Abemaciclib in Newly Diagnosed Meningioma Patients

Inclusion Criteria: * Participants with newly diagnosed intracranial WHO Grade 3 meningioma; or, * Participants with previous lower grade meningioma and histopathologically confirmed newly transformation to Grade 3. * Plan to receive or have received upfront standard of care radiation therapy (RT) for the newly diagnosed WHO Grade 3 meningioma. * No prior treatment for Grade 3 meningioma other than surgical resection or biopsy and upfront RT. If previously diagnosed with a lower grade meningioma, no prior treatment other than surgical resection or biopsy and no prior 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). Written informed consent for the protocol must be obtained prior to any screening procedures. If consent cannot be expressed in writing, it must be formally document and witnessed, ideally via an independent trusted witness. 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 as defi…