Approximately 20% of patients with recurrent high-grade glioma will undergo surgical resection of their tumor typically at the time of first recurrence. These patients will be eligible for this study. An extra 10 mL of blood will be collected during a routine clinical procedure prior to initiation of ribociclib treatment (i.e., baseline blood sample), separated into plasma and buffy coat fractions, and frozen for pharmacokinetic (PK) analysis. Patients will be treated with ribociclib for 8-21 days prior to surgery to identify drug effects on tumor cells, and to determine drug PK. The last presurgical dose of ribociclib will be administered on the day of surgery at 4-8 hours prior to surgery to allow sufficient time for the drug to enter tumor cells, inhibit CDK4/6, and modulate downstream effectors. Blood will be acquired within 1 hour before surgery (as close to the time of surgery as possible). Blood will be separated into plasma and buffy coat fractions, and frozen. During surgery, samples of brain tumor core and infiltrating brain tumor will be acquired. Tissue samples will be frozen or fixed in paraffin embedded blocks and histology for PK and molecular analyses.