Cytotoxic agents

A compound of formula I: (see formula) in which A and B are independently N or CH; Y and Z are independently hydroxy, (C1-C6) alkyl (C1-C6) alkyl, alkanoyloxy (C1-C6) alkanoyloxy (C1-C6) alkyl, -OP (= O) (OH) 2 or -OC (= O) NRcRd; or Y and Z together with the ring carbon atoms to which they are attached form an alkylenedioxy ring with from 5 to 7 ring atoms; R1 is (C1-C6); and Rc and Rd are each independently (C1-C6) alkyl or (CI-C4) alkyl; or Rc and Rd together with the nitrogen to which they are attached form a N '- piperazino (C1-C6), pyrrolidino or piperidino, which ring may be optionally substituted with one or more arito, heteroaryl or heterocycle; or a pharmaceutically acceptable salt thereof.