Pyrrolobenzodiazepines

Compounds of the formula I: or solvate thereof, wherein: R 2 is an optionally substituted C 5-20 aryl group; R 6 and R 9 are independently selected from H, R, OH, OR, SH, SR, NH 2 , NHR, NRR′, nitro, Me 3 Sn and halo; where R and R′ are independently selected from optionally substituted C 1-12 alkyl, C 3-20 heterocyclyl and C 5-20 aryl groups; R 7 is selected from H, R, OH, OR, SH, SR, NH 2 , NHR, NHRR′, nitro, Me 3 Sn and halo; R″ is a C 3-12 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.