A compound of formula (I): R1 is hydrogen or alkyl optionally substituted by hydroxy, alkoxy, or alkylthio, R2 is hydrogen, alkyl, hydroxyalkyl, alkylcarbonyloxyalkyl, alkoxyalkyl, alkylthioalkyl, alkenyl, cycloalkylalkyl, heterocyclylalkyl, aralkyl in which the aryl ring thereof is optionally fused to a 5-membered heterocyclic group or is optionally substituted by one or more substituents selected from alkoxy, amino, alkylamino, dialkylamino, acylamino, halogen, hydroxy, aminosulfonyl, alkylaminosulfonyl, dialkylaminosulfonyl, alkylsulfonylamino or dialkylaminosulfonylamino, R3 is hydrogen or alkyl optionally substituted by hydroxy, alkoxy, or alkylthio, R4 is hydrogen or alkyl, R5 is a quinolinyl, isoquinolinyl or oxodihydroisoquinolinyl group optionally fused to a 5-membered heterocyclic group and optionally substituted by one or more substituents selected from halogen, cyano, hydroxy, alkyl, hydroxyalkyl, alkoxyalkyl, alkylthioalkyl, alkoxy, alkylthio, alkenyl, alkoxycarbonyl, alkynyl, carboxyl, acyl, a group of formula —N(R6)R7, aryl optionally substituted by one or more substituents selected from halogen or alkoxy, or heteroaryl having 5 or 6 ring atoms attached through a ring carbon atom to the indicated carbon atom, and R6 and R7 are each independently hydrogen or alkyl optionally substituted by hydroxy or alkoxy or one of R6 and R7 is hydrogen and the other is acyl, or R6 and R7 together with the nitrogen atom to which they are attached denote a 5- or 6-membered heterocyclyl group.