Preliminary evaluation of efficacy and determination of the ribociclib safety profile in patients with brain tumors will be studied. All patients will be treated with ribociclib (600 mg/day) for 8-21 days before surgical resection of the recurrent tumor. Rb status of the recurrent tumor will be determined by immunohistochemistry within 2 weeks after surgery. Patients with Rb-positive tumors will continue treatment with ribociclib (21 days on, 7 days off) after surgery. Patients will be treated until unacceptable toxicity is observed, or until disease progression as assessed by radiographic or clinical metrics. We will determine whether molecular markers associated with changes induced by ribociclib treatment in tumors (matching initial vs. recurrent tumors) correlate with progression-free survival.