Pyrrolobenzodiazepines

Compounds of the formulae Ia and Ib: wherein: A is CH 2 , or a single bond; R 2 is selected from: R, OH, OR, CO 2 H, CO 2 R, COH, COR, SO 2 R, CN; R 6 , R 7 and R 9 are independently selected from H, R, OH, OR, halo, amino, NHR, nitro, Me 3 Sn; and R 8 is selected from H, R, OH, OR, halo, amino, NHR, nitro, Me 3 Sn, where R is as defined above, or the compound is a dimer with each monomer being the same or different and being of formula Ia or Ib, where the R 8 groups of the monomers form together a bridge having the formula —X—R′—X— linking the monomers, where R′ is an alkylene chain containing from 3 to 12 carbon atoms, which chain may be interrupted by one or more hetero-atoms and/or aromatic rings and may contain one or more carbon-carbon double or triple bonds, and each X is independently selected from O, S, or N; except that in a compound of formula Ia when A is a single bond, then R 2 is not CH═CH(CONH 2 ) or CH═CH(CONMe 2 ). Other related compounds are also disclosed.