A compound of formula (I); or a salt or solvate thereof, wherein: the dotted line indicates the optional presence of a double bond between C2 and C3; R2 is selected from —H, —OH, =0, ═CH2, —CN, —R, OR, halo, ═CH—R, O—SO2—R, CO2R and COR; R7 is 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 C1-7 alkyl, C3-20 heterocyclyl and C5-20 aryl groups; R10 and R11 either together form a double bond, or are selected from H and YRY, where Y is selected from O, S and NH and R is H or C1-7 alkyl or H and SOxM, where x is 2 or 3 and M is a monovalent pharmaceutically acceptable cation; each X is independently a heteroarylene group; n is from 1 to 6; and RE is C1-4 alkyl. The compound is useful for the treatment of proliferative diseases.