A compound with the formula I: wherein: R 2 is of formula II: where A is a C 5-7 aryl group, X is selected from the group consisting of: OH, SH, CO 2 H, COH, N=C=O, NHNH 2, CONHNH 2, and NHR N, wherein R N is selected from the group comprising H and C 1-4 alkyl, and either: (i) Q 1 is a single bond, and Q 2 is selected from a single bond and -Z-(CH 2) n -, where Z is selected from a single bond, O, S and NH and is from 1 to 3; or (ii) Q 1 is -CH=CH-, and Q 2 is a single bond; R 12 is selected from: (iia) C 1-5 saturated aliphatic alkyl; (iib) C 3-6 saturated cycloalkyl; (iic) wherein each of R 21, R 22 and R 23 are independently selected from H, C 1-3 saturated alkyl, C 2-3 alkenyl, C 2-3 alkynyl and cyclopropyl, where the total number of carbon atoms in the R 12 group is no more than 5; (iid) wherein one of R 25a and R 25b is H and the other is selected from: phenyl, which phenyl is optionally substituted by a group selected from halo, methyl, methoxy; pyridyl; and thiophenyl;and (iie) where R 24 is selected from H, C 1-3 saturated alkyl, C 2-3 alkenyl, C 2-3 alkynyl; cyclopropyl; phenyl, which phenyl is optionally substituted by a group selected from halo, methyl, methoxy; pyridyl; and thiophenyl; R 6 and R 9 are independently selected from H, R, OH, OR, SH, SR, NH 2, NHR, NRR', nitro, Me 3 Sn and halo; R 7 is selected from H, R, OH, OR, SH, SR, NH 2, NHR, NHRR', nitro, Me 3 Sn and halo; where R and R' are independently selected from optionally substituted C 1-12 alkyl, C 3-20 heterocyclyl and C 5-20 aryl groups; either: (a) R 10 is H, and R 11 is OH, OR A, where R A is C 1-4 alkyl; (b) R 10 and R 11 form a nitrogen-carbon double bond between the nitrogen and carbon atoms to which they are bound; or (c) R 10 is H and R 11 is SO z M, where z is 2 or 3 and M is a monovalent pharmaceutically acceptable cation; R" is a C 3-12 alkylene group, which chain may be interrupted by one or more heteroatoms, and/or aromatic rings; Y and Y' are selected from O, S, or NH; R 6', R 7', R 9' are selected from the same groups as R 6, R 7 and R 9 respectively and R 10' and R 11' are the same as R 10 and R 11, wherein if R 11 and R 11' are SO z M, M may also represent a divalent pharmaceutically acceptable cation. R 2 is of formula II: where A is a C 5-7 aryl group, X is selected from the group consisting of: OH, SH, CO 2 H, COH, N=C=O, NHNH 2, CONHNH 2, and NHR N, wherein R N is selected from the group comprising H and C 1-4 alkyl, and either: (i) Q 1 is a single bond, and Q 2 is selected from a single bond and -Z-(CH 2) n -, where Z is selected from a single bond, O, S and NH and is from 1 to 3; or (ii) Q 1 is -CH=CH-, and Q 2 is a single bond; (i) Q 1 is a single bond, and Q 2 is selected from a single bond and -Z-(CH 2) n -, where Z is selected from a single bond, O, S and NH and is from 1 to 3; or (ii) Q 1 is -CH=CH-, and Q 2 is a single bond; R 12 is selected from: (iia) C 1-5 saturated aliphatic alkyl; (iib) C 3-6 saturated cycloalkyl; (iic) wherein each of R 21, R 22 and R 23 are independently selected from H, C 1-3 saturated alkyl, C 2-3 alkenyl, C 2-3 alkynyl and cyclopropyl, where the total number of carbon atoms in the R 12 group is no more than 5; (iid) wherein one of R 25a and R 25b is H and the other is selected from: phenyl, which phenyl is optionally substituted by a group selected from halo, methyl, methoxy; pyridyl; and thiophenyl;and (iie) where R 24 is selected from H, C 1-3 saturated alkyl, C 2-3 alkenyl, C 2-3 alkynyl; cyclopropyl; phenyl, which phenyl is optionally substituted by a group selected from halo, methyl, methoxy; pyridyl; and thiophenyl; (iia) C 1-5 saturated aliphatic alkyl; (iib) C 3-6 saturated cycloalkyl; (iic) wherein each of R 21, R 22 and R 23 are independently selected from H, C 1-3 saturated alkyl, C 2-3 alkenyl, C 2-3 alkynyl and cyclopropyl, where the total number of carbon atoms in the R 12 group is no more than 5; (iid) wherein one of R 25a and R 25b is H and the other is selected from: phenyl, which phenyl is optionally substituted by a group selected from halo, methyl, methoxy; pyridyl; and thiophenyl;and (iie) where R 24 is selected from H, C 1-3 saturated alkyl, C 2-3 alkenyl, C 2-3 alkynyl; cyclopropyl; phenyl, which phenyl is optionally substituted by a group selected from halo, methyl, methoxy; pyridyl; and thiophenyl; R 6 and R 9 are independently selected from H, R, OH, OR, SH, SR, NH 2, NHR, NRR', nitro, Me 3 Sn and halo; R 7 is selected from H, R, OH, OR, SH, SR, NH 2, NHR, NHRR', nitro, Me 3 Sn and halo; where R and R' are independently selected from optionally substituted C 1-12 alkyl, C 3-20 heterocyclyl and C 5-20 aryl groups; either: (a) R 10 is H, and R 11 is OH, OR A, where R A is C 1-4 alkyl; (b) R 10 and R 11 form a nitrogen-carbon double bond between the nitrogen and carbon atoms to which they are bound; or (c) R 10 is H and R 11 is SO z M, where z is 2 or 3 and M is a monovalent pharmaceutically acceptable cation; (a) R 10 is H, and R 11 is OH, OR A, where R A is C 1-4 alkyl; (b) R 10 and R 11 form a nitrogen-carbon double bond between the nitrogen and carbon atoms to which they are bound; or (c) R 10 is H and R 11 is SO z M, where z is 2 or 3 and M is a monovalent pharmaceutically acceptable cation; R" is a C 3-12 alkylene group, which chain may be interrupted by one or more heteroatoms, and/or aromatic rings; Y and Y' are selected from O, S, or NH; R 6', R 7', R 9' are selected from the same groups as R 6, R 7 and R 9 respectively and R 10' and R 11' are the same as R 10 and R 11, wherein if R 11 and R 11' are SO z M, M may also represent a divalent pharmaceutically acceptable cation.