The present invention relates to a compound of formula (1), or a pharmaceutically acceptable salt thereof, wherein: Ar is a substituted aryl or heteroaryl group bearing at least one nitro or azido group or is a group of formula (2) or (3) wherein R1 and R2, which may be the same or different are independently optionally substituted alkyl, optionally substituted alkenyl, optionally substituted alkynyl, aryl, COR3 or, together with the intervening carbon atom, form an optionally substituted heterocycloalkyl or carbocyclic ring; L is -OC(O)- or -OP(0)(OR6)-; n isOorl; X is O, S, NR7 or a single covalent bond; R3 is OR4 or NR4R5; R4, R5, R6 and R7 are each independently hydrogen or optionally substituted alkyl or, where R3 is NR4R5 and R5 can be joined to form, together with the intervening nitrogen atom, a heterocycloalkyl ring; R8 is hydrogen, alkoxy or dialkylaminoalkyl; R9 is optionally substituted alkyl; Rio is hydrogen, alkyl, alkoxy or dialkylaminoalkyl; Rn and R12 are independently hydrogen, alkyl, alkoxy, thioalkoxy, amino, alkylamino, dialkylamino, morpholino, piperidino,piperazino or 1-aziridinyl; A is an optionally substituted aryl or heteroaryl ring; and Dr is a moiety such that DrXH represents a cytotoxic or cytostatic compound.