Drug class: CDK4/6 inhibitor Mechanism of Action Ribociclib is an orally administered, highly selective small molecule inhibitor of cyclin dependent kinases 4 and 6 (CDK4/6). It is approved in combination with other agents for the treatment of HR positive, HER2 negative advanced or metastatic breast cancer (1). Inside cells, CDK4/6 act as “ignition switches” for cell division. When bound to Cyclin D, they activate signaling that drives the cell cycle forward. In HR positive b