The invention relates to a pyrimidone derivative represented by formula (I) or a salt thereof wherein: (1) X represents two hydrogen atoms, a sulphur atom, an oxygen atom or a C1-2 alkyl group and a hydrogen atom; Y represents a bond, an ethenylene group, an ethynylene group or a methylene group optionally substituted; R1 represents a 2, 3 or 4-pyridine ring or a 2, 4 or 5-pyrimidine ring optionally substituted; R2 represents a heterocyclic bicyclic rings, having 1-4 heteroatoms selected from an oxygen atom, a sulfur atom and a nitrogen atom and having 5-9 carbon atoms, of formula (II) R3 represents a hydrogen atom, a C1-6 alkyl group, a hydroxy group, a C1-4 alkoxy group or a halogen atom; R4 represents a hydrogen atom, a C1-6 alkyl group, a C1-4 alkoxy group or a halogen atom. The invention relates also to a medicament comprising the said derivative or a salt thereof as an active ingredient which is used for preventive and/or therapeutic treatment of a neurodegenerative disease caused by abnormal activity of GSK3β or GSK3β and cdk5/p25, such as Alzheimer disease.