Compounds of the formula: (I) or solvate thereof, wherein: R2 is an optionally substituted C5-20 aryl group; R6 and R9 are independently selected from H, R, OH, OR, SH, SR, NH2, NHR, NRR′, nitro, Me3Sn and halo; where R and R′ are independently selected from optionally substituted C1-12 alkyl, C3-20 heterocyclyl and C5-20 aryl groups; R7 is selected from H, R, OH, OR, SH, SR, NH2, NHR, NHRR′, nitro, Me3Sn and halo; R″ is a C3-12 alkylene group, which chain may be interrupted by one or more heteroatoms and/or aromatic rings; X is selected from O, S, or NH; z is 2 or 3; M is a monovalent pharmaceutically acceptable cation; R2′, R6′, R7′, R9′, X′ and M′ are selected from the same groups as R2, R6, R7, R9, X and M respectively, or M and M′ may together represent a divalent pharmaceutically acceptable cation.