The invention provides compounds or salts thereof of the general formula (I): wherein each of R1 and R2 independently represents a C1-6 alkyl or C2-7 acyl group; X represents OCH2 or a group CR3R4; wherein each of R3 or R4 independently represents a hydrogen atom or a C1-3 alkyl group; R5 represents a hydrogen atom or a C1-3 alkyl, C2-3 alkenyl or C2-3 alkynyl group; R6 represents a hydrogen atom or a C1-6 alkyl, C2-6 alkenyl, C2-6 alkynyl, amino, C1-6 alkylamino, di(C1-6) alkylamino or C2-7 acylamino group; each of R7 and R8 independently represents a hydrogen or halogen atom or a hydroxy, trifluoromethyl, C1-6 alkyl, C2-6 alkenyl, C2-6 alkynyl, C2-7 acyl, C1-6 alkylthio, C1-6 alkoxy, C3-6 cycloalkyl; and R9 represents a hydrogen or halogen atom or a hydroxy, trifluoromethyl, C1-6 alkyl, C2-6 alkenyl, C2-6 alkynyl, C2-7 acyl, C1-6 alkylthio, C1-6 alkoxy or C3-6 cycloalkyl group. The compounds or salts thereof are useful for treatment of respiratory disorders such as asthma. Compounds of the invention have a longer duration of action than the known compound trequinsin (9,10-dimethoxy-3 methyl-2-mesitylimino-2,3,6,7-tetrahydro-4-H-pyrimido[6,1-a]isoquinolin-4-one).