TFOs such as 2-Methyl-2-thiazoline (2MT), 4-Ethyl-2-methyl-2-thiazoline (4E2MT), and 5-Methyl-2-thiazoline (5MT) not only generate TRPA1-mediated ionic currents, but also act as agonists by inducing c-fos expression in TRPA1-positive neurons in the trigeminal and vagus nerves. All of these ligands possess TRPA1 agonist properties, but while AITC and CNA promote inflammation via TRPA1, TFOs such as 2MT, 4E2MT, and 5MT also suppress inflammation via TRPA1, inducing the exact opposite physiological response.

Generally, the activity of ligands that bind to TRPA1 has been measured using patch clamp analysis and calcium imaging, using the activity of inducing ionic currents as an indicator. TRPA1 agonists are thought to induce pain, while TRPA1 antagonists are thought to have analgesic effects. However, as shown above, this model does not necessarily hold true, as some TRPA1 agonists induce both inflammatory and anti-inflammatory effects. The physiological effects induced by ligands that interact with TRPA1 cannot be classified as agonists or antagonists, but are thought to be diverse depending on the properties of the ligand. Although several TRPA1 antagonists have been developed as analgesics, none have been established as analgesics to date. The criterion of being a TRPA1 antagonist may not be sufficient to demonstrate analgesic effects in humans. To overcome this issue and develop effective analgesics, it is necessary to develop appropriate ligands that induce analgesic effects via TRPA1.

The object of the present invention is to provide a novel analgesic.

Based on this background, the inventors conducted behavioral pharmacological analysis using mice to verify the analgesic effects of TFO. The results confirmed that TFO exhibits significant analgesic effects in multiple pain models, including the formalin test, capsaicin test, hot plate test, and neuropathic pain model.

When AITC is injected into the sole of a mouse's foot, it senses pain and causes the animal to lick its paw. The activity of a candidate analgesic compound is measured as the effect of reducing the number of licks. It is known that the response specificity of TRPA1 to ligands differs between humans and mice. Therefore, there are concerns that animal experiments alone are insufficient to identify desirable candidates for human analgesics. Furthermore, as mentioned above, it is difficult to evaluate its pharmacological effects solely by measuring the agonist or antagonist activity of human TRPA1 using in vitro systems. AITC is the pungent component of wasabi and mustard, and its volatilized odor molecules bind to TRPA1 in the trigeminal nerve in the oral cavity and nose, inducing a sharp pain sensation in humans. Odor molecules that suppress this pain sensation could be candidates for analgesics. Using this and other experimental systems, this technology has also succeeded in developing several molecules that exert potent analgesic effects in humans, utilizing TRPA1 agonists, which have previously been thought to be a potential pain stimulus.

This technology has utilized experimental systems in humans and mice to successfully develop several types of molecules that exert powerful analgesic effects using TFOs, a TRPA1 agonist that has previously been thought to be a potential pain stimulus. These molecules selectively regulate the transmission of sensory information via TRPA1, and while avoiding the side effects and dependency seen in existing analgesics, they exhibit effective analgesic effects against a variety of pains, making them of extremely high clinical and practical value as analgesics based on a new mechanism. The molecules developed here can be used as analgesics by inhaling them as a vaporized gas, injecting them into the skin or veins, or administering them orally or transdermally.

That is, the present invention relates to the following: [1] Formula (I)

(In the formula,
Ring A is a 5- to 7-membered heterocycle containing 1 or 2 heteroatoms selected from a nitrogen atom, an optionally oxidized sulfur atom, and an oxygen atom;
R ¹ , R ² , R ³ , and R ⁴ are each independently a hydrogen atom, a C _1-6 alkyl group, a C _1-6 alkoxy group, a halogen atom, an amino group, —SH, a C _1-6 alkylthio group, a C _2-6 alkenylthio group, a C _1-6 alkyl-carbonyl group, a formyl group, a C _6-10 aryl group, a C _1-6 alkoxycarbonyl group, a 5- or 6-membered heteroaryl group, or an oxo group;
R ¹ and R ² may be bonded to each other to form an optionally substituted 5- or 6-membered ring;
n is 0, 1, or 2.
An analgesic containing a heterocyclic compound represented by the following formula (I) or a salt thereof as an active ingredient.
[2] The analgesic according to [1], wherein ring A is thiazoline, thiazole, thiazolidine, thiomorpholine, thiophene, pyrrole, morpholine, azepane, pyridine, pyrazine, furan, 2,3-dihydro-4H-1,4-thiazine, or imidazole.
[3] The analgesic according to [1] or [2] for nasal administration.
[4] Use of a heterocyclic compound represented by formula (I) or a salt thereof for producing an analgesic.
[5] The use according to [4], wherein ring A is thiazoline, thiazole, thiazolidine, thiomorpholine, thiophene, pyrrole, morpholine, azepane, pyridine, pyrazine, furan, 2,3-dihydro-4H-1,4-thiazine, or imidazole.
[6] The use according to [4] or [5], wherein the analgesic is for nasal administration.
[7] A method for preventing or treating pain in a mammal, which comprises administering to the mammal an effective amount of a heterocyclic compound represented by formula (I) or a salt thereof.
[8] The method according to [7], wherein ring A is thiazoline, thiazole, thiazolidine, thiomorpholine, thiophene, pyrrole, morpholine, azepane, pyridine, pyrazine, furan, 2,3-dihydro-4H-1,4-thiazine, or imidazole.
[9] The method according to [7] or [8], wherein the heterocyclic compound or a salt thereof is administered intranasally.
[10] A heterocyclic compound represented by formula (I) or a salt thereof for use in the prevention or treatment of pain.
[11] The heterocyclic compound represented by formula (I) or a salt thereof for use according to [10], wherein ring A is thiazoline, thiazole, thiazolidine, thiomorpholine, thiophene, pyrrole, morpholine, azepane, pyridine, pyrazine, furan, 2,3-dihydro-4H-1,4-thiazine, or imidazole.
[12] The heterocyclic compound represented by formula (I) or a salt thereof for use according to [10] or [11], which is for nasal administration.
[13] The analgesic according to any one of [1] to [3], for use in the prevention or treatment of neuropathic pain.
[14] The use according to any one of [4] to [6], wherein the analgesic is an analgesic for use in the prevention or treatment of neuropathic pain.
[15] The method according to any one of [7] to [9], wherein the pain is neuropathic pain.
[16] The heterocyclic compound represented by formula (I) or a salt thereof for use according to any one of [10] to [12], wherein the pain is neuropathic pain.

The present invention provides compounds that act directly on TRPA1 and other receptors that detect noxious stimuli to suppress the generation of pain sensations, or that suppress the recognition of pain by transmitting sensory information to the brain. The analgesic of the present invention can be used as an agent for the prevention or treatment of pain.

1 shows the results of an experiment evaluating the analgesic effect of TFO on a formalin-induced pain model. 1 shows the results of an experiment evaluating the analgesic effect of TFO on a capsaicin-induced pain model. 1 shows the results of an experiment evaluating whether TFO induces analgesic activity in an opioid receptor-independent manner. 1 shows the results of an experiment evaluating whether TFO induces analgesic effects in a _CB1 receptor-independent manner. 1 shows the results of an experiment evaluating the inhibitory effect of TFO on sensory transmission at the spinal cord level using c-fos mapping. 1 shows the results of an experiment evaluating the allodynia suppressing effect of TFO in a neuropathic pain model. 1 shows the results of an experiment evaluating the centrally mediated analgesic effect of TFO. 1 shows the results of an experiment evaluating the analgesic effect of TFO on acute pain caused by thermal stimulation. The chemical structures of the test substances used in Example 8 are shown below. The results of a human sensory test evaluating the pain intensity caused by AITC in the presence of TFO (2-methyl-2-thiazoline (2MT), thiomorpholine (TMO)) are shown.

Ring A in formula (I) represents a 5- to 7-membered heterocycle containing one or two heteroatoms selected from a nitrogen atom, an optionally oxidized sulfur atom, and an oxygen atom. Ring A is preferably a 5- to 7-membered heterocycle containing one or two heteroatoms selected from a nitrogen atom and an optionally oxidized sulfur atom. Ring A is more preferably a 5- to 7-membered heterocycle containing a nitrogen atom and an optionally oxidized sulfur atom. The number of members in ring A is more preferably 5 or 6.

Examples of the heterocycle include, but are not limited to, pyrrole, pyridine, pyridazine, pyrimidine, pyrazine, piperazine, pyrrolidine, hexahydropyridazine, imidazole, imidazolidine, piperidine, thiophene, thiolane, tetrahydro-2H-thiopyran, thiazoline (e.g., 2-thiazoline, 3-thiazoline, 4-thiazoline), thiazole, thiazolidine, isothiazole, isothiazoline, thiomorpholine, thiadiazoline, thiadiazole, thiadiazolidine, 1,3-thiazinane, 5,6-dihydro-4H-1,3-thiazine, 2,3-dihydro-4H-1,4-thiazine, furan, 2H-pyran, 4H-pyran, oxazole, isoxazole, morpholine, oxazoline, and azepane. Preferably, it is thiazoline (e.g., 2-thiazoline, 3-thiazoline, 4-thiazoline), thiazole, thiazolidine, thiomorpholine, thiophene, pyrrole, morpholine, azepane, pyridine, pyrazine, furan, 2,3-dihydro-4H-1,4-thiazine, or imidazole; more preferably, it is thiazoline (e.g., 2-thiazoline), thiazole, thiazolidine, thiomorpholine, thiophene, pyrrole, pyridine, pyrazine, or 2,3-dihydro-4H-1,4-thiazine; even more preferably, it is thiazoline (e.g., 2-thiazoline), thiazole, thiazolidine, thiomorpholine, thiophene, pyrrole, pyridine, or pyrazine; and particularly preferably, it is thiazoline (e.g., 2-thiazoline) or thiomorpholine.

As used herein, "halogen atom" is preferably selected from fluorine atom, chlorine atom, bromine atom, and iodine atom.

As used herein, a "C _1-6 alkyl group" (when used as a group or part of a group) means a straight or branched chain alkyl group having from 1 to 6 carbon atoms. Examples of C _1-6 alkyl groups include, but are not limited to, a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, a 1-methylpropyl group (sec-butyl group), a 2-methylpropyl group (isobutyl group), a tert-butyl group, a pentyl group, a 1-methylbutyl group, a 2-methylbutyl group, a 3-methylbutyl group, a 1,1-dimethylpropyl group, a 2,2-dimethylpropyl group, a 1,2-dimethylpropyl group, a 1-ethylpropyl group, a hexyl group, a 1-methylpentyl group, a 2-methylpentyl group, a 3-methylpentyl group, a 4-methylpentyl group, a 1,1-dimethylbutyl group, a 2,2-dimethylbutyl group, a 3,3-dimethylbutyl group, a 1,2-dimethylbutyl group, a 1,3-dimethylbutyl group, a 2,3-dimethylbutyl group, a 1-ethylbutyl group, a 2-ethylbutyl group, and a 1-ethyl-2-methylpropyl group. Preferred examples of the C _1-6 alkyl group include a C _1-4 alkyl group (a linear or branched alkyl group having 1 to 4 carbon atoms), with a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, and a sec-butyl group being more preferred, and a methyl group being particularly preferred.

The term "C _1-6 haloalkyl group" as used herein means a C _1-6 alkyl group substituted with 1 to 5 halogeno groups, and when there are two or more halogeno groups, the types of the halogeno groups may be the same or different. Examples of the halogeno group include a fluoro group, a chloro group, and a bromo group. Examples of _C1-6 haloalkyl groups include, but are not limited to, a fluoromethyl group, a difluoromethyl group, a trifluoromethyl group, a chlorodifluoromethyl group, a 1-fluoroethyl group, a 2-fluoroethyl group, a 2-chloroethyl group, a 2-bromoethyl group, a 1,1-difluoroethyl group, a 1,2-difluoroethyl group, a 2,2,2-trifluoroethyl group, a 1,1,2,2-tetrafluoroethyl group, a 1,1,2,2,2-pentafluoroethyl group, a 1-fluoropropyl group, a 1,1-difluoropropyl group, a 2,2-difluoropropyl group, a 3-fluoropropyl group, a 3,3,3-trifluoropropyl group, a 4-fluorobutyl group, a 4,4,4-trifluorobutyl group, a 5-fluoropentyl group, a 5,5,5-trifluoropentyl group, a 6-fluorohexyl group, and a 6,6,6-trifluorohexyl group.

As used herein, a " _C2-6 alkenyl group" (when used as a group or part of a group) refers to a straight or branched chain alkenyl group having from 2 to 6 carbon atoms. Examples of _C2-6 alkenyl groups include, but are not limited to, vinyl, allyl, prop-1-enyl, but-1-en-1-yl, but-2-en-1-yl, pent-4-en-1-yl, 2-methylallyl, and the like.

As used herein, a "C _1-6 alkoxy group" (when used as a group or part of a group) refers to a straight or branched chain alkoxy group having 1 to 6 carbon atoms. Examples of C _1-6 alkoxy groups include, but are not limited to, methoxy, ethoxy, propoxy, isopropoxy, butoxy, 1-methylpropoxy, 2-methylpropoxy, tert-butoxy, pentyloxy, 1-methylbutoxy, 2-methylbutoxy, 3-methylbutoxy, 1,1-dimethylpropoxy, 2,2-dimethylpropoxy, 1,2-dimethylpropoxy, 1-ethylpropoxy, and hexyloxy.

The term "C _1-6 alkylthio group" used herein refers to a -SH group substituted with a C _1-6 alkyl group. Examples of C _1-6 alkylthio groups include, but are not limited to, methylthio, ethylthio, propylthio, and butylthio groups.

As used herein, a " _C2-6 alkenylthio group" refers to a -SH group substituted by a _C2-6 alkenyl. Examples of _C2-6 alkenylthio groups include, but are not limited to, vinylthio, allylthio, prop-1-enylthio, but-1-en-1-ylthio, but-2-en-1-ylthio, pent-4-en-1-ylthio, and 2-methylallylthio groups.

As used herein, the term "C _1-6 alkyl-carbonyl group" refers to a carbonyl group to which a C _1-6 alkyl group is bonded. Examples of C _1-6 alkyl-carbonyl groups include, but are not limited to, acetyl, propionyl, butyryl, isobutyryl, valeryl, and hexanoyl groups.

As used herein, the term "C _1-6 alkoxycarbonyl group" refers to a carbonyl group bonded to a C _1-6 alkoxy group. Examples of C _1-6 alkoxycarbonyl groups include, but are not limited to, methoxycarbonyl, ethoxycarbonyl, propoxycarbonyl, isopropoxycarbonyl, and butoxycarbonyl groups.

As used herein, the term "C _6-10 aryl group" refers to an aromatic hydrocarbon group having 6 to 10 carbon atoms. Examples of _{C 6-10} aryl groups include, but are not limited to, phenyl groups and naphthyl groups (1-naphthyl groups, 2-naphthyl groups).

As used herein, the term "5- or 6-membered heteroaryl group" refers to a 5- or 6-membered heteroaryl group containing at least one heteroatom (preferably 1 to 3, more preferably 1 or 2) selected from a nitrogen atom, an optionally oxidized sulfur atom, and an oxygen atom. As the 5- or 6-membered heteroaryl group, a 5- or 6-membered heteroaryl group containing 1 or 2 heteroatoms selected from a nitrogen atom and an optionally oxidized sulfur atom is preferred.

Examples of 5- or 6-membered heteroaryl groups include, but are not limited to, pyrrolyl, pyridyl, pyridazinyl, pyrimidinyl, pyrazinyl, imidazolyl, thienyl, thiazolyl, isothiazolyl, thiadiazolyl, furyl, oxazolyl, and isoxazolyl groups. Preference is given to pyridyl and thienyl groups.

As used herein, the term "oxo" (when used as a group or part of a group) refers to the =O group.

As used herein, "optionally oxidized sulfur atom" means S, SO, or _SO2 .

The "5- or 6-membered ring" in the "optionally substituted 5- or 6-membered ring" formed by ^R1 and ^R2 bonding to each other refers to a 5- or 6-membered ring which may contain at least one heteroatom (preferably 1 to 3, more preferably 1 or 2) selected from a nitrogen atom, an optionally oxidized sulfur atom, and an oxygen atom. Examples of the 5- or 6-membered ring include a benzene ring and a tetrahydropyrimidine ring. The 5- or 6-membered ring may be substituted, and examples of the substituent include 1 to 4 (preferably 1 or 2) substituents selected from a _C1-6 alkyl group, a halogen atom, an amino group, -SH, a _C1-6 alkylthio group, _{a C2-6 alkenylthio group, a C1-6 alkyl -} carbonyl group, a formyl group, a C1-6 alkoxycarbonyl group, an oxo group, and the like. The substituents are preferably 1 to 4 substituents selected from a _C1-6 alkyl group (e.g., methyl) and an oxo group.

In formula (I), preferably, R ¹ , R ² , R ³ , and R ⁴ are each independently a hydrogen atom, a C _1-6 alkyl group (e.g., methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl), a C _1-6 alkoxy group (e.g., methoxy, ethoxy), a halogen atom (e.g., chlorine atom), an amino group, —SH, a C _1-6 alkylthio group (e.g., methylthio), a C _2-6 alkenylthio group (e.g., allylthio), a C _1-6 alkyl-carbonyl group (e.g., acetyl), a formyl group, a C _6-10 aryl group (e.g., phenyl), a 5- or 6-membered heteroaryl group (e.g., thienyl), or an oxo group; R ¹ and R ² may bond to each other to form an optionally substituted 5- or 6-membered ring (e.g., a benzene ring, a tetrahydropyrimidine ring).

In formula (I), when n=1 or 2, it is preferable that at least one of R ¹ , R ² , R ³ , and R ⁴ is not a hydrogen atom. In formula (I), when n=0, it is preferable that at least one of R ¹ , R ² , and R ³ is not a hydrogen atom.

In the present invention, examples of the heterocyclic compound of formula (I) suitable for use as an active ingredient include, but are not limited to, the following compounds:
2-Methyl-2-thiazoline (2MT)
Thiomorpholine (TMO)
2-acetylthiophene
2,5-Dimethylpyrrole
2-Ethylpyrrole
2-Chlorothiazole
4-Methylthiazole
2,3-Diethylpyrazine
2-ethoxythiazole
2-(Methylthio)-2-thiazolinethiomorpholine 1,1-dioxide
2,4,5-trimethylthiazole
2-acetyl-3,5-dimethylpyrazine
2-Methylthiazolethiomorpholine
2-Methylthiomorpholine
2,6-dimethylpyrazine
2-amino-2-thiazoline
2,6-Dimethylpyridine
2-aminothiazole
5-Acetyl-2,4-dimethylthiazole
2-Isobutylthiazole
2-Ethyl-3-(methylthio)pyrazine
2-acetylpyrrole
4-Ethyl-2-methyl-2-thiazoline
2,4-Dimethylpyrrole
2-Ethyl-3,5-dimethylpyrazine
2-Ethyl-3,6-dimethylpyrazine
2-Ethyl-3,5(6)-dimethylpyrazine (mixture of 2-ethyl-3,5-dimethylpyrazine and 2-ethyl-3,6-dimethylpyrazine)
2,2-dimethylthiazolidine
2-acetyl-3-ethylpyrazine

In the present invention, the heterocyclic compounds of formula (I) used as active ingredients include substances generally known as reagents, and are commercially available or can be obtained by methods known per se. The use of heterocyclic compounds of formula (I) as analgesics (drugs for the prevention or treatment of pain) has not been disclosed or suggested to date.

Preferred examples of the heterocyclic compound represented by formula (I) include compounds represented by the following formulas (A) to (D) or salts thereof.

(In the formula,
X ¹ is S, O, or N(R ¹⁶ );
^X2 is N or ^CR12 ;
X ³ is S, SO ₂ , O, or —(CH ₂ ) ₂ —;
^X4 is N or ^CR15 ;

represents a single or double bond;
R ¹¹ , R ¹² , R ¹³ , R ¹⁴ , R ¹⁵ , and R ¹⁶ are each independently a hydrogen atom, a C _1-6 alkyl group, a C _1-6 alkoxy group, a halogen atom, an amino group, —SH, a C _1-6 alkylthio group, a C _2-6 alkenylthio group, a C _1-6 alkyl-carbonyl group, a formyl group, a C _6-10 aryl group, a C _1-6 alkoxycarbonyl group, a 5- or 6-membered heteroaryl group, or an oxo group;
R ¹³ and R ¹⁴ may be bonded to each other to form a benzene ring or a tetrahydropyrimidine ring optionally substituted with 1 to 4 substituents selected from a C _1-6 alkyl group and an oxo group;
In formula (A), R ¹¹ and R ¹² are not oxo groups;
In formula (A),

represents a double bond, R ¹³ and R ¹⁴ are not oxo groups;
In formula (D), R ¹¹ , R ¹² , R ¹³ , R ¹⁴ , and R ¹⁵ are not oxo groups, and in formula (B), R ¹¹ and R ¹² may combine together to form an oxo group.

In formulae (A) to (D), preferably, R ¹¹ , R ¹² , R ¹³ , R ¹⁴ , R ¹⁵ and R ¹⁶ are each independently a hydrogen atom, a C _1-6 alkyl group (e.g., methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl), a C _1-6 alkoxy group (e.g., methoxy, ethoxy), a halogen atom (e.g., chlorine atom), an amino group, —SH, a C _1-6 alkylthio group (e.g., methylthio), a C _2-6 alkenylthio group (e.g., allylthio), a C _1-6 alkyl-carbonyl group (e.g., acetyl), a formyl group, a C _6-10 aryl group (e.g., phenyl), a 5- or 6-membered heteroaryl group (e.g., thienyl), or an oxo group; R ¹³ and R ¹⁴ are bonded to each other to form a benzene ring, or A tetrahydropyrimidine ring may be formed which may be substituted with 1 to 4 substituents selected from _1-6 alkyl groups and oxo groups.

Salts of the compounds according to the present invention may be any pharmaceutically acceptable salt, and examples include alkali metal salts such as sodium salts and potassium salts; alkaline earth metal salts such as magnesium salts and calcium salts; ammonium salts such as dimethylammonium salts and triethylammonium salts; inorganic acid salts such as hydrochlorides, perchlorates, sulfates and nitrates; and organic acid salts such as acetates and methanesulfonates.

Preferred examples of heterocyclic compounds represented by formula (I) include compounds represented by the following formula (A-1) or (C-1) or salts thereof.

(wherein R ^11A is a hydrogen atom, a C _1-6 alkyl group, a C _1-6 alkoxy group, a halogen atom, an amino group, —SH, a C _1-6 alkylthio group, or a C _1-6 alkenylthio group; R ^13A is a hydrogen atom or a C _1-6 alkyl group; R ^14A is a hydrogen atom, a C _1-6 alkyl group, or a C _1-6 alkyl-carbonyl group; R ^13A and R ^14A may be bonded to each other to form a benzene ring;

indicates a single or double bond)

In a preferred embodiment of formula (A-1), R ^11A is a hydrogen atom, a C _1-6 alkyl group, a C _1-6 alkoxy group, an amino group, —SH, or a C _1-6 alkenylthio group; R ^13A is a hydrogen atom or a C _1-6 alkyl group; R ^14A is a hydrogen atom or a C _1-6 alkyl group; R ^13A and R ^14A may be bonded to each other to form a benzene ring;

Indicates a single or double bond.

In another preferred embodiment of formula (A-1), R ^11A is a C _1-6 alkyl group, a C _1-6 alkoxy group, an amino group, —SH, or a C _1-6 alkenylthio group; R ^13A is a hydrogen atom or a C _1-6 alkyl group; R ^14A is a hydrogen atom or a C _1-6 alkyl group; and R ^13A and R ^14A may be bonded to each other to form a benzene ring.
In another preferred embodiment of formula (A-1), R ^11A is a hydrogen atom or a C _1-6 alkyl group; R ^13A is a hydrogen atom or a C _1-6 alkyl group; and R ^14A is a hydrogen atom or a C _1-6 alkyl group.

In formula (A-1), preferably, R ^11A is a hydrogen atom or a C _1-4 alkyl group; R ^13A is a hydrogen atom or a C _1-4 alkyl group; and R ^14A is a hydrogen atom or a C _1-4 alkyl group.
In formula (A-1), it is more preferable that at least one of R ^11A , R ^13A and R ^14A is not a hydrogen atom.
In formula (A-1), more preferably, R ^11A is a C _1-4 alkyl group; R ^13A is a hydrogen atom or a C _1-4 alkyl group; and R ^14A is a hydrogen atom;

indicates a single bond.

(wherein ^X3 is S or _SO2 ; ^R11A is a hydrogen atom or a _C1-6 alkyl group; ^R12A is a hydrogen atom or a _C1-6 alkyl group; R13A is a hydrogen atom or a _C1-6 alkyl group; ^R14A is a hydrogen atom or a _C1-6 alkyl group; ^{and R16A} is a hydrogen atom or a _C1-6 alkyl group).
In formula (C-1), preferably, R ^11A is a hydrogen atom or a C _1-4 alkyl group; R ^12A is a hydrogen atom or a C _1-4 alkyl group; R ^13A is a hydrogen atom or a C _1-4 alkyl group; R ^14A is a hydrogen atom or a C _1-4 alkyl group; and R ^16A is a hydrogen atom or a C _1-4 alkyl group.
In formula (C-1), more preferably, X ³ is S or SO ₂ ; R ^11A is a hydrogen atom or a C _1-4 alkyl group; and R ^12A , R ^13A , R ^14A and R ^16A are each a hydrogen atom.

The analgesic provided by the present invention can be used as a preventive or therapeutic agent for pain. An analgesic refers to a medicine used to prevent or treat (including alleviate) pain.

Pain includes nociceptive pain and neuropathic pain. Exemplary types of pain include acute pain, chronic pain, mild pain, moderate pain, severe pain, musculoskeletal pain, complex regional pain syndrome, neuropathic pain, postoperative pain, inflammatory pain, rheumatoid arthritis pain, osteoarthritis pain, pain associated with temporomandibular joint disorders, back pain (e.g., acute low back pain), trigeminal neuralgia, postherpetic neuralgia, sciatica, visceral pain, cancer pain, burn pain, oral pain, neuralgia, migraine, neuropathy, pain associated with acute trauma, chemotherapy-induced mononeuropathy pain states, polyneuropathy pain states (e.g., diabetic peripheral neuropathy and chemotherapy-induced neuropathy), autonomic neuropathic pain states, pain states associated with peripheral nervous system (PNS) lesions or central nervous system (CNS) lesions or diseases, polyneuropathy of the neck, lower back, or sciatica type. These include, but are not limited to, pain conditions associated with radiculopathy, cauda equina syndrome, piriformis syndrome, paraplegia, quadriplegia, various infections, chemical injury, radiation exposure, underlying disease or deficiency states (e.g., beriberi, vitamin deficiency, hypothyroidism, porphyria, cancer, HIV, autoimmune diseases such as multiple sclerosis, and spinal cord injury), fibromyalgia, pain conditions associated with various diseases (e.g., nerve injury, ischemia, neurodegeneration, stroke, post-stroke pain, inflammatory disorders, esophagitis, gastroesophageal reflux disorder (GERD), irritable bowel syndrome, inflammatory bowel disease, pelvic hypersensitivity, urinary incontinence, cystitis, gastric and duodenal ulcers), crush and injury-induced pain, incisional pain, bone pain, pain associated with sickle cell disease, muscle pain, pain due to colic, pain from hyperalgesia or allodynia, and referred pain.

A heterocyclic compound represented by formula (I) or a salt thereof (hereinafter also referred to as the compound of the present invention) can be administered to animals, including humans, that have developed or are at risk of developing pain, for the purpose of preventing pain onset or alleviating symptoms. A gas generated from the compound of the present invention at a concentration of 0.1 to 100,000 ppm can be inhaled through the nasal cavity or lungs using a gas mask or a device with a similar function. Alternatively, the compound of the present invention can be administered orally at a dose of 1 μg/kg to 5,000 mg/kg. Alternatively, the compound of the present invention can be injected into the body at a dose of 1 μg/kg to 5,000 mg/kg by intradermal, subcutaneous, intramuscular, intravenous, intraarterial, intrathecal, intraperitoneal, or other methods. Alternatively, the compounds of the present invention can be administered at a dose of 1 μg/kg to 5,000 mg/kg by methods such as transdermal administration, transmucosal administration, buccal administration, sublingual administration, ocular administration, ear drop administration, nasal administration (intranasal administration), rectal administration, and vaginal administration. The frequency of administration can be a single dose, or continuous administration at regular intervals or continuous administration at different time intervals. Animals to which this administration can be administered include mammals (humans, mice, rats, hamsters, rabbits, cats, dogs, cows, sheep, pigs, horses, monkeys, etc.).

When the compound of the present invention is used as an analgesic (hereinafter also referred to as the agent of the present invention), pharmaceutically acceptable additives can be added as needed.

Specific examples of pharmaceutically acceptable additives include, but are not limited to, antioxidants, preservatives, colorants, flavorants, and diluents, emulsifiers, suspending agents, solvents, fillers, bulking agents, buffers, delivery vehicles, diluents, carriers, excipients and/or pharmaceutical adjuvants.

The formulation form of the agent of the present invention is not particularly limited, but examples include liquids, injections, sustained-release preparations, lotions, creams, gels, sprays, patches (e.g., tapes, poultices), ointments, suspensions, emulsions, syrups, capsules, granules, powders, tablets, orally disintegrating tablets, chewable tablets, effervescent tablets, pills, sublingual tablets, troches, drops, buccal tablets, inhalants, eye drops, ear drops, nasal drops, suppositories, enemas, vaginal suppositories, and vaginal tablets. The above formulations can be prepared by methods known in the art. The solvent used to formulate the agent of the present invention into the above formulations may be either aqueous or non-aqueous.

Injections can be prepared by methods known in the art. For example, the compound is dissolved in an appropriate solvent (such as physiological saline, a buffer solution such as PBS, or sterile water), sterilized by filtration using a filter, and then filled into a sterile container (such as an ampoule) to prepare an injection. If necessary, the injection may contain a conventional pharmaceutical carrier. Administration using a non-invasive catheter may also be used. Carriers that can be used in the present invention include neutral buffered saline, saline containing serum albumin, etc.

The present invention will be explained in more detail and specifically using the following examples, but the present invention is not limited to these examples.

Example 1: Analgesic Effect of TFO on Formalin-Induced Pain Model Experimental Method: C57BL/6 mice (male, 10-12 weeks old) were intraperitoneally (ip) administered saline or 40 mg/kg 2MT. Thirty minutes later, 20 μl of 5% formalin was administered to the footpad of the mouse, which was then placed in an observation chamber. The mouse's behavior was videotaped for 60 minutes after administration. Spontaneous pain behavior was assessed by licking or biting the footpad from the videotaped images.

Results The results are shown in FIG.
The time (seconds) spent exhibiting pain behavior (licking and biting time) every 5 minutes after formalin injection is shown for the saline and 2MT groups (A). The periods 0-5 minutes and 10-60 minutes after formalin injection were defined as the first and second phases, respectively. The mean ± standard error of the time spent exhibiting pain behavior during each period is shown for the saline and 2MT groups (B). Statistical calculations were performed using Student's t-test to examine the change in the time spent exhibiting pain behavior between the saline and 2MT groups for each period during the first and second phases. **** indicates a p<0.0001, indicating a statistically significant difference.

Phase 1 is generally called the acute phase and is thought to represent acute peripheral pain, while phase 2 is called the inflammatory phase and is thought to represent inflammatory and central pain induced by the production of inflammatory mediators. 2MT suppressed pain behavior in both phases 1 and 2, demonstrating its analgesic effects on both acute peripheral pain, inflammatory and central pain.

Example 2: Analgesic Effect of TFO on Capsaicin-Induced Pain Model Experimental Method: C57BL/6 mice (male, 10-11 weeks old) were administered saline or 10 mg/kg 2MT i.p. 30 minutes later, 20 μl of capsaicin solution (1.6 μg capsaicin dissolved in 20 μl of saline containing 0.4% DMSO) was administered to the paw pad. The mice were then placed in an observation chamber and their behavior was videotaped for 5 minutes after administration. Spontaneous pain behavior was assessed by licking or biting the paw pad from the video footage.

Results The results are shown in FIG.
The mean ± standard error of pain behavior is shown for the saline and 2MT groups. Statistical calculations were performed using Student's t-test to examine the change in the time it took for pain behavior to be exhibited between the saline and 2MT groups. ** indicates p<0.01, indicating a statistically significant difference.

Formalin is thought to induce acute peripheral pain via TRPA1, whereas capsaicin induces acute peripheral pain via TRPV1. Although 2MT has been suggested to bind to TRPA1 but not TRPV1, it was shown to have an analgesic effect not only on the acute peripheral pain induced by formalin via TRPA1, but also on the acute peripheral pain induced by capsaicin via TRPV1. These results suggest that 2MT may be useful as a novel analgesic that acts on the nervous system above primary nerves.

Example 3: TFO induces analgesia independently of opioid receptors. Experimental Method: C57BL/6 mice (male, 9-14 weeks old) were given an i.p. injection of vehicle (saline) or 7 mg/kg naltrexone hydrochloride (Naltx), followed 30 minutes later by an i.p. injection of saline or 10 mg/kg 2MT. Thirty minutes later, 20 μl of 5% formalin was administered to the footpad of the mice, who were then placed in an observation chamber. Behavioral analysis was performed as in Example 1.

Results The results are shown in FIG.
Bar graphs show the mean ± standard error of pain behavior for the four groups: vehicle-saline, naltrexone-saline, vehicle-2MT, and naltrexone-2MT. A two-way ANOVA was performed to examine the changes in the time during which pain behavior was observed in these four groups, and the significance of main effects and interactions was confirmed. Post-hoc comparisons between groups were then performed using Fisher's least significant difference test without correction for multiple comparisons. * indicates p<0.05, ** indicates p<0.01, and **** indicates p<0.0001, indicating a statistically significant difference.

The inhibitory effect of 2MT on Formalin-induced pain behavior was not inhibited by administration of the opioid receptor antagonist naltrexone in either phase 1 or phase 2, and instead tended to be enhanced. This suggests that the analgesic effect of 2MT is induced by an opioid-independent mechanism, demonstrating the potential for 2MT to be a useful novel non-opioid analgesic.

Example 4: TFO induces analgesia independent of CB1 receptors. _Experimental Method: C57BL/6 mice (male, 11-14 weeks old) were given an i.p. injection of vehicle (10% DMSO in saline) or 2 mg/kg AM251, followed 30 minutes later by an i.p. injection of saline or 10 mg/kg 2MT. Thirty minutes later, 20 μl of 5% formalin was administered to the footpad of the mice, which were then placed in an observation chamber. Behavioral analysis was performed as in Example 1. AM251 (CAS Registry Number 183232-66-8) is a cannabinoid ( _CB1 ) receptor antagonist.

Results The results are shown in FIG.
Bar graphs show the mean ± standard error of pain behavior for the four groups: vehicle-saline, AM251-saline, vehicle-2MT, and AM251-2MT. A two-way ANOVA was performed on the changes in the time period during which pain behavior was observed in these four groups to confirm the significance of main effects and interactions. Post-hoc comparisons between groups were then performed using Fisher's least significant difference test without correction for multiple comparisons. *** indicates p<0.001, and **** indicates p<0.0001, indicating a statistically significant difference.

The inhibitory effect of 2MT on Formalin-induced pain behavior was not affected by the administration of AM251, a cannabinoid ( _CB1 ) receptor antagonist, in either phase 1 or phase 2. These results suggest that the analgesic effect of 2MT is induced by a mechanism independent of cannabinoid receptors, and suggest that 2MT may be useful as a novel non-cannabinoid analgesic agent.

Example 5: Evaluation of the inhibitory effect of TFO on sensory transmission at the spinal cord level using c-fos mapping Experimental Method: C57BL/6 mice (male, 9-12 weeks old) were transferred to a new cage and allowed to acclimate for 2 hours. Then, saline or 2MT (40 mg/kg) was administered intraperitoneally. Thirty minutes after administration, 5% formalin (20 μl) was injected into the plantar surface of the hind paw. Thirty minutes after formalin administration, the mice were anesthetized with isoflurane, and the spinal cords were removed and fixed overnight in 4% PFA solution at 4°C. After fixation, the spinal cord tissue was dehydrated with ethanol and xylene and embedded in paraffin. Five-micrometer-thick sections were prepared from the spinal cords, and c-fos mRNA was detected by in situ hybridization using a DIG-labeled RNA probe. The sections were reacted with anti-DIG antibody at 37°C, stained for 6 hours, and then nuclear stained for an additional 4 minutes. After staining, the specimens were mounted and images were acquired using a virtual slide scanner.

Results The results are shown in FIG.
Representative staining images for the saline- and 2MT-treated groups are shown in Figures 5A and 5B, respectively. The number of c-fos-positive cells (c-fos+ cells/section) in the spinal dorsal horn of each group was counted and the results are shown in Figure 5C. The graph in Figure 5C shows the mean ± standard error. Statistical calculations were performed using the Mann-Whitney test for the changes in c-fos-positive cell foci between the saline and 2MT groups. **** indicates p<0.0001, indicating a statistically significant difference.

c-fos gene expression is widely used as an indicator of neural activity. It is known that noxious stimuli induced by formalin transmit nociceptive information to the dorsal horn of the spinal cord via C fibers and Aδ fibers. In this example, we confirmed that c-fos expression was induced in spinal dorsal horn neurons activated by input from these fibers. In contrast, c-fos expression in spinal dorsal horn neurons was significantly reduced in the group administered both formalin and 2MT. This result suggests that 2MT has the effect of suppressing the activation of nociceptive neurons in the spinal dorsal horn, i.e., possesses an analgesic effect. Therefore, this compound may be useful as a novel analgesic that can inhibit pain processing through the inhibition of sensory transmission at the spinal cord level.

Example 6: Anti-allodynic effect of TFO in a neuropathic pain model Experimental method: C57BL/6 mice (male, 9-12 weeks old) were immobilized in the dorsal recumbent position under triple anesthesia (a mixture of medetomidine, midazolam, and butorphanol). The skin was incised along the midline slightly to the left of the dorsal midline (the injured side). After incising the fascia, the surgical field was opened using a retractor, and the muscle above the L5 transverse process was dissected to expose the transverse process. Next, the base of the L5 transverse process was removed using a drill, revealing the L3 and L4 spinal nerves running below. Of the exposed nerves, the L4 spinal nerve was completely transected with scissors, leaving the L3 spinal nerve intact. After checking for bleeding, the muscle layer and skin were sutured with 4-0 silk thread, and the surgery was completed. After surgery, atipamezole was administered intraperitoneally to reverse the anesthesia, and the mice were kept warm on a heating pad until they woke up.

The neuropathic pain model created using the above method was evaluated for mechanical sensitivity using Von Frey filaments on day 0 and day 7 after surgery. The test began with mice individually placed on an aluminum mesh plate and covered with an opaque mouse cage. After an acclimation period of 30 minutes to 1 hour, the test was conducted. A 0.02-2.0 g Von Frey filament was applied perpendicularly to the central plantar area of the hind paw on both the nerve-transected and uninjured sides, and pain-related behaviors such as rapid withdrawal of the hind paw or licking of the paw were observed. The stimulus intensity threshold was calculated as the 50% response threshold using Dixon's up-and-down method. For the test on day 7 after surgery, baseline values were measured after acclimation, followed by intraperitoneal administration of saline or 40 mg/kg 2MT, with further measurements taken 30, 60, and 120 minutes after administration.

Results The results are shown in FIG.
Mechanical sensitivity was assessed in the hind paws of the nerve-transected side (A) and the uninjured side (B) in the saline- and 2MT-treated groups. Results are shown in line graphs as the mean ± standard error of the scores at baseline, before drug administration on day 7 after surgery (Pre), and 30, 60, and 120 minutes after drug administration (Time after ip injection). Statistical calculations were performed using two-way ANOVA for the saline and 2MT groups to assess the significance of main effects and interactions. Post-hoc evaluation between groups was then performed using Sidak's multiple comparison test. ** indicates p<0.01, and **** indicates p<0.0001, indicating a statistically significant difference between the saline and 2MT groups.

In mice administered 2MT, the response threshold to mechanical stimulation in the hind paw on the nerve-transected side was significantly higher than in the control group, suppressing hypersensitivity to mechanical stimulation (A). On the other hand, no significant change was observed in the response threshold in the uninjured (contralateral) hind paw, demonstrating that 2MT does not affect normal sensory function (B). These results indicate that 2MT selectively suppresses abnormal pain sensitivity (allodynia) caused by nerve injury, without affecting physiological sensory thresholds.

Example 7: Evaluation of the centrally mediated analgesic effect of TFO Experimental Method: C57BL/6 mice (male, 13-14 weeks old) were placed in the analysis chamber and allowed to acclimate for 10-15 minutes. After this, TFO was administered to the plantar surface under the following three conditions:
1) 20 μl of saline was administered to the sole of the right hind paw, and 20 μl of 5% formalin was administered to the sole of the left hind paw (saline group).
2) 20 μl of 0.12% 2MT solution was administered to the sole of the right hind paw, and 20 μl of 5% formalin was administered to the sole of the left hind paw (2MT_R group).
3) 20 μl of saline solution was administered to the sole of the right hind paw, and 20 μl of 5% formalin/0.12% 2MT solution was administered to the sole of the left hind paw (2MT_L group).
The behavior of mice in each group was videotaped for 5 minutes after administration, and spontaneous pain behavior, such as licking or biting the sole of the foot, was measured from the video.

Results The results are shown in FIG.
The bar graph shows the mean ± standard error of the time that pain behavior was observed for each group. Statistical calculations were performed using one-way ANOVA and Tukey's multiple comparison test for the three groups. *** indicates p<0.001, **** indicates p<0.0001, indicating a statistically significant difference, and ns indicates p>0.05, indicating no statistically significant difference.

Formalin-induced acute pain was suppressed not only when Formalin was administered to the ipsilateral leg (2MT_L) but also when it was administered to the contralateral leg (2MT_R), and no statistical difference was observed in the analgesic effect when administered ipsilaterally or contralaterally. Therefore, the analgesic effect of 2MT cannot be explained solely by its action as a direct antagonist of TRPA1, the receptor for formalin, suggesting that 2MT may be useful as a novel analgesic that transmits analgesic signals more centrally.

Example 8: Analgesic Effect of TFO on Acute Pain Induced by Heat Stimulation Experimental Method: C57BL/6 mice (male, 8-12 weeks old) were allowed to acclimate to the laboratory for at least 30 minutes, after which saline or each test substance was subcutaneously injected into the dorsal skin at a dose of 40 mg/kg. Thirty minutes after administration, the mice were placed on a 52°C hot plate, and their behavior was recorded by videotape recording. The video images were used to measure the time from when the mice were placed on the hot plate until they exhibited pain-related behaviors (e.g., licking their hind paws, jumping, or shaking their hind paws). The maximum cutoff time was 60 seconds; if the mice did not exhibit pain-related behaviors, they were removed from the hot plate at that point.

Results The results are shown in Figure 8A. The chemical structures of the test substances are shown in Figure 8B.
The graph shows the mean ± standard error of the latency (seconds) until the mice exhibited pain-related behavior after administration of each compound. Statistical analysis was performed using Student's t-test against the saline condition, with * indicating p<0.05, ** indicating p<0.01, *** indicating p<0.001, and **** indicating p<0.0001, indicating a statistically significant difference.
Administration of each of the following compounds prolonged the latency to exhibit pain-related behavior in response to heat stimulation, indicating that these compounds have analgesic effects against acute pain induced by heat stimulation.
2-Acetylthiophene
2,5-Dimethylpyrrole
2-Ethylpyrrole
2-Chlorothiazole
4-Methylthiazole
2,3-Diethylpyrazine
2-Ethoxythiazole
2-(Methylthio)-2-thiazoline
Thiomorpholine 1,1-dioxide
2,4,5-Trimethylthiazole
2-Acetyl-3,5-dimethylpyrazine
2-Methylthiazole
Thiomorpholine
2-Methylthiomorpholine
2,6-Dimethylpyrazine
2-Amino-2-thiazoline
2,6-Dimethylpyridine
2-Aminothiazole
5-Acetyl-2,4-dimethylthiazole
2-Isobutylthiazole
2-Methyl-2-thiazoline
2-Ethyl-3-(methylthio)pyrazine
2-Acetylpyrrole
4-Ethyl-2-methyl-2-thiazoline
2,4-Dimethylpyrrole
2-Ethyl-3,5(6)-dimethylpyrazine (mixture of 2-ethyl-3,5-dimethylpyrazine and 2-ethyl-3,6-dimethylpyrazine)
2,2-Dimethylthiazolidine
2-Acetyl-3-ethylpyrazine

Example 9: Analgesic Effect of TFO in Humans Experimental Method: 5 μl of allyl isothiocyanate (AITC) was applied to the tip of a cotton swab. Next, 0 μl, 5 μl, or 10 μl of 2-methyl-2-thiazoline (2MT), thiomorpholine (TMO), or trans-cinnamaldehyde (CNA) was applied to the tip of the AITC-applied swab. Pain felt when sniffing the cotton swab containing only AITC was scored as 10 points, and pain felt when sniffing the cotton swab containing no other swab was scored as 0 points. The pain felt from 0 to 30 seconds after sniffing the cotton swabs containing AITC, 2MT, TMO, or CNA was measured by a human sensory test (three subjects).

Results The results are shown in FIG.
When AITC was administered in combination with 2MT and TMO, the pain score induced by AITC was reduced in a dose-dependent manner (A, B). On the other hand, when AITC was administered in combination with the TRPA1 agonist CNA, no reduction in the pain score induced by AITC was observed (C). Comparisons of pain scores between the control condition (AITC only) and the conditions where each compound was administered in combination with AITC were evaluated using the Kruskal-Wallis test. *, p<0.05; ***, p<0.001; ns, p>0.05.

AITC is known to induce pain in humans and mice via TRPA1. Sensory testing suggested that 2MT and TMO have the effect of suppressing AITC-induced pain. This effect was not observed with the existing TRPA1 agonist CNA. In rodent studies, the monophasic pain-like behavior that occurs immediately after AITC administration is evaluated as an acute pain response. Furthermore, because AITC itself induces local inflammation, it is known that hyperalgesia develops a certain time after administration. Therefore, while it can be used as an acute pain model, it is also often used as a model to evaluate the formation of post-inflammatory hyperalgesia. In this example, the pain felt between 0 and 30 seconds after sniffing AITC was scored, which is thought to be a measurement of its effect on acute pain. TFO was shown to have the effect of suppressing TRPA1-induced acute pain not only in mice but also in humans.

The analgesic of the present invention can suppress the occurrence of pain sensations by directly acting on TRPA1 and other receptors that detect noxious stimuli, or can suppress the perception of pain by transmitting sensory information to the brain, and can be used to prevent or treat pain.

This application is based on patent application No. 2024-084662 filed in Japan, the contents of which are incorporated in their entirety into this specification.

Claims (9)

Formula (I)
(In the formula,
Ring A is a 5- to 7-membered heterocycle containing 1 or 2 heteroatoms selected from a nitrogen atom, an optionally oxidized sulfur atom, and an oxygen atom;
R ¹ , R ² , R ³ , and R ⁴ are each independently a hydrogen atom, a C _1-6 alkyl group, a C _1-6 alkoxy group, a halogen atom, an amino group, —SH, a C _1-6 alkylthio group, a C _2-6 alkenylthio group, a C _1-6 alkyl-carbonyl group, a formyl group, a C _6-10 aryl group, a C _1-6 alkoxycarbonyl group, a 5- or 6-membered heteroaryl group, or an oxo group;
R ¹ and R ² may be bonded to each other to form an optionally substituted 5- or 6-membered ring;
n is 0, 1, or 2.
An analgesic containing a heterocyclic compound represented by the following formula (I) or a salt thereof as an active ingredient. The analgesic of claim 1, wherein ring A is thiazoline, thiazole, thiazolidine, thiomorpholine, thiophene, pyrrole, morpholine, azepane, pyridine, pyrazine, furan, 2,3-dihydro-4H-1,4-thiazine, or imidazole. The analgesic described in claim 1 or 2 for nasal administration. for producing an analgesic,
(In the formula,
Ring A is a 5- to 7-membered heterocycle containing 1 or 2 heteroatoms selected from a nitrogen atom, an optionally oxidized sulfur atom, and an oxygen atom;
R ¹ , R ² , R ³ , and R ⁴ are each independently a hydrogen atom, a C _1-6 alkyl group, a C _1-6 alkoxy group, a halogen atom, an amino group, —SH, a C _1-6 alkylthio group, a C _2-6 alkenylthio group, a C _1-6 alkyl-carbonyl group, a formyl group, a C _6-10 aryl group, a C _1-6 alkoxycarbonyl group, a 5- or 6-membered heteroaryl group, or an oxo group;
R ¹ and R ² may be bonded to each other to form an optionally substituted 5- or 6-membered ring;
n is 0, 1, or 2.
Use of a heterocyclic compound represented by the formula: or a salt thereof. The use according to claim 4, wherein ring A is thiazoline, thiazole, thiazolidine, thiomorpholine, thiophene, pyrrole, morpholine, azepane, pyridine, pyrazine, furan, 2,3-dihydro-4H-1,4-thiazine, or imidazole. The use according to claim 4 or 5, wherein the analgesic is for nasal administration. an effective amount of a compound of formula (I)
(In the formula,
Ring A is a 5- to 7-membered heterocycle containing 1 or 2 heteroatoms selected from a nitrogen atom, an optionally oxidized sulfur atom, and an oxygen atom;
R ¹ , R ² , R ³ , and R ⁴ are each independently a hydrogen atom, a C _1-6 alkyl group, a C _1-6 alkoxy group, a halogen atom, an amino group, —SH, a C _1-6 alkylthio group, a C _2-6 alkenylthio group, a C _1-6 alkyl-carbonyl group, a formyl group, a C _6-10 aryl group, a C _1-6 alkoxycarbonyl group, a 5- or 6-membered heteroaryl group, or an oxo group;
R ¹ and R ² may be bonded to each other to form an optionally substituted 5- or 6-membered ring;
n is 0, 1, or 2.
A method for preventing or treating pain in a mammal, comprising administering to the mammal a heterocyclic compound represented by the following formula (I): or a salt thereof. The method of claim 7, wherein ring A is thiazoline, thiazole, thiazolidine, thiomorpholine, thiophene, pyrrole, morpholine, azepane, pyridine, pyrazine, furan, 2,3-dihydro-4H-1,4-thiazine, or imidazole. The method of claim 7 or 8, wherein the heterocyclic compound or its salt is administered nasally.