Abstract: A pyrrolobenzodiazepine dimer compound of Formula (I): or pharmaceutically acceptable salt or solvate thereof is useful as a therapeutic agent for the treatment of leukaemias, especially B-cell leukaemias, that exhibit resistance to other chemotherapeutic drugs, wherein: the dotted lines indicate the optional presence of a double bond between C1 and C2 or C2 and C3; R2 and R3 are independently selected from —H, ?O, ?CH2, —CN, —R, OR, halo, ?CH—R, O—SO2—R, CO2R and COR; R6, R7 and R9 are independently selected from II, R, OII, OR, SII, SR, NII2, NIIR, 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; R10 is a carbamate-based nitrogen protecting group and R15 is either O—R11, wherein R is an oxygen protecting group, or OH, or R10 and R15 together form a double bond between N10 and C11; R? is a C3-12 alkylene group, which chain may be interrupted by one or more heteroatoms and/or aromatic rings, and each X is