A compound of formula, or a pharmaceutically acceptable salt thereof, wherein: R 1 is selected from: - H; - CO-NR 8 R 9, wherein R 8 and R 9 are each independently selected from H, alkyl, cycloalkyl and mono or bicyclic heterocycloalkyl, wherein said alkyl group is optionally substituted by one or more R 12 groups, and said heterocycloalkyl is optionally substituted by one or more groups selected from R 10 and R 12; or R 8 and R 9 are linked, together with the nitrogen to which they are attached, to form a heterocycloalkyl group optionally containing one or more additional heteroatoms, and optionally substituted by one or more groups select from R 10 and (CH 2) m R 12; R 2 is selected from H and alkyl, wherein said alkyl group is optionally substituted by one or more R 12 groups; R 3 is selected from alkyl, cycloalkyl and heterocycloalkyl, each of which may be optionally substituted by one or more groups selected from halo, OH and alkoxy; Z 1, Z 2, Z 3 and Z 4 are all C; R 4, R 5, R 6 and R 7 are each independently selected from H, alkyl, CN, NO 2, OH, alkoxy, NHCO-alkyl, halo and haloalkyl; or Z 1, Z 3 and Z 4 are all C, Z 2 is N, R 5 is absent and R 4, R 6 and R 7 are as defined above; or Z 2, Z 3 and Z 4 are all C, Z 1 is N, R 4 is absent and R 5, R 6 and R 7 are as defined above; each R 10 and R 11 is independently alkyl; each R 12 is independently selected from CO 2 R 10, COOH, OH, alkoxy, haloalkyl, NH 2, NHR 10, NR 10 R 11, heteroaryl and heterocycloalkyl, wherein said heterocycloalkyl is optionally further substituted by one or more R 10 groups; R 13 is H or halo; and wherein "alkyl" denotes a C 1 -C 12 -alkyl group. R 1 is selected from: - H; - CO-NR 8 R 9, wherein R 8 and R 9 are each independently selected from H, alkyl, cycloalkyl and mono or bicyclic heterocycloalkyl, wherein said alkyl group is optionally substituted by one or more R 12 groups, and said heterocycloalkyl is optionally substituted by one or more groups selected from R 10 and R 12; or R 8 and R 9 are linked, together with the nitrogen to which they are attached, to form a heterocycloalkyl group optionally containing one or more additional heteroatoms, and optionally substituted by one or more groups select from R 10 and (CH 2) m R 12; - H; - CO-NR 8 R 9, wherein R 8 and R 9 are each independently selected from H, alkyl, cycloalkyl and mono or bicyclic heterocycloalkyl, wherein said alkyl group is optionally substituted by one or more R 12 groups, and said heterocycloalkyl is optionally substituted by one or more groups selected from R 10 and R 12; or R 8 and R 9 are linked, together with the nitrogen to which they are attached, to form a heterocycloalkyl group optionally containing one or more additional heteroatoms, and optionally substituted by one or more groups select from R 10 and (CH 2) m R 12; R 2 is selected from H and alkyl, wherein said alkyl group is optionally substituted by one or more R 12 groups; R 3 is selected from alkyl, cycloalkyl and heterocycloalkyl, each of which may be optionally substituted by one or more groups selected from halo, OH and alkoxy; Z 1, Z 2, Z 3 and Z 4 are all C; R 4, R 5, R 6 and R 7 are each independently selected from H, alkyl, CN, NO 2, OH, alkoxy, NHCO-alkyl, halo and haloalkyl; or Z 1, Z 3 and Z 4 are all C, Z 2 is N, R 5 is absent and R 4, R 6 and R 7 are as defined above; or Z 2, Z 3 and Z 4 are all C, Z 1 is N, R 4 is absent and R 5, R 6 and R 7 are as defined above; each R 10 and R 11 is independently alkyl; each R 12 is independently selected from CO 2 R 10, COOH, OH, alkoxy, haloalkyl, NH 2, NHR 10, NR 10 R 11, heteroaryl and heterocycloalkyl, wherein said heterocycloalkyl is optionally further substituted by one or more R 10 groups; R 13 is H or halo; and wherein "alkyl" denotes a C 1 -C 12 -alkyl group.