Pabcicilib is an inhibitor of cyclin dependent kinase (CDK) 4 and 6. Cyclin D1 and CDK4 / 6 are downstream signaling pathways leading to cell proliferation. In vitro, the proliferation of ER positive breast cancer cells was inhibited by blocking the progression from G1 to S phase. Compared with the treatment of each drug alone, the combination of pabacib and anti estrogen resulted in decreased phosphorylation of retinoblastoma protein (RB), decreased E2F expression and signal, and prevented growth. In vitro treatment of ER positive breast cancer cell lines with combination of pabacib and anti estrogen resulted in increased cell senescence, which lasted up to 6 days after drug removal.