RecruitingPhase 2Drug trialPalbociclib and Pembrolizumab In Central Nervous System MetastasesThis research study is studying This research study is studying the efficacy and safety of the following study drugs as a possible treatment for recurrent central nervous system (CNS) metastases: * Palbociclib alone (Cohort 1) * The combination of palbociclib and pembrolizumab (Cohort 2) Pfizer and Merck, pharmaceutical companies, are supporting this research study by providing the study drugs as well as funding for research activities.