Palbociclib and Pembrolizumab In Central Nervous System Metastases

Inclusion Criteria: * Participants must have histologically or cytologically confirmed disease from any solid tumor (Cohort 1) * Participants must have histologically or cytologically confirmed disease from breast cancer or melanoma (Cohort 2). * Participants must have measurable disease in the CNS, defined as at least one lesion that can be accurately measured in at least one dimension as ≥10 mm. * Participants must have progressive CNS lesions, as defined by one of the following: * Patients may have multiple progressive CNS lesions, some of which have been treated by SRS or surgery. Patients are eligible if they have one or more un-treated (by surgery or SRS) progressive lesions that is measurable. * Patients have measurable residual or progressive lesions after surgery. * Patients who have had prior WBRT and/or SRS are eligible but there needs to be unequivocal evidence of progression of at least one lesion treated by radiation (e.g. tissue diagnosis). Biopsy can be considered for definitive diagnosis. * Patients who have previously been treated with systemic therapy for CNS metastases are eligible. * Age \> 18 years. The toxicity of palbociclib in children is unknown. * ECOG performance status of 0, 1 or 2 (Karnofsky ≥ 60, see Appendix A). * Participants must have normal organ and marrow function as defined below: * leukocytes ≥3,000/mcL * absolute neutrophil count ≥1,500/mcL * platelets ≥100,000/mcL * hemoglobin ≥9g/dL * total bilirubin \< 1.5 x inst…