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.
