Abemaciclib in Newly Diagnosed Meningioma Patients

Inclusion Criteria: Participant with an intracranial WHO Grade 3 meningioma or lower grade meningioma that has progressed to WHO grade 3 that have received prior radiation therapy. Resected tissue must demonstrate: (a) RB positivity on immunohistochemistry (IHC); or, no RB mutations on next-generation sequencing (NGS). A washout period of 14 to 35 days is required between end of RT and Day 1 and must be fully recovered from the acute effects of RT. Ability to understand and the willingness to sign a written informed consent document (personally or by the legally authorized representative, if applicable). Participant has voluntarily agreed to participate by giving written informed consent (personally or via legally authorized representative(s), and assent if applicable). Participant must be willing and able to comply with scheduled visits, treatment plans, laboratory tests and other procedures. Age ≥18 years at time of consent. Have a performance status (PS) of ≤2 on the Eastern Cooperative Oncology Group (ECOG) scale. Ability to swallow oral medications. Participant has adequate bone marrow and organ function Confirmed negative serum pregnancy test (β-hCG) before starting study treatment or participant who is no longer of childbearing potential due to surgical, chemical, or natural menopause. For females of reproductive potential: use of highly effective contraception during study participation and for an additional 3 weeks after the end of treatment administration. For males of reproductive potential: use of condoms or other methods to ensure effective contraception with partner and for an additional 3 weeks after the end of treatment administration. Exclusion Criteria: Prior history of cancer with ongoing treatment of disease. Current use of coumarin-derived anticoagulant for treatment, prophylaxis or otherwise. Therapy with heparin, low molecular weight heparin (LMWH) or fondaparinux is allowed. Pregnancy, breastfeeding or lactation. Known allergic reactions to components of the abemaciclib. Active infection or fever >38.5°C requiring systemic antibiotic, antifungal or antiviral therapy within 4 weeks of Day 1. Known to have active (acute or chronic) or uncontrolled severe infection, liver disease such as cirrhosis, decompensated liver disease, and active and chronic hepatitis. Known active systemic bacterial infection, fungal infection, or detectable viral infection . Participant has serious and/or uncontrolled preexisting medical condition(s) that, in the judgment of the investigator, would preclude participation in this study. Prior therapy with any CDK4/6 inhibitor. Treatment with another investigational drug within 5 half-lives of the investigational product. The patient has a personal history of any of the following conditions: syncope of cardiovascular etiology, ventricular arrhythmia of pathological origin (including, but not limited to, ventricular tachycardia and ventricular fibrillation), or sudden cardiac arrest.