A compound of formula I and salts and solvates thereof, wherein: 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, NRR′, nitro, Me3Sn and halo; R″ is a C3-12 alkylene group, which chain may be interrupted by one or more heteroatoms, e.g. O, S, NRN2 (where RN2 is H or C1-4 alkyl), and/or aromatic rings, e.g. benzene or pyridine; Y and Y′ are selected from O, S, or NH; R6, R7, R9 are selected from the same groups as R6, R7 and R9 respectively; R11b is selected from OH, ORA, where RA is C1-4 alkyl; and RL is a linker for connection to a cell binding agent.