A compound of formula, or a pharmaceutically acceptable salt thereof, wherein: R 1 is selected from: - CO-NR 12 R 13, wherein R 12 and R 13 are each independently selected from H, alkyl, cycloalkyl and heterocycloalkyl, wherein said alkyl group is optionally substituted by one or more R 14 groups, and said heterocycloalkyl is optionally substituted by one or more R 10 groups; or R 12 and R 13 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 R 10 groups; - hydroxyalkyl; - H; - NH 2; - NH-alkyl, wherein said alkyl group is optionally substituted with one or more R 14 groups; - NH-CO-heterocycloalkyl; - heterocycloalkyl optionally substituted by one or more R 10 groups; and - alkoxy optionally substituted with one or more R 14 groups; R 2, R 3, R 4 and R 5 are each independently selected from H, alkyl, hydroxyalkyl and (CH 2) n -R 12'; or R 2 and R 3 are linked to form a cycloalkyl or heterocycloalkyl group each of which may be optionally further substituted with one or more R 10 groups; or R 4 and R 5 are linked to form a cycloalkyl or heterocycloalkyl group each of which may be optionally further substituted with one or more R 10 groups; or one of R 2 and R 3 is absent, one of R 4 and R 5 is absent, and the dashed line is a double bond; Z 1, Z 2, Z 3 and Z 4 are all C; R 6, R 7, R 8 and R 9 are each independently selected from H, 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 7 is absent and R 6, R 8 and R 9 are as defined above; or Z 2, Z 3 and Z 4 are all C, Z 1 is N, R 6 is absent and R 7, R 8 and R 9 are as defined above; n is an integer from 1 to 10; each R 12' is independently selected from NH 2, NHR 10, NR 10 R 11 and heterocycloalkyl, wherein said heterocycloalkyl is optionally further substituted by one or more R 10 groups; each R 10 and R 11 is independently alkyl; and each R 14 is independently selected from 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; wherein "alkyl" denotes a C 1 -C 12 -alkyl group. R 1 is selected from: - CO-NR 12 R 13, wherein R 12 and R 13 are each independently selected from H, alkyl, cycloalkyl and heterocycloalkyl, wherein said alkyl group is optionally substituted by one or more R 14 groups, and said heterocycloalkyl is optionally substituted by one or more R 10 groups; or R 12 and R 13 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 R 10 groups; - hydroxyalkyl; - H; - NH 2; - NH-alkyl, wherein said alkyl group is optionally substituted with one or more R 14 groups; - NH-CO-heterocycloalkyl; - heterocycloalkyl optionally substituted by one or more R 10 groups; and - alkoxy optionally substituted with one or more R 14 groups; - CO-NR 12 R 13, wherein R 12 and R 13 are each independently selected from H, alkyl, cycloalkyl and heterocycloalkyl, wherein said alkyl group is optionally substituted by one or more R 14 groups, and said heterocycloalkyl is optionally substituted by one or more R 10 groups; or R 12 and R 13 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 R 10 groups; - hydroxyalkyl; - H; - NH 2; - NH-alkyl, wherein said alkyl group is optionally substituted with one or more R 14 groups; - NH-CO-heterocycloalkyl; - heterocycloalkyl optionally substituted by one or more R 10 groups; and - alkoxy optionally substituted with one or more R 14 groups; R 2, R 3, R 4 and R 5 are each independently selected from H, alkyl, hydroxyalkyl and (CH 2) n -R 12'; or R 2 and R 3 are linked to form a cycloalkyl or heterocycloalkyl group each of which may be optionally further substituted with one or more R 10 groups; or R 4 and R 5 are linked to form a cycloalkyl or heterocycloalkyl group each of which may be optionally further substituted with one or more R 10 groups; or one of R 2 and R 3 is absent, one of R 4 and R 5 is absent, and the dashed line is a double bond; Z 1, Z 2, Z 3 and Z 4 are all C; R 6, R 7, R 8 and R 9 are each independently selected from H, 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 7 is absent and R 6, R 8 and R 9 are as defined above; or Z 2, Z 3 and Z 4 are all C, Z 1 is N, R 6 is absent and R 7, R 8 and R 9 are as defined above; n is an integer from 1 to 10; each R 12' is independently selected from NH 2, NHR 10, NR 10 R 11 and heterocycloalkyl, wherein said heterocycloalkyl is optionally further substituted by one or more R 10 groups; each R 10 and R 11 is independently alkyl; and each R 14 is independently selected from 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; wherein "alkyl" denotes a C 1 -C 12 -alkyl group.