A compound of the formula: or a salt or solvate thereof, wherein: X 1 is CR 11 or N; X 2 is O or S; R 1, R 2, R 4 and R 5 are independently hydrogen, halo, nitro, azido, hydroxy, C 1 -C 10 alkoxy, halo-substituted C 1 -C 10 alkoxy, acyloxy, C 1 -C 10 alkylthio, halo-substituted C 1 -C 10 alkylthio, amino, carboxy, C 1 -C 10 alkyl, C 2 -C 10 alkenyl, C 2 -C 10 alkynyl or C 3 -C 10 cycloalkyl; R 3 is hydrogen, halo, cyano, nitro, azido, hydroxy, C 1 -C 10 alkoxy, halo-substituted C 1 -C 10 alkoxy, acyloxy, mercapto, C 1 -C 10 alkylthio, halo-substituted C 1 -C 10 alkylthio, -SO 2 R a, -SO 2 N(R c)R d, -N(R c)R d, -C OR b, C 1 -C 10 alkyl, C 2 -C 10 alkenyl, C 2 -C 10 alkynyl or C 3 -C 10 cycloalkyl; R a is C 1 -C 10 alkyl or C 6 -C 14 aryl; each R b, R c and R d is independently hydrogen or C 1 -C 10 alkyl; R 6 is -C OR 7, -C NR 7 R 8, -C N(R 8)OR 7, -C R 9 or -NHSO 2 R 10; each R 7 and R 8 is independently hydrogen, C 1 -C 10 alkyl, C 2 -C 10 alkenyl, C 2 -C 10 alkynyl, C 3 -C 10 cycloalkyl, C 3 -C 10 cycloalkyl C 1 -C 10 alkyl or C 1 -C 10 alkyl C 3 -C 10 cycloalkyl; R 9 is C 1 -C 10 alkyl, C 3 -C 10 cycloalkyl or C 6 -C 14 aryl; R 10 is C 1 -C 10 alkyl, C 2 -C 10 alkenyl, C 2 -C 10 alkynyl, C 3 -C 10 cycloalkyl, C 3 -C 10 cycloalkyl C 1 -C 10 alkyl or C 1 -C 10 alkyl C 3 -C 10 cycloalkyl; and R 11 is hydrogen, C 1 -C 10 alkyl, C 2 -C 10 alkenyl, C 2 -C 10 alkynyl, C 3 -C 10 cycloalkyl or C 3 -C 10 cycloalkyl C 1 -C 10 alkyl; wherein each C 1 -C 10 alkyl, C 2 -C 10 alkenyl, C 2 -C 10 alkynyl or C 3 -C 10 cycloalkyl moiety may be unsubstituted or substituted with one or more groups independently selected from hydroxy, oxo, halo, cyano, nitro, trifluoromethyl, azido, amino, carboxy and mercapto; provided that if X 1 is CH, one of R 1 and R 2 is methyl or chloro, the other of R 1 and R 2 is hydrogen and R 3 is iodo, then R 6 is -NHSO 2 R 10 or -C N(R 8)OR 7 where R 7 is C 1 -C 10 alkyl substituted with at least one hydroxy group. X 1 is CR 11 or N; X 2 is O or S; R 1, R 2, R 4 and R 5 are independently hydrogen, halo, nitro, azido, hydroxy, C 1 -C 10 alkoxy, halo-substituted C 1 -C 10 alkoxy, acyloxy, C 1 -C 10 alkylthio, halo-substituted C 1 -C 10 alkylthio, amino, carboxy, C 1 -C 10 alkyl, C 2 -C 10 alkenyl, C 2 -C 10 alkynyl or C 3 -C 10 cycloalkyl; R 3 is hydrogen, halo, cyano, nitro, azido, hydroxy, C 1 -C 10 alkoxy, halo-substituted C 1 -C 10 alkoxy, acyloxy, mercapto, C 1 -C 10 alkylthio, halo-substituted C 1 -C 10 alkylthio, -SO 2 R a, -SO 2 N(R c)R d, -N(R c)R d, -C OR b, C 1 -C 10 alkyl, C 2 -C 10 alkenyl, C 2 -C 10 alkynyl or C 3 -C 10 cycloalkyl; R a is C 1 -C 10 alkyl or C 6 -C 14 aryl; each R b, R c and R d is independently hydrogen or C 1 -C 10 alkyl; R 6 is -C OR 7, -C NR 7 R 8, -C N(R 8)OR 7, -C R 9 or -NHSO 2 R 10; each R 7 and R 8 is independently hydrogen, C 1 -C 10 alkyl, C 2 -C 10 alkenyl, C 2 -C 10 alkynyl, C 3 -C 10 cycloalkyl, C 3 -C 10 cycloalkyl C 1 -C 10 alkyl or C 1 -C 10 alkyl C 3 -C 10 cycloalkyl; R 9 is C 1 -C 10 alkyl, C 3 -C 10 cycloalkyl or C 6 -C 14 aryl; R 10 is C 1 -C 10 alkyl, C 2 -C 10 alkenyl, C 2 -C 10 alkynyl, C 3 -C 10 cycloalkyl, C 3 -C 10 cycloalkyl C 1 -C 10 alkyl or C 1 -C 10 alkyl C 3 -C 10 cycloalkyl; and R 11 is hydrogen, C 1 -C 10 alkyl, C 2 -C 10 alkenyl, C 2 -C 10 alkynyl, C 3 -C 10 cycloalkyl or C 3 -C 10 cycloalkyl C 1 -C 10 alkyl; wherein each C 1 -C 10 alkyl, C 2 -C 10 alkenyl, C 2 -C 10 alkynyl or C 3 -C 10 cycloalkyl moiety may be unsubstituted or substituted with one or more groups independently selected from hydroxy, oxo, halo, cyano, nitro, trifluoromethyl, azido, amino, carboxy and mercapto; provided that if X 1 is CH, one of R 1 and R 2 is methyl or chloro, the other of R 1 and R 2 is hydrogen and R 3 is iodo, then R 6 is -NHSO 2 R 10 or -C N(R 8)OR 7 where R 7 is C 1 -C 10 alkyl substituted with at least one hydroxy group.