A N-oxide pyrazine derivative represented by formula (I) or a salt thereof, or a solvate thereof or a hydrate thereof: wherein: R1 represents a 4-pyridine ring, a halogen atom R2 represents a hydrogen atom R3 represents a hydrogen atom R4 represents: a phenylC 1-3 alkyl group, this group being optionally substituted R5 represents a hydrogen atom in the form of a free base or of an addition salt with an acid. Use in Therapy