The invention relates to 4-((6-(2,2-Difluoroethyl)-8-(2-hydroxy-2-methylcyclopentyl)-7-oxo-7,8-dihydropyrido[2,3-d]pyrimidin-2-yl)amino)piperidine-1-sulfonamide or pharmaceutically acceptable salts thereof, to compositions containing them, to processes for their preparation, to intermediates used in such processes, and to methods of using such compounds, salts and compositions for the treatment of abnormal cell growth, including cancer. The disclosure also relates to its crystalline form 1, to pharmaceutical compositions comprising form 1, and to the use of form 1 for the treatment of cancers.