A compound represented by the formula (I) :
wherein, for example, R1, R2, R3, and R4 are each independently hydrogen atom, alkyl, and the like, R5 and R6 are each independently hydrogen atom, alkyl, and the like, X is -O-, -S-, and the like, Y is 5- membered heteroaryl-diyl and the like, Z is optionally substituted aryl and the like, the prodrugs thereof, or their pharmaceutically acceptable salts, or their solvates.