The present invention provides a compound of the formula (I): (wherein, R.sup.1 is ethyl optionally substituted by halogen or amino optionally substituted by lower alkyl, R.sup.2 and R.sup.3 are each independently hydrogen or lower alkyl, X is cycloalkylene, lower alkylene or piperidinediyl, Z is optionally substituted phenyl or optionally substituted heterocyclyl) prodrug, pharmaceutically acceptable salt or solvate thereof.