A compound of formula, or a pharmaceutically acceptable salt thereof, wherein: R b is selected from alkyl, cycloalkyl and heterocycloalkyl, each of which may be optionally substituted by one or more groups selected from halo and alkoxy; R 1a is selected from: - CO-NR 12a R 13a, wherein R 12a and R 13a are each independently selected from H, alkyl, cycloalkyl and mono or bicyclic heterocycloalkyl, wherein said alkyl group is optionally substituted by one or more (CH 2) m R 14a groups, and said heterocycloalkyl is optionally substituted by one or more groups selected from R 10 and (CH 2) m R 14a; or R 12a and R 13a 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)mR 14 a; - hydroxyalkyl; and - COOH; 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; m is an integer from 1 to 10; each R 10 and R 11 is independently alkyl; each R 14a 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; wherein "alkyl" denotes a C 1 -C 12 -alkyl group. R b is selected from alkyl, cycloalkyl and heterocycloalkyl, each of which may be optionally substituted by one or more groups selected from halo and alkoxy; R 1a is selected from: - CO-NR 12a R 13a, wherein R 12a and R 13a are each independently selected from H, alkyl, cycloalkyl and mono or bicyclic heterocycloalkyl, wherein said alkyl group is optionally substituted by one or more (CH 2) m R 14a groups, and said heterocycloalkyl is optionally substituted by one or more groups selected from R 10 and (CH 2) m R 14a; or R 12a and R 13a 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)mR 14 a; - hydroxyalkyl; and - COOH; - CO-NR 12a R 13a, wherein R 12a and R 13a are each independently selected from H, alkyl, cycloalkyl and mono or bicyclic heterocycloalkyl, wherein said alkyl group is optionally substituted by one or more (CH 2) m R 14a groups, and said heterocycloalkyl is optionally substituted by one or more groups selected from R 10 and (CH 2) m R 14a; or R 12a and R 13a 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)mR 14 a; - hydroxyalkyl; and - COOH; 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; m is an integer from 1 to 10; each R 10 and R 11 is independently alkyl; each R 14a 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; wherein "alkyl" denotes a C 1 -C 12 -alkyl group.