solid forms of a selective inhibitor of CDK4 / 6

A crystalline freebase of 6-acetyl-8-cyclopentyl-5-methyl-2- (5-piperazin-1-yl-pyridin-2-ylamino) -8H-pyrido [2,3-d] pyrimidin-7-one having a specific surface area of ​​<= 2 m2 / g, and wherein the crystalline freebase is a Form a polymorph of the free base having a pattern of X-ray diffraction powder comprising a peak at angle diffraction (2θ) of 10.1 ± 0.2.