Compounds of formula (I), wherein A represents N or CH; R.sup.1 and R.sup.2 independently represent C.sup.1-4 alkyl, halo or CF.sub.3 ; R.sup.3 represents C.sub.1-4 alkyl (optionally substituted), C.sub.3-7 cycloalkyl, CF.sub.3 or aryl; R.sup.4 represents H, C.sub.3-7 cycloalkyl or C.sub.1-6 alkyl (optionally substituted); and their pharmaceutically acceptable derivatives; are useful in the treatment of, inter alia, neurodogenerative disorders. ##STR1##