Fused tricyclic compound having aldose reductase inhibitory activity

An aldose reductase inhibitor comprising a fused tricyclic compound shown by the following formula:

• wherein R1 independently represents 1 to 3 atoms or substituents selected from a hydrogen atom, a halogen atom, a substituted or unsubstituted alkyl, cycloalkyl, alkylene, or alkoxy group, and a protected or unprotected hydroxyl or carboxyl group, R2 represents a protected or unprotected carboxyl group, R3 independently represents 1 or 2 atoms or substituents selected from a hydrogen atom, a halogen atom, an oxo group, a substituted or unsubstituted alkyl or alkoxy group, and a protected or unprotected carboxyl group, A represents an alkylene group, B represents an oxygen atom, a sulfur atom, or a group shown by the following formula:
• wherein R4 represents an alkyl or aryl group substituted with an aryl, cycloalkyl, or heterocyclic group, and X represents an oxygen atom or a sulfur atom, provided that, when B represents a group shown by the following formula:
• wherein R4 represents an alkyl or aryl group substituted with an aryl, cycloalkyl, or heterocyclic group, X represents a sulfur atom.

A therapeutic agent comprising a fused tricyclic compound shown by the following formula as a main ingredient:

• wherein R1 independently represents 1 to 3 atoms or substituents selected from a hydrogen atom, a halogen atom, a substituted or unsubstituted alkyl, cycloalkyl, alkylene, or alkoxy group, and a protected or unprotected hydroxyl or carboxyl group, R2 represents a protected or unprotected carboxyl group, R3 independently represents 1 or 2 atoms or substituents selected from a hydrogen atom, a halogen atom, an oxo group, a substituted or unsubstituted alkyl or alkoxy group, and a protected or unprotected carboxyl group, A represents an alkylene group, B represents an oxygen atom, a sulfur atom, or a group shown by the following formula:
• wherein R4 represents an alkyl or aryl group substituted with an aryl, cycloalkyl, or heterocyclic group, and X represents an oxygen atom or a sulfur atom, provided that, when B represents a group shown by the following formula:
• wherein R4 represents an alkyl or aryl group substituted with an aryl, cycloalkyl, or heterocyclic group, X represents a sulfur atom.