This invention is directed to prodrugs that can be activated at the preferred site of action in order to selectively deliver the corresponding therapeutic parent drugs to target cells or to the target site. This invention will therefore primarily but not exclusively relate to tumor cells as target cells. More specifically the prodrugs are compounds of the formula V-(W)k-(X)l-A-Z, wherein: V is a specifier W and X are each a 1, (4+2n) electronic cascade spacer, being the same or different A is either a spacer group of formula (Y)m, wherein: Y is a 1, (4+2n) electronic cascade spacer, or a group of formula U being a cyclisation elimination spacer Z is a therapeutic drug k, l and m are integers from 0 to 5 n is an integer of 0 to 10, with the provisos that: when A is (Y)m: k+l+m > 1, and if k+l+m=1, then n > 1 when A is U: k+l > 1.
