RB1 and glioblastoma: CDK4/6 inhibitor therapy has potential for the treatment of GBM as the CCND–CDK4/6–CDKN2A–Rb pathway is one of the three pathways commonly disrupted in GBM, and in preclinical and phase 1 trials, palbociclib, abemaciclib, and ribociclib have been shown to cross the BBB at levels that would be expected to elicit kinase inhibition (20–22).