A pyrimidone derivative represented by formula (I) or a salt thereof, or a solvate thereof or a hydrate thereof: wherein: Y represents two hydrogen atoms, a sulphur atom, an oxygen atom or a C 1-2 alkyl group and a hydrogen atom； Z represents a bond, an oxygen atom, a nitrogen atom substituted by a hydrogen atom or a C 1-3 alkyl group, a sulphur atom, a methylene group optionally substituted by one or two groups chosen from a C 1-6 alkyl group, a hydroxyl group, a C 1-6 alkoxy group, a C 1-2 perhalogenated alkyl group or an amino group； R1 represents a 2, 3 or 4-pyridine ring or a 2, 4 or 5-pyrimidine ring, the ring being optionally substituted by a C 1-6 alkyl group, a C 1-6 alkoxy group or a halogen atom； R2 represents a hydrogen atom, a C 1-6 alkyl group or a halogen atom； R3 represents a 4-15 membered heterocyclic group, this group being optionally substituted； R4 represents a hydrogen atom or a C 1-6 alkyl group； and n represents 0 to 3 and their therapeutic use.
