A compound of formula I: or a pharmaceutically acceptable salt thereof wherein A is selected from S and O; each of X 1, X 2, X 3, X 4 and X 5 is independently selected from C and N; R 1 is selected from the group consisting of -H, -C 1 - 6 alkyl, -C 1-6 alkyl-amino wherein the amino group is optionally substituted with one or twoC 1-6 acyl or C 1-6 alkyl groups, and -C 1-6 alkyl-heterocyclyl wherein the heterocyclyl group is a 5- or 6- membered aliphatic or aromatic heterocycle, optionally benzo-fused, and is optionally substituted with one or more R 6 groups; R 2 is selected from the group consisting of NH 2, -NH R 5, -N( R 5)C N( R 5) 2 and -N( R 5)C R 5; R 3 is-CF 3; each of R 4 and R 8 is selected from H, -CN, -halo, -CF 3, -C 1 - 6 alkoxy, -CO 2 -C 1 - 6 alkyl, -NO 2, -C 1-6 alkyl-NH 2, -heterocyclyl, and -CONH m [(CH 2) n NH 2] 2-m, wherein the heterocyclyl group is a 5- or 6- membered aliphatic or aromatic optionally benzo-fused heterocycle; each instance of R 5 is independently selected from the group consisting of -H, -C 1-6 alkyl optionally substituted with one or more R 6 groups, -C 2 - 6 alkenyl optionally substituted with one or more R 6 groups, -C 0 - 3 alkyl-C 3 - 6 cycloalkyl-C 0 - 3 alkyl optionally substituted with one or more R 6 groups, -phenyl optionally substituted with one or more R 6 groups, -C=C-Ph optionally substituted with one or more R 6 groups, and -C 0 - 3 alkyl-heterocyclyl-C 0 - 3 alkyl optionally substituted with one or more R 6 groups, wherein the heterocyclyl group is a 5-, 6- or 7-membered aliphatic or aromatic optionally benzo-fused heterocycle; each instance of R 6 is independently selected from the group consisting of -halo, -CN, -C 1-6 alkyl, -OH, -C 1-6 alkoxy, -C 1-6 alkyl-NH 2, -NH m [(CH 2) n NH 2] 2-m, -NH 2, -NH-C 1-6 alkyl, and -N-C 1-6 dialkyl; n and m are integers, wherein each instance of n is independently chosen from 2 or 3, and each instance of m is independently chosen from 0 or 1. A is selected from S and O; each of X 1, X 2, X 3, X 4 and X 5 is independently selected from C and N; R 1 is selected from the group consisting of -H, -C 1 - 6 alkyl, -C 1-6 alkyl-amino wherein the amino group is optionally substituted with one or twoC 1-6 acyl or C 1-6 alkyl groups, and -C 1-6 alkyl-heterocyclyl wherein the heterocyclyl group is a 5- or 6- membered aliphatic or aromatic heterocycle, optionally benzo-fused, and is optionally substituted with one or more -H, -C 1 - 6 alkyl, -C 1-6 alkyl-amino wherein the amino group is optionally substituted with one or twoC 1-6 acyl or C 1-6 alkyl groups, and -C 1-6 alkyl-heterocyclyl wherein the heterocyclyl group is a 5- or 6- membered aliphatic or aromatic heterocycle, optionally benzo-fused, and is optionally substituted with one or more R 6 groups; R 2 is selected from the group consisting of NH 2, -NH R 5, -N( R 5)C N( R 5) 2 and -N( R 5)C R 5; R 3 is-CF 3; each of R 4 and R 8 is selected from H, -CN, -halo, -CF 3, -C 1 - 6 alkoxy, -CO 2 -C 1 - 6 alkyl, -NO 2, -C 1-6 alkyl-NH 2, -heterocyclyl, and -CONH m [(CH 2) n NH 2] 2-m, wherein the heterocyclyl group is a 5- or 6- membered aliphatic or aromatic optionally benzo-fused heterocycle; each instance of R 5 is independently selected from the group consisting of -H, -C 1-6 alkyl optionally substituted with one or more R 6 groups, -C 2 - 6 alkenyl optionally substituted with one or more R 6 groups, -C 0 - 3 alkyl-C 3 - 6 cycloalkyl-C 0 - 3 alkyl optionally substituted with one or more R 6 groups, -phenyl optionally substituted with one or more R 6 groups, -C=C-Ph optionally substituted with one or more R 6 groups, and -C 0 - 3 alkyl-heterocyclyl-C 0 - 3 alkyl optionally substituted with one or more R 6 groups, wherein the heterocyclyl group is a 5-, 6- or 7-membered aliphatic or aromatic optionally benzo-fused heterocycle; -H, -C 1-6 alkyl optionally substituted with one or more R 6 groups, -C 2 - 6 alkenyl optionally substituted with one or more R 6 groups, -C 0 - 3 alkyl-C 3 - 6 cycloalkyl-C 0 - 3 alkyl optionally substituted with one or more R 6 groups, -phenyl optionally substituted with one or more R 6 groups, -C=C-Ph optionally substituted with one or more R 6 groups, and -C 0 - 3 alkyl-heterocyclyl-C 0 - 3 alkyl optionally substituted with one or more R 6 groups, wherein the heterocyclyl group is a 5-, 6- or 7-membered aliphatic or aromatic optionally benzo-fused heterocycle; each instance of R 6 is independently selected from the group consisting of -halo, -CN, -C 1-6 alkyl, -OH, -C 1-6 alkoxy, -C 1-6 alkyl-NH 2, -NH m [(CH 2) n NH 2] 2-m, -NH 2, -NH-C 1-6 alkyl, and -N-C 1-6 dialkyl; n and m are integers, wherein each instance of n is independently chosen from 2 or 3, and each instance of m is independently chosen from 0 or 1.