A compound of the formula: ##STR1## wherein R.sup.1 is pyridyl,R.sup.2 is hydrogen, lower alkyl or hydroxy (lower) alkyl,R.sup.3 is hydrogen, hydroxy or lower alkyl, andR.sup.4 is aryl optionally substituted with substituent(s) selected from the group consisting of lower alkylthio, lower alkylsulfinyl, lower alkylsulfonyl, nitro, amino, substituted amino, hydroxy, lower alkoxy, lower alkynyloxy, substituted or unsubstituted ar(lower)alkoxy, halogen, halo(lower)alkyl, carboxy and esterified carboxy,and a pharmaceutically acceptable salt thereof, processes for the preparation thereof and pharmaceutical composition comprising the same.