Palbociclib Tri-O-isobutyryl-β-D-glucuronic Acid Methyl Ester is an intermediate in synthesizing Palbociclib N-β-D-Glucuronide Sodium Salt (P139910), a derivative of Palbociclib (P139900) which is an experimental drug for the treatment of breast cancer being developed by Pfizer. It is a selective inhibitor of the cyclin-dependent kinases CDK4 and CDK6.