CDK4 and breast cancer: Pyrido[2,3-d]pyrimidine is a critical component in various bioactive compounds, including anticancer medicines, attributed to cyclin-dependent kinase inhibition.100,101 Palbociclib is a CDK4/6 inhibitor licensed by the US Food and Drug Administration to treat breast cancer.102 Safinaz et al., in 2019, prepared pyrido[2,3-d]pyrimidines derivatives and evaluated their cytotoxicity activity against various cell lines, including MCF-7, PC-3, and luA-549 and compared the results with doxorubicin.