A selective inhibitor of Aurora A kinase for use in treating a proliferative disorder by administration in combination with another therapy or one or more therapeutic agents, wherein the selective inhibitor of Aurora A kinase is a compound of formula: or a pharmaceutically acceptable salt thereof, wherein: R a is selected from the group consisting of C 1-3 aliphatic, C 1-3 fluoroaliphatic, -R 1, -T-R 1 -R 2, and -T-R 2; T is a C 1-3 alkylene chain optionally substituted with fluoro; R 1 is a 5- or 6-membered aryl, heteroaryl, or heterocyclyl ring optionally substituted with one or two substituents independently selected from the group consisting of halo, C 1-3 aliphatic, and C 1-3 fluoroaliphatic; R 2 is selected from the group consisting of halo, -C≡C-R 3, -CH=CH-R 3, -N(R 4) 2, and -OR 5; R 3 is hydrogen, C 1-3 aliphatic, C 1-3 fluoroaliphatic, -CH 2 OCH 3 or an aryl, heteroaryl, or heterocyclyl group; each R 4 independently is hydrogen or an aliphatic, aryl, heteroaryl, or heterocyclyl group; or two R 4 on the same nitrogen atom, taken together with the nitrogen atom form a 5- to 6-membered heteroaryl or 4- to 8-membered heterocyclyl ring having, in addition to the nitrogen atom, 0-2 ring heteroatoms selected from N, O, and S; R 5 is hydrogen, C 1-3 aliphatic, C 1-3 fluoroaliphatic or an aryl, heteroaryl, or heterocyclyl group; and R b is selected from the group consisting of fluoro, chloro, -CH 3, -CF 3, -OH, -OCH 3, -OCF 3, -OCH 2 CH 3, and -OCH 2 CF 3; and wherein said therapy or one or more therapeutic agents is selected from cytotoxic agents, radiotherapy and immunotherapy. R a is selected from the group consisting of C 1-3 aliphatic, C 1-3 fluoroaliphatic, -R 1, -T-R 1 -R 2, and -T-R 2; T is a C 1-3 alkylene chain optionally substituted with fluoro; R 1 is a 5- or 6-membered aryl, heteroaryl, or heterocyclyl ring optionally substituted with one or two substituents independently selected from the group consisting of halo, C 1-3 aliphatic, and C 1-3 fluoroaliphatic; R 2 is selected from the group consisting of halo, -C≡C-R 3, -CH=CH-R 3, -N(R 4) 2, and -OR 5; R 3 is hydrogen, C 1-3 aliphatic, C 1-3 fluoroaliphatic, -CH 2 OCH 3 or an aryl, heteroaryl, or heterocyclyl group; each R 4 independently is hydrogen or an aliphatic, aryl, heteroaryl, or heterocyclyl group; or two R 4 on the same nitrogen atom, taken together with the nitrogen atom form a 5- to 6-membered heteroaryl or 4- to 8-membered heterocyclyl ring having, in addition to the nitrogen atom, 0-2 ring heteroatoms selected from N, O, and S; R 5 is hydrogen, C 1-3 aliphatic, C 1-3 fluoroaliphatic or an aryl, heteroaryl, or heterocyclyl group; and T is a C 1-3 alkylene chain optionally substituted with fluoro; R 1 is a 5- or 6-membered aryl, heteroaryl, or heterocyclyl ring optionally substituted with one or two substituents independently selected from the group consisting of halo, C 1-3 aliphatic, and C 1-3 fluoroaliphatic; R 2 is selected from the group consisting of halo, -C≡C-R 3, -CH=CH-R 3, -N(R 4) 2, and -OR 5; R 3 is hydrogen, C 1-3 aliphatic, C 1-3 fluoroaliphatic, -CH 2 OCH 3 or an aryl, heteroaryl, or heterocyclyl group; each R 4 independently is hydrogen or an aliphatic, aryl, heteroaryl, or heterocyclyl group; or two R 4 on the same nitrogen atom, taken together with the nitrogen atom form a 5- to 6-membered heteroaryl or 4- to 8-membered heterocyclyl ring having, in addition to the nitrogen atom, 0-2 ring heteroatoms selected from N, O, and S; R 5 is hydrogen, C 1-3 aliphatic, C 1-3 fluoroaliphatic or an aryl, heteroaryl, or heterocyclyl group; and R b is selected from the group consisting of fluoro, chloro, -CH 3, -CF 3, -OH, -OCH 3, -OCF 3, -OCH 2 CH 3, and -OCH 2 CF 3; and