CDK4 and cancer: Notably, they found that palbociclib had synergistic growth inhibitory activity when combined with the anti-estrogen drug tamoxifen and demonstrated efficacy in a model of acquired tamoxifen resistance.638 Further in vitro studies showed that palbociclib, as well as other CDK4/6 inhibitors such as ribociclib and abemaciclib, were effective in blocking cell proliferation in several RB-positive cancer cell lines, but not in cell lines that had lost RB expression.