Solid forms of a selective cdk4/6 inhibitor compound acetyl-8-cyclopentyl-5-methyl-2-(5-piperazin-1-yl-pyridin-2-ylamino)-8h-pyrido[2,3-d]pyrimidin-7-one

This invention relates to the crystalline free base of acetyl-8- cyclopentyl-5-methyl-2-(5-piperazin-l-yl-pyridin-2-ylamino)-8H- pyrido[2,3-d]pyrimidin-7-one, formula (1) having improved properties, to pharmaceutical compositions and dosage forms comprising the free base, and to methods for making and using such compounds, compositions and dosage forms in the treatment of cell proliferative diseases, such as cancer.