Compounds of the formula: (I) or solvate thereof, wherein: R<2> is an optionally substituted C5-20
aryl group; R<6> and R<9> 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; R<7> 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; R<2>', R<6>', R<7>', R<9>', X' and M' are selected from the same groups as R<2>, R<6>, R<7>, R<9>, X and M respectively, or M and M' may together represent a
divalent pharmaceutically acceptable cation.