CDK9 and glioblastoma: Abemaciclib, a CDK4/6 inhibitor that is currently being tested in phase I trials for adult and pediatric patients with glioblastoma or other brain tumors (see Introduction), has also been reported to demonstrate potent inhibition against CDK9 and more efficient BBB penetration compared to ribociclib and palbociclib, two other CDK4/6 inhibitors in phase I trials [85,86,87].