Pembrolizumab And Lenvatinib In Leptomeningeal Metastases

Inclusion Criteria: * Participants must have histologically or cytologically confirmed solid malignancy. * Leptomeningeal metastases, as determined by: 1) positive CSF cytology, or 2) MRI suggestive of leptomeningeal metastases and atypical cytology. * Age ≥18 years. Because no dosing or adverse event data are currently available on the use of pembrolizumab in combination with lenvatinib in participants \<18 years of age, children are excluded from this study, but will be eligible for future pediatric trials. * ECOG performance status ≤ 1 (Karnofsky ≥70%, see Appendix A) * Participants must have normal organ and marrow function; all screening labs should be performed within 14 days of treatment initiation. * Eligibility Criteria for Organ and Marrow Function * Hematological * Absolute neutrophil count (ANC) ≥1500/μL * Platelets ≥100 000/μL * Hemoglobin ≥9.0 g/dL or ≥5.6 mmol/La * Renal \--- Creatinine ≤1.5 × ULN OR Measured or calculatedb creatinine clearance (GFR can also be used in place of creatinine or CrCl) ≥30 mL/min for participant with creatinine levels \>1.5 × institutional ULN * Hepatic * Total bilirubin ≤1.5 ×ULN OR direct bilirubin ≤ULN for participants with total bilirubin levels \>1.5 × ULN * AST (SGOT) and ALT (SGPT) ≤2.5 × ULN (≤5 × ULN for participants with liver metastases) * Coagulation * International normalized ratio (INR) OR prothrombin time (PT) Activated partial thromboplastin time (aPTT) ≤1.5 × ULN unless part…