A compound of formula, or a pharmaceutically acceptable salt thereof: wherein R 1 is independently at each occurrence selected from: C 1 -C 24 -alkyl, C 3 -C 24 -alkenyl, C 3 -C 24 -alkynyl, C 0 -C 4 -alkylene-C 3 -C 8 -cycloalkyl and C 0 -C 4 -alkylene-aryl; R 2 and R 3 are each independently at each occurrence selected from H, C 1 -C 6 -alkyl and C 1 -C 3 -alkylene-R 7; or R 2 and R 3 together with the atom to which they are attached form a 3- to 6-membered cycloalkyl or heterocycloalkyl group; R 4 is independently at each occurrence H or C 1 -C 4 -alkyl; or R 4, a group selected from R 2 and R 3 and the atoms to which they are attached may form a 3- to 6-membered heterocycloalkyl group; R 5 is independently at each occurrence selected from aryl, 5-, 6-, 9- or 10-membered heteroaryl, C 3 -C 8 -cycloalkyl, 3- to 7-membered heterocycloalkyl, C 1 -C 3 -alkylene-R 5a and C 1 -C 8 -alkyl, said aryl being optionally fused to C 6 -C 8 -cycloalkyl; R 5a is independently at each occurrence selected from aryl, 5-, 6-, 9- or 10-membered heteroaryl, C 3 -C 8 -cycloalkyl, 3- to 7-membered heterocycloalkyl, said aryl being optionally fused to C 6 -C 8 -cycloalkyl; R 6 is independently selected from: and R 7 is independently at each occurrence selected from aryl, imidazole, indole, SR a, OR a, CO 2 R a, CO 2 NR a R a, NR a R b and NH(=NH)NH 2; R 8 is independently selected from H and Z 1 and Z 2 are each independently selected from O and S; R 9 is independently selected from H and Me; Y is independently selected from H, F, Cl and OMe; X is at each occurrence selected from H and a pharmaceutically acceptable cation; provided that in at least one occurrence X is a pharmaceutically acceptable cation; wherein any aryl group is either phenyl or naphthyl; wherein where any of R 1, R 2, R 3, R 4, R 5 or R 7 is an alkyl, cycloalkyl, heterocycloalkyl, aryl or heteroaryl, that alkyl, cycloalkyl, heterocycloalkyl, aryl or heteroaryl group is optionally substituted with from 1 to 4 substituents selected from: halo, nitro, cyano, NR a R a, NR a S 2 R a, NR a C R a, NR a CONR a R a, NR a CO 2 R a, OR a; SR a, SOR a, SO 3 R a, SO 2 R a, SO 2 NR a R a, CO 2 R a C R a, CONR a R a, CR a R a NR a R a, C 1 -C 4 -alkyl, C 2 -C 4 -alkenyl, C 2 -C 4 -alkynyl and C 1 -C 4 -haloalkyl; wherein R a is independently at each occurrence selected from: H and C 1 -C 4 -alkyl; and R b is independently at each occurrence selected from: H, and C 1 -C 4 -alkyl and C -C 1 -C 4 -alkyl. R 1 is independently at each occurrence selected from: C 1 -C 24 -alkyl, C 3 -C 24 -alkenyl, C 3 -C 24 -alkynyl, C 0 -C 4 -alkylene-C 3 -C 8 -cycloalkyl and C 0 -C 4 -alkylene-aryl; R 2 and R 3 are each independently at each occurrence selected from H, C 1 -C 6 -alkyl and C 1 -C 3 -alkylene-R 7; or R 2 and R 3 together with the atom to which they are attached form a 3- to 6-membered cycloalkyl or heterocycloalkyl group; R 4 is independently at each occurrence H or C 1 -C 4 -alkyl; or R 4, a group selected from R 2 and R 3 and the atoms to which they are attached may form a 3- to 6-membered heterocycloalkyl group; R 5 is independently at each occurrence selected from aryl, 5-, 6-, 9- or 10-membered heteroaryl, C 3 -C 8 -cycloalkyl, 3- to 7-membered heterocycloalkyl, C 1 -C 3 -alkylene-R 5a and C 1 -C 8 -alkyl, said aryl being optionally fused to C 6 -C 8 -cycloalkyl; R 5a is independently at each occurrence selected from aryl, 5-, 6-, 9- or 10-membered heteroaryl, C 3 -C 8 -cycloalkyl, 3- to 7-membered heterocycloalkyl, said aryl being optionally fused to C 6 -C 8 -cycloalkyl; R 6 is independently selected from: and R 7 is independently at each occurrence selected from aryl, imidazole, indole, SR a, OR a, CO 2 R a, CO 2 NR a R a, NR a R b and NH(=NH)NH 2; R 8 is independently selected from H and Z 1 and Z 2 are each independently selected from O and S; R 9 is independently selected from H and Me; Y is independently selected from H, F, Cl and OMe; X is at each occurrence selected from H and a pharmaceutically acceptable cation; provided that in at least one occurrence X is a pharmaceutically acceptable cation; wherein any aryl group is either phenyl or naphthyl; wherein where any of R 1, R 2, R 3, R 4, R 5 or R 7 is an alkyl, cycloalkyl, heterocycloalkyl, aryl or heteroaryl, that alkyl, cycloalkyl, heterocycloalkyl, aryl or heteroaryl group is optionally substituted with from 1 to 4 substituents selected from: halo, nitro, cyano, NR a R a, NR a S 2 R a, NR a C R a, NR a CONR a R a, NR a CO 2 R a, OR a; SR a, SOR a, SO 3 R a, SO 2 R a, SO 2 NR a R a, CO 2 R a C R a, CONR a R a, CR a R a NR a R a, C 1 -C 4 -alkyl, C 2 -C 4 -alkenyl, C 2 -C 4 -alkynyl and C 1 -C 4 -haloalkyl; wherein R a is independently at each occurrence selected from: H and C 1 -C 4 -alkyl; and R b is independently at each occurrence selected from: H, and C 1 -C 4 -alkyl and C -C 1 -C 4 -alkyl.