Paxalisib (GDC-0084) In Recurrent Or Refractory PCNSL

This is an open-label, phase 2 study to determine the efficacy of Paxalisib (GDC-0084) in 25 patients with recurrent or refractory primary central nervous system lymphoma (R/R PCNSL. * The name of the study drug involved in this study is Paxalisib (GDC-0084). * The research study procedures include: screening for eligibility and study treatment including evaluations and follow up visits. * It is expected that about 25 participants will take part in this research study for up to 24 months as long as there is no serious side effects and disease progression. This research study is a Phase 2 clinical trial. Phase 2 clinical trials test the safety and effectiveness of an investigational drug to learn whether the drug works in treating a specific disease. "Investigational" means that the drug is being studied. The U.S. Food and Drug Administration (FDA) has not approved Paxalisib for this specific disease but it has been approved for other uses.