A compound of formula or a pharmaceutically acceptable salt thereof: R 1 is aryl; R 2 is selected from C 1 -C 24 -alkyl, C 3 -C 24 -alkenyl, C3-C24-alkynyl, C 0 -C 4 -alkylene-C 3 -C 7 -cycloalkyl or C 0 -C 4 -alkylene-aryl; R 3 and R 4 are each independently selected from H, C 1 -C 6 -alkyl and C 1 -C 3 -alkylene-R 9; or wherein R 3 and R 4 together with the atom to which they are attached form a 3- to 6-membered cycloalkyl or heterocycloalkyl group; R 5 and R 7 are each independently selected from H and C 1 -C 4 -alkyl; R 6 is independently selected from H and C R 10; R 8 is independently selected from H, C OR 10 and C R 10; R 9 is independently selected from aryl (e.g. phenyl), 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 10 is independently at each occurrence selected from C 1 -C 24 -alkyl, C 3 -C 24 -alkenyl, C3-C24-alkynyl, C 0 -C 4 -alkylene-C 3 -C 7 -cycloalkyl or C 0 -C 4 -alkylene-aryl; wherein any aryl group is phenyl, naphthyl or tetrahydronaphthyl and wherein any phenyl, alkyl, alkyne, alkene, alkylene, cycloalkyl, naphthyl or tetrahydronaphthyl 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, C -C 1 -C 4 -alkyl, S 2 -C 1 -C 4 -alkyl. R 1 is aryl; R 2 is selected from C 1 -C 24 -alkyl, C 3 -C 24 -alkenyl, C3-C24-alkynyl, C 0 -C 4 -alkylene-C 3 -C 7 -cycloalkyl or C 0 -C 4 -alkylene-aryl; R 3 and R 4 are each independently selected from H, C 1 -C 6 -alkyl and C 1 -C 3 -alkylene-R 9; or wherein R 3 and R 4 together with the atom to which they are attached form a 3- to 6-membered cycloalkyl or heterocycloalkyl group; R 5 and R 7 are each independently selected from H and C 1 -C 4 -alkyl; R 6 is independently selected from H and C R 10; R 8 is independently selected from H, C OR 10 and C R 10; R 9 is independently selected from aryl (e.g. phenyl), 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 10 is independently at each occurrence selected from C 1 -C 24 -alkyl, C 3 -C 24 -alkenyl, C3-C24-alkynyl, C 0 -C 4 -alkylene-C 3 -C 7 -cycloalkyl or C 0 -C 4 -alkylene-aryl; wherein any aryl group is phenyl, naphthyl or tetrahydronaphthyl and wherein any phenyl, alkyl, alkyne, alkene, alkylene, cycloalkyl, naphthyl or tetrahydronaphthyl 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, C -C 1 -C 4 -alkyl, S 2 -C 1 -C 4 -alkyl.