Glycine transporter-inhibiting substances

A compound of formula [I] or a pharmaceutically acceptable salt thereof:

• wherein
• R1 and R1′ are the same or different, and each represent a hydrogen atom, a halogen atom, a C1-6 alkoxy group, a haloC1-6 alkyl group, a cyano group, a heteroaryl group (which may be substituted by a C1-6 alkyl group), a C1-6 alkyl group, a C3-6 cycloalkyl group, a C1-6 alkylamino group, or the formula CONR7R8 (R7 and R8 are the same or different, and each represent a hydrogen atom or a C1-6 alkyl group),
• R2 represents a hydrogen atom or a C1-6 alkyl group,
• R3 represents a phenyl group (which may be substituted by 1 to 3 substituents selected from halogen atoms, cyano groups, C1-6 alkyl groups, C1-6 alkoxy groups, C1-6 alkylamino groups, C1-6 alkylsulfonyl groups, haloC1-6 alkyl groups, haloC1-6 alkoxy groups, haloC1-6 alkylsulfanyl groups, phenyl groups, phenoxy groups, heteroaryl groups (which may be substituted by a C1-6 alkyl group), and the formula —SO2NR9R10 (R9 and R10 are the same or different, and each represent a hydrogen atom or a C1-6 alkyl group)) or a heteroaryl group or a bicyclic heteroaryl group (the each heteroaryl group may be substituted by 1 to 3 substituents selected from halogen atoms, C1-6 alkyl groups, C3-6 cycloalkyl groups, C1-6 alkoxy groups, cyano groups, C1-6 alkanoyl groups, and haloC1-6 alkyl groups),
• R4 represents a C1-6 alkyl group (which may be substituted by 1 to 3 halogen atoms, a C1-6 alkoxy group, a C3-6 cycloalkyl group, or a phenyl group), a C3-6 cycloalkyl group, or a phenyl group, R5 and R6 are the same or different, and each represent a hydrogen atom or a C1-6 alkyl group, and
• A1, A2, A3, and A4 are the same or different, and each represent the formula CH or a nitrogen atom, provided that one or two of A1, A2, A3, and A4 represent a nitrogen atom.