Prodrug of amino acid derivative

A compound represented by formula (I-A):

• or pharmaceutically acceptable salt thereof,
• wherein
• R1 represents a C1-6 alkyl group, a heteroaryl group optionally substituted by one halogen atom, or the following formula (IIIA):
• where Rx represents a hydrogen atom, a halogen atom, a C1-6 alkyl group, or a C1-6 alkoxy group, wherein the C1-6 alkyl group and the C1-6 alkoxy group are each optionally substituted by one to three halogen atoms, and
• Ry represents a hydrogen atom, a fluorine atom, a C1-6 alkyl group, or a C1-6 alkoxy group, wherein the C1-6 alkyl group and the C1-6 alkoxy group are each optionally substituted by one to three halogen atoms,
• R1′ represents a hydrogen atom or a C1-6 alkyl group,
• or R1 and R1′ optionally form a C3-6 cycloalkane together with the carbon atom adjacent thereto,
• R2 represents a C3-6 alkyl group, a C3-8 cycloalkyl group optionally substituted by one to three C1-6 alkyl groups, a C3-8 cycloalkoxy group optionally substituted by one to three C1-6 alkyl groups and optionally having a C1-5 alkylene group crosslinking two different carbon atoms in the ring, an adamantyl group optionally substituted by one to three C1-6 alkyl groups, or a phenyl group,
• R3 represents a hydrogen atom or a C1-6 alkyl group, and
• R4 represents a hydrogen atom or a fluorine atom.