Primary and metastatic brain tumors have limited treatment options and long term survival is rare. Cyclin-dependent kinases (CDKs) are major regulators of the cell cycle and are commonly altered in tumors. The CDK4/6 pathway regulates the checkpoint between G1 and S phase of the cell cycle. When altered, cells are able to proliferate rapidly and independent of this checkpoint. The blood-brain barrier (BBB) is a network of cells and proteins that prevent the paracellular and transcellular passage of many therapeutic agents from systemic circulation into the brain. The efflux transporters P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) actively transport substrates back into systemic circulation. Previous studies have demonstrated that numerous molecularly-targeted agents are substrates of P-gp and/or BCRP and that these transporters are responsible for the limited brain delivery. The objective of this work was to evaluate the role of efflux transport at the BBB in the brain delivery of CDK4/6 inhibitors and assess the relationship between brain delivery and efficacy in a glioblastoma (GBM) patient-derived xenograft. There are three CDK4/6 inhibitors, palbociclib, ribociclib and abemaciclib, currently in clinical development for the treatment of a variety of solid tumors. We show that palbociclib has limited brain delivery to active efflux transport by P-gp and BCRP at the BBB. Furthermore, we demonstrate that the concentrations of palbociclib reaching the brain are also subtherapeutic in the treatment of a subcutaneous GBM tumor. We then used a pharmacological inhibitor of efflux transport and improved the brain delivery of palbociclib in tumor-naïve and tumor-bearing mice. We demonstrated that chronic use of this combination therapy was well tolerated and significantly improved the brain delivery of palbociclib in an intracranial tumor model to the same levels that were achieved in the subcutaneous GBM model. Despite improving the brain delivery of palbociclib, there was no improvement in efficacy. Using the in situ brain perfusion technique, we compared the brain delivery of palbociclib, ribociclib and abemaciclib and the data suggest that abemaciclib may saturate efflux at lower concentrations that palbociclib or ribociclib and have improved brain delivery. These studies show that improving the brain delivery of palbociclib alone is not sufficient to improve survival in the intracranial GBM model. Future studies that reveal other factors besides delivery that are altered in subcutaneous models of brain tumors will be essential in understanding the use of preclinical models to study experimental GBM therapies.