The invention relates to a pyrimidone derivative represented by formula
(I) or a salt thereof wherein: (I) X represents two hydrogen atoms, a
sulphur atom, an oxygen atom or a C.sub.1-2 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 C.sub.1-6
alkyl group, a hydroxy group, a C.sub.1-4 alkoxy group or a halogen atom;
R4 represents a hydrogen atom, a C.sub.1-6 alkyl group, a C.sub.1-4
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 GSK3.beta. or
GSK3.beta. and cdk5/p25, such as Alzheimer disease ##STR00001##