Amino substituted pyrazine derivatives for the treatment of pain

1. A compound of formula (I):

or a pharmaceutically acceptable salt thereof;

wherein R 1 is a 5-membered heteroaryl group comprising either (a) from 1 to 4 nitrogen atoms or (b) one oxygen or one sulphur atom and 0, 1 or 2 nitrogen atoms, optionally each independently substituted by one or more substituents selected from (C 1 -C 4 )alkyl, (C 1 -C 4 )alkoxy, halo(C 1 -C 4 alkyl, (C 1 -C 4 )alkoxy(C 1 -C 4 alkyl, amino(C 1 -C 4 )alkyl, amino, (C 1 -C 4 )alkylamino, di-((C 1 -C 4 )alkyl)amino, (C 1 -C 4 alkylamino(C 1 -C 4 )alkyl and di-((C 1 -C 4 alkyl)amino(C 1 -C 4 alkyl; with the proviso that R 1 is not imidazolyl, oxazolyl or 1,2,4-triazolyl; Ar is

and → indicates the point of attachment to the pyrazine ring; and

each R 2 is independently selected from hydrogen, (C 1 -C 4 alkyl, (C 1 -C 4 alkoxy, halo(C 1 -C 4 )alkyl, halo(C 1 -C 4 alkoxy, cyano and halo.

2. A compound of formula (I), or a pharmaceutically acceptable salt thereof, according to claim 1 , wherein Ar is

3. A compound of formula (I), or a pharmaceutically acceptable salt thereof, according to claim 1 , wherein each R 2 is independently selected from hydrogen, methoxy, ethoxy, cyano, methyl, ethyl, trifluoromethyl, trifluomethoxy, chloro and fluoro.

4. A compound of formula (I), or a pharmaceutically acceptable salt thereof, according to claim 1 , wherein each R 2 is independently selected from hydrogen, methoxy, cyano, trifluoromethyl, chloro and fluoro.

5. A compound of formula (I), or a pharmaceutically acceptable salt thereof, according to claim 1 , wherein Ar is 2-chlorophenyl, 2,3-dichlorophenyl, 2,5-dichlorophenyl, 2,5-dichloro-3-methoxyphenyl, 2,3,5-trichlorophenyl, 2-chloro-5-methoxyphenyl, 2,3-dichloro-5-methoxyphenyl or 2-chloro-5-cyanophenyl.

6. A compound of formula (I), or a pharmaceutically acceptable salt thereof, according to claim 1 , wherein R 1 is pyrazolyl or isoxazolyl, each being optionally substituted with (C 1 -C 4 alkyl or (C 1 -C 4 alkoxy(C 1 -C 4 alkyl.

7. A compound of formula (I), or a pharmaceutically acceptable salt thereof, according to claim 1 , wherein R 1 is pyrazolyl or isoxazolyl, each being substituted with one, two or three substituents independently selected from methyl, ethyl, and isopropyl.

8. A compound of formula (I), or a pharmaceutically acceptable salt thereof, according to claim 1 , wherein R 1 is 3-methylisoxazol-4-yl, 1-methyl-1H-pyrazol-5-yl, 5-isopropylisoxazol-4-yl, 5-methylisoxazol-4-yl or 3-ethyl-5-methyl-isoxazol-4-yl.

9. A compound of formula (I) according to claim 1 , selected from:

N-[6-amino-5-(2,3-dichlorophenyl)pyrazin-2-yl]-3-methylisoxazole-4-carboxamide;

N-[6-amino-5-(2,5-dichlorophenyl)pyrazin-2-yl]-3-methylisoxazole-4-carboxamide;

N-[6-amino-5-(2,5-dichloro-3-methoxyphenyl)pyrazin-2-yl]-3-methylisoxazole-4-carboxamide;

N-[6-amino-5-(2,3-dichloro-5-methoxyphenyl)pyrazin-2-yl]-1-methyl-1H-pyrazole-5-carboxamide;

N-[6-amino-5-(2-chlorophenyl)pyrazin-2-yl]-5-isopropylisoxazole-4-carboxamide;

N-[6-amino-5-(2-chlorophenyl)pyrazin-2-yl]-3-methylisoxazole-4-carboxamide;

N-[6-amino-5-(2,3,5-trichlorophenyl)pyrazin-2-yl]-5-methylisoxazole-4-carboxamide;

N-[6-amino-5-(2-chloro-5-methoxyphenyl)pyrazin-2-yl]-3-methylisoxazole-4-carboxamide;

N-[6-amino-5-(2-chloro-5-cyanophenyl)pyrazin-2-yl]-1-methyl-1H-pyrazole-5-carboxamide;

N-[6-amino-5-(2-chloro-5-methoxyphenyl)pyrazin-2-yl]-3-ethyl-5-methylisoxazole-4-carboxamide;

N-[6-amino-5-(2-chloro-5-methoxyphenyl)pyrazin-2-yl]-1-methyl-1H-pyrazole-5-carboxamide;

N-[6-amino-5-(2,5-dichlorophenyl)pyrazin-2-yl]-1-methyl-1H-pyrazole-5-carboxamide; and

N-[6-amino—5-(2-chloro-5-methoxyphenyl)pyrazin-2-yl]-5-isopropylisoxazole-4-carboxamide;

or a pharmaceutically acceptable salt.

10. A pharmaceutical composition including a compound of the formula (I) or a pharmaceutically acceptable salt thereof, according to claim 1 , together with one or more pharmaceutically acceptable excipients.