Disclosed are polymorphs of the isethionate salt of 6-acetyl-8-cyclopentyl-5-methyl-2-(5-piperazin-1-yl-pyridin-2-ylamino)-8H- -pyrido[2,3-d]pyrimidin-7-one, ##STR00001## which is a selective cyclin-dependent kinase 4 (CDK4) inhibitor useful for treating inflammation and cell proliferative diseases such as cancer and restenosis.