The invention relates to a CDK4/6 kinase inhibitor, or a pharmaceutically acceptable salt, ester, or solvate thereof, or their isomers； a pharmaceutical formulation, pharmaceutical composition and kit comprising said CDK4/6 kinase inhibitor, or a pharmaceutically acceptable salt, ester, or solvate thereof, or their isomers, and use of said CDK4/6 kinase inhibitor, or a pharmaceutically acceptable salt, ester, or solvate thereof, or their isomers. For example, the compounds of the invention are useful for reducing or inhibiting the activity of CDK4/6 kinase in a cell, and/or treating and/or preventing a cancer-related disease mediated by CDK4/6 kinase.
