Inhibitor of pain threshold decrease

1. A method for treating chronic pain comprising administering to a patient an effective amount of a κ-opioid receptor agonist which is a compound represented by the following formula or a pharmaceutically acceptable salt thereof:

wherein R 1 represents an acyl group;

R 2 and R 3 , which are the same or different, represent a hydrogen atom, a halogen atom, an alkyl group, a cycloalkyl group, an aryl group, a hydroxyl group or its ester, an alkoxy group, an aryloxy group, a carboxy group or its ester, an alkylcarbonyl group, an arylcarbonyl group, an amino group, an alkylamino group, an arylamino group, a cyano group or a nitro group, the alkyl group, the cycloalkyl group, the aryl group, the alkoxy group, the aryloxy group, the alkylcarbonyl group, the arylcarbonyl group, the alkylamino group or the arylamino group can be substituted with a halogen atom, an alkyl group, a cycloalkyl group, an aryl group, a hydroxyl group or its ester, an alkoxy group, an aryloxy group, a carboxy group or its ester, an alkylcarbonyl group, an arylcarbonyl group, an amino group, an alkylamino group, an arylamino group, a cyano group or a nitro group;

R 4 and R 5 , which are the same or different, represent a hydrogen atom, an alkyl group, a cycloalkyl group, an aryl group, a hydroxyl group or its ester, an alkoxy group, an aryloxy group or an acyl group, the alkyl group, the cycloalkyl group, the aryl group, the alkoxy group, the aryloxy group or the acyl group can be substituted with a halogen atom, an alkyl group, a cycloalkyl group, an aryl group, a hydroxyl group or its ester, an alkoxy group, an aryloxy group, a carboxy group or its ester, an alkylcarbonyl group, an arylcarbonyl group, an amino group, an alkylamino group, an arylamino group, a mercapto group, an alkylthio group, an arylthio group, a cyano group, a nitro group or a heterocycle, and further the alkyl group, the cycloalkyl group, the aryl group, the alkoxy group, the aryloxy group, the alkylcarbonyl group, the arylcarbonyl group, the alkylamino group, the arylamino group, the alkylthio group, the arylthio group or the heterocycle can be substituted with an aryl group, a hydroxyl group or its ester, an alkoxy group, an aryloxy group, an alkoxyalkoxy group, a carboxy group or its ester;

R 4 and R 5 can be bound to form a heterocycle, the heterocycle can be substituted with a halogen atom, an alkyl group, a cycloalkyl group, an aryl group, a hydroxyl group or its ester, an alkoxy group, an aryloxy group, a carboxy group or its ester, and further the alkyl group, the cycloalkyl group, the aryl group, the alkoxy group or the aryloxy group can be substituted with an aryl group, a hydroxyl group or its ester, an alkoxy group, an aryloxy group, an alkoxyalkoxy group, a carboxy group or its ester; and

A 1 represents an alkylene group.

2. The method as claimed in claim 1 , wherein the κ-opioid receptor agonist is selected from the group consisting of

(+)-3-acetyl-6-chloro-2-[2-(3-(N-(2-hydroxyethyl)-N-isopropylamino)propoxy)-5-methoxyphenyl]benzothiazoline,

(+)-3-acetyl-6-chloro-2-[2-(3-(N-(2-methoxyethyl)-N-isopropylamino)propoxy)-5-methoxyphenyl]benzothiazoline,

(+)-3-acetyl-6-chloro-2-[2-(3-(N-(2-ethoxyethyl)-N-isopropylamino)propoxy)-5-methoxyphenyl]benzothiazoline,

(+)-3-acetyl-6-chloro-2-[2-(3-chloropropoxy)-5-methoxyphenyl]benzothiazoline,

3-acetyl-6-chloro-2-[2-(3-(N-(2-hydroxyethyl)-N-isopropylamino)-1-methylpropoxy)-5-methoxyphenyl]benzothiazoline,

(+)-2-[2-(3-(N-(2-acetoxyethyl)-N-isopropylamino)propoxy)-5-methoxyphenyl]-3-acetyl-6-chlorobenzothiazoline,

(+)-3-acetyl-6-chloro-2-[2-(3-(N-isopropyl-(N-methoxymethyloxyethyl)amino)propoxy)-5-methoxyphenyl]benzothiazoline and

3-acetyl-6-chloro-2-[2-(3-(N-(2-ethoxyethyl)-N-isopropylamino)propoxy)-5-methoxyphenyl]benzothiazoline, or a pharmaceutically acceptable salt thereof.

3. The method as claimed in claim 1 , wherein the κ-opioid receptor agonist is continuously administered.

4. The method as claimed in claim 1 , wherein the κ-opioid receptor agonist is

(+)-3-acetyl-6-chloro-2-[2-(3-(N-(2-hydroxyethyl)-N-isopropylamino)propoxy)-5-methoxyphenyl]benzothiazoline.

5. The method as claimed in claim 1 , wherein the κ-opioid receptor agonist is

(+)-3-acetyl-6-chloro-2-[2-(3-(N-(2-methoxyethyl)-N-isopropylamino)propoxy)-5-methoxyphenyl]benzothiazoline.

6. The method as claimed in claim 1 , wherein the κ-opioid receptor agonist is

(+)-3-acetyl-6-chloro-2-[2-(3-(N-(2-ethoxyethyl)-N-isopropylamino)propoxy)-5-methoxyphenyl]benzothiazoline.

7. The method as claimed in claim 1 , wherein the κ-opioid receptor agonist is

3-acetyl-6-chloro-2-[2-(3-(N-(2-ethoxyethyl)-N-isopropylamino)propoxy)-5-methoxyphenyl]benzothiazoline.