A compound, which is a compound of formula or a pharmacologically acceptable salt thereof: wherein R 1 -R 4 are each independently selected from (i) H, halogen, hydroxy, cyano, di-(C 1-6 -alkyl) amino, mono- and di-(C 1-6 -alkyl)aminocarbonyl, and (ii) C 1-6 -alkyl, C 3-7 -cycloalkyl, mono- or polycyclic C 6-10 -aryl, mono- or polycyclic C 7-11 -aralkyl, mono- or bicyclic 4- to 10-membered heterocyclyl containing 1-4 heteroatoms selected from O, N and S, C 3-7 -cycloalkylamino, C 1-6 -alkyloxy, C 3-7 -cycloalkyloxy, mono- or polycyclic C 6 - aryloxy, mono- or polycyclic C 7-11 -aralkyloxy, mono- or bicyclic 4- to 10-membered heterocyclyloxy containing 1-4 heteroatoms selected from O, N and S, C 1-6 -alkylthio, C 1-6 -alkylsulfonyl, C 1-6 -alkylcarbonyl, and hydroxyaminocarbonyl; each optionally substituted with a substituent selected from halogen, cyano, hydroxy, C 1-6 -alkyl, C 3-6 -cycloalkyl, hydroxy-C 1-6 -alkyl, C 1-6 -alkyloxy, aminocarbonyl, oxo, nitro, C 1-6 -alkylthio, C 1-6 -alkylsulfonyl and C 6-10 -aryl; and R 2 -R 4 may further be carboxy, or C 1-6 -alkyloxycarbonyl optionally substituted with a substituent as defined above; and R 5 and R 6 each independently are H or C 1-6 -alkyl, provided that (i) R 1 -R 4 all being H is excluded and (ii) R 5 and R 6 are both H is excluded. R 1 -R 4 are each independently selected from (i) H, halogen, hydroxy, cyano, di-(C 1-6 -alkyl) amino, mono- and di-(C 1-6 -alkyl)aminocarbonyl, and (ii) C 1-6 -alkyl, C 3-7 -cycloalkyl, mono- or polycyclic C 6-10 -aryl, mono- or polycyclic C 7-11 -aralkyl, mono- or bicyclic 4- to 10-membered heterocyclyl containing 1-4 heteroatoms selected from O, N and S, C 3-7 -cycloalkylamino, C 1-6 -alkyloxy, C 3-7 -cycloalkyloxy, mono- or polycyclic C 6 - aryloxy, mono- or polycyclic C 7-11 -aralkyloxy, mono- or bicyclic 4- to 10-membered heterocyclyloxy containing 1-4 heteroatoms selected from O, N and S, C 1-6 -alkylthio, C 1-6 -alkylsulfonyl, C 1-6 -alkylcarbonyl, and hydroxyaminocarbonyl; each optionally substituted with a substituent selected from halogen, cyano, hydroxy, C 1-6 -alkyl, C 3-6 -cycloalkyl, hydroxy-C 1-6 -alkyl, C 1-6 -alkyloxy, aminocarbonyl, oxo, nitro, C 1-6 -alkylthio, C 1-6 -alkylsulfonyl and C 6-10 -aryl; and R 2 -R 4 may further be carboxy, or C 1-6 -alkyloxycarbonyl optionally substituted with a substituent as defined above; and R 5 and R 6 each independently are H or C 1-6 -alkyl, provided that (i) R 1 -R 4 all being H is excluded and (ii) R 5 and R 6 are both H is excluded. (i) H, halogen, hydroxy, cyano, di-(C 1-6 -alkyl) amino, mono- and di-(C 1-6 -alkyl)aminocarbonyl, and (ii) C 1-6 -alkyl, C 3-7 -cycloalkyl, mono- or polycyclic C 6-10 -aryl, mono- or polycyclic C 7-11 -aralkyl, mono- or bicyclic 4- to 10-membered heterocyclyl containing 1-4 heteroatoms selected from O, N and S, C 3-7 -cycloalkylamino, C 1-6 -alkyloxy, C 3-7 -cycloalkyloxy, mono- or polycyclic C 6 - aryloxy, mono- or polycyclic C 7-11 -aralkyloxy, mono- or bicyclic 4- to 10-membered heterocyclyloxy containing 1-4 heteroatoms selected from O, N and S, C 1-6 -alkylthio, C 1-6 -alkylsulfonyl, C 1-6 -alkylcarbonyl, and hydroxyaminocarbonyl; each optionally substituted with a substituent selected from halogen, cyano, hydroxy, C 1-6 -alkyl, C 3-6 -cycloalkyl, hydroxy-C 1-6 -alkyl, C 1-6 -alkyloxy, aminocarbonyl, oxo, nitro, C 1-6 -alkylthio, C 1-6 -alkylsulfonyl and C 6-10 -aryl; and R 2 -R 4 may further be carboxy, or C 1-6 -alkyloxycarbonyl optionally substituted with a substituent as defined above; and R 5 and R 6 each independently are H or C 1-6 -alkyl,