1. Pathology report confirming the diagnosis of B-cell PCNSL
2. Relapsed or refractory B-cell PCNSL with at least 1 prior high-dose methotrexate (HD-MTX) based therapy for PCNSL:
* Relapsed disease: Participants who achieved a response (CR, CRu, PR) to the last treatment and subsequently experienced disease progression
* Refractory disease: Participants whose best response to the last treatment was stable disease or PD
3. One or more measurable brain lesions with a diameter in at least one dimension greater than or equal to (≥) 1 centimeter (cm) in gadolinium-enhanced magnetic resonance imaging (MRI)
4. Eastern Cooperative Oncology Group Performance Status (ECOG PS) of 0-2
5. Adequate bone marrow, renal, and hepatic function per central lab values
6. Participants must agree to comply with all defined contraceptive requirements
You may not be if
1. Participants with isolated intraocular PCNSL or spinal PCNSL with no brain lesions
2. Participants with non-B-cell PCNSL
3. Participants with systemic presence of lymphoma
4. Refractory to temozolomide with or without rituximab-containing regimens in the last PCNSL treatment
5. Concomitant systemic corticosteroid exposure within 14 days before starting study drug per Investigator assessment with the exception of the following:
* Equivalent of up to 10 milligram per day (mg/day) of prednisone for a disease other than PCNSL
* Equivalent of up to 50 mg/day of prednisone (equal to 8 mg/day dexamethasone) for participants with lesions of the brain and/or spinal cord
6. Active malignancy, other than PCNSL requiring systemic therapy
7. Poorly controlled comorbidity, or history of medical conditions contraindicated per Investigator assessment
8. Participants who are unable to swallow oral medication
