A compound of the general formula or a salt, solvate or prodrug thereof, wherein A 1 is selected from the group consisting of N and C-R 8; A 2 is selected from the group consisting of N and C-R 3; A 3 is selected from the group consisting of N and C-R 9; A 4 and A 5 and A 6 are independently selected from the group consisting of N and C-R 1; R 1 is selected from the group consisting of hydrogen, (C 1 -C 3)alkyl, halogen, (C 1 -C3)alkoxy and (C 1 -C 3)haloalkyl; R 2 is selected from the group consisting of hydrogen, halogen and (C 1 -C 3)alkyl; R 3 is selected from the group consisting of hydrogen, (C 1 -C 3)alkyl, NR 4 R 5, (C 1 -C 3)alkyl-NR 4 R 5 and cyano; wherein R 4 and R 5 are independently selected from the group consisting of hydrogen, (C 3 -C 5)cycloalkyl, (C 3 -C 5)heterocycloalkyl, (C 1 -C 3)alkyl, or R 4 and R 5 together with the nitrogen atom to which they are attached form a 5- to 7-membered heterocyclic ring optionally comprising in addition to the aforementioned nitrogen atom a further heteroatom group selected from the group consisting of O and NR 6, wherein R 6 is selected from the group consisting of hydrogen, methyl, acetyl and formyl; Y is selected from the group consisting of O and S; R 7 is selected from the group consisting of hydrogen, and (C 1 -C 3)alkyl; R 8 is selected from the group consisting of (C 1 -C 4)alkyl, (C 3 -C 5)cycloalkyl, and (C 3 -C 5)heterocycloalkyl; R 9 is selected from the group consisting of hydrogen, (C 1 -C 3)alkyl, (C 1 -C 3)alkoxy; and wherein said prodrug has either of the following formulae: wherein P is and wherein Q, Q 1 and Q 2 are each independently selected from the group consisting of H and (C 1 -C 3)alkyl and M + is a cationic counterion, wherein alkyl includes alkyl, alkenyl and alkynyl, and wherein (C 3 -C 5)cycloalkyl and (C 3 -C 5)heterocycloalkyl are optionally substituted with one or more (C 1 -C 3)alkoxy and/or (C 1 -C 3)alkyl groups. A 1 is selected from the group consisting of N and C-R 8; A 2 is selected from the group consisting of N and C-R 3; A 3 is selected from the group consisting of N and C-R 9; A 4 and A 5 and A 6 are independently selected from the group consisting of N and C-R 1; R 1 is selected from the group consisting of hydrogen, (C 1 -C 3)alkyl, halogen, (C 1 -C3)alkoxy and (C 1 -C 3)haloalkyl; R 2 is selected from the group consisting of hydrogen, halogen and (C 1 -C 3)alkyl; R 3 is selected from the group consisting of hydrogen, (C 1 -C 3)alkyl, NR 4 R 5, (C 1 -C 3)alkyl-NR 4 R 5 and cyano; wherein R 4 and R 5 are independently selected from the group consisting of hydrogen, (C 3 -C 5)cycloalkyl, (C 3 -C 5)heterocycloalkyl, (C 1 -C 3)alkyl, or R 4 and R 5 together with the nitrogen atom to which they are attached form a 5- to 7-membered heterocyclic ring optionally comprising in addition to the aforementioned nitrogen atom a further heteroatom group selected from the group consisting of O and NR 6, wherein R 6 is selected from the group consisting of hydrogen, methyl, acetyl and formyl; Y is selected from the group consisting of O and S; R 7 is selected from the group consisting of hydrogen, and (C 1 -C 3)alkyl; R 8 is selected from the group consisting of (C 1 -C 4)alkyl, (C 3 -C 5)cycloalkyl, and (C 3 -C 5)heterocycloalkyl; R 9 is selected from the group consisting of hydrogen, (C 1 -C 3)alkyl, (C 1 -C 3)alkoxy; and wherein said prodrug has either of the following formulae: P is and wherein Q, Q 1 and Q 2 are each independently selected from the group consisting of H and (C 1 -C 3)alkyl and M + is a cationic counterion, wherein alkyl includes alkyl, alkenyl and alkynyl, and wherein (C 3 -C 5)cycloalkyl and (C 3 -C 5)heterocycloalkyl are optionally substituted with one or more (C 1 -C 3)alkoxy and/or (C 1 -C 3)alkyl groups.