A conjugate according to claim 2, wherein the moiety A--O-- represents ##STR26## in which R10 is a hydrogen atom or a hydroxy or methoxy group, one of R11 and R12 is a hydrogen atom and the other is a hydroxy group or R11 is a hydrogen or iodine atom and R12 is a hydrogen atom, and R13 is an amino group or represents a nitrogen atom enclosed in a morpholino ring.