This disclosure relates to the crystalline fumarate salt of (S)-[3,4-difluoro-2-(2-fluoro-4-iodophenylamino)phenyl] [3-hydroxy-3-(piperidin-2-yl) azetidin-l-yl]-methanone. The disclosure also relates to pharmaceutical compositions comprising the crystalline fumarate salt of (S)-[3,4-difluoro-2-(2-fluoro-4-iodophenylamino)phenyl] [3-hydroxy-3-(piperidin-2-yl) azetidin-l-yl]-methanone. The disclosure also relates to methods of treating cancers comprising administering to a patient in need thereof the crystalline fumarate salt of (S)-[3,4-difluoro-2-(2-fluoro-4-iodophenylamino)phenyl] [3-hydroxy-3-(piperidin-2-yl) azetidin-1-yl]-methanone.