A method of inhibiting a resistant neoplasm, or a neoplasm susceptible to resistance, in a mammal which comprises administering to a mammal in need thereof an effective amount of a compound of formula I: ##STR20##where: A is ##STR21## ##STR22## or ##STR23## Y is --E--C(O)R.sup.1 or --E--NR.sup.2 R.sup.3 ; E is a bond or --CH.sub.2 --; R.sup.1 is independently at each occurrence hydrogen, C.sub.1 -