R² is an optionally substituted aryl or heteroaryl group having 5 to 20 ring atoms, the heteroaryl groups having one or more heteroatoms independently selected from the group consisting of N, O and S, wherein the optional substituents are independently selected from C₁-C₁₂ alkyl, heterocyclyl groups having 3 to 20 ring atoms of which 1 to 10 are heteroatoms independently selected from the group consisting of N, O and S, aryl groups having 5 to 20 ring atoms, halo, hydroxy, alkoxy, acetal, hemiacetal, ketal, hemiketal, oxo, thione, imino, formyl, acyl, carboxy, thiocarboxy, thiolcarboxy, imidic acid, hydroxamic acid, ester, acyloxy, oxycaroyloxy, amino, amido, thioamido, acylamido, aminocaronyloxy, ureido, guanidino, tetrazolyl, imino, amidino, nitro, nitroso, azido, cyano, isocyano, cyanato, isocyanato, thiocyano, isothiocyano, sulfhydryl, thioether, disulfide, sulfine, sulfone, sulfinic acid, sulfonic acid, sulfinate, sulfonate, sulfinyloxy, sulfonyloxy, sulfate, sulfamyl, sulfonamido, sulfamino, sulfinamino, phosphino, phospho, phosphinyl, phosphonic acid, phosphonate, phosphoric acid, phosphate, phosphorous acid, phosphite, phosphoramidite, phosphoramidate or bisoxyalkylene;

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 C_1-12 alkyl, heterocyclyl groups having 3 to 20 ring atoms of which 1 to 10 are heteroatoms independently selected from the group consisting of N, O and S and aryl groups having 5 to 20 ring atoms wherein the optional substituents are independently selected from C₁-C₁₂ alkyl, heterocyclyl groups having 3 to 20 ring atoms of which 1 to 10 are heteroatoms independently selected from the group consisting of N, O and S, aryl groups having 5 to 20 ring atoms, halo, hydroxy, alkoxy, acetal, hemiacetal, ketal, hemiketal, oxo, thione, imino, formyl, acyl, carboxy, thiocarboxy, thiolcarboxy, imidic acid, hydroxamic acid, ester, acyloxy, oxycaroyloxy, amino, amido, thioamido, acylamido, aminocaronyloxy, ureido, guanidino, tetrazolyl, imino, amidino, nitro, nitroso, azido, cyano, isocyano, cyanato, isocyanato, thiocyano, isothiocyano, sulfhydryl, thioether, disulfide, sulfine, sulfone, sulfinic acid, sulfonic acid, sulfinate, sulfonate, sulfinyloxy, sulfonyloxy, sulfate, sulfamyl, sulfonamido, sulfamino, sulfinamino, phosphino, phospho, phosphinyl, phosphonic acid, phosphonate, phosphoric acid, phosphate, phosphorous acid, phosphite, phosphoramidite, phosphoramidate or bisoxyalkylene;

R⁷ is selected from H, R, OH, OR, SH, SR, NH₂, NHR, NHRR′, nitro, Me₃Sn and halo;

R″ is a C_3-12 alkylene group, which chain may be interrupted by one or more heteroatoms and/or aromatic rings;

X is selected from O, S, or NH;

z is 2 or 3;

M is a monovalent pharmaceutically acceptable cation;

R^2′, R^6′, R^7′, R^9′, X′ and M′ are selected from the same groups as R², R⁶, R⁷, R⁹, X and M respectively, or M and M′ may together represent a divalent pharmaceutically acceptable cation.

R⁶ is the same as R^6′;

R⁷ is the same as R^7′;

R⁹ is the same as R^9′;

M is the same as M′; and

X is the same as X′.