CDK4 and pneumonia: In September 2019, the FDA issued a drug safety newsletter and warned that palbociclib, ribociclib, and abemaciclib used to treat advanced breast cancer could trigger rare but serious pneumonia called interstitial lung disease.381 This risk warning information must be added to the drug labels and patient instructions of all approved CDK4/6 inhibitors.382 Moreover, understanding the mechanisms of resistance to CDK4/6 inhibitors is another issue that needs to be considered.