Background: Primary CNS Lymphoma (PCNSL) is an aggressive primary brain tumor. Outcome and treatment options for patients with recurrent/refractory (r/r) disease are poor. We have observed promising efficacy of single agent ibrutinib in r/r PCNSL and secondary CNS lymphoma (SCNSL). In this phase 1B trial, we investigate the toxicity of ibrutinib in combination with high-dose methotrexate (HD-MTX) in r/r PCNSL/SCNSL. Methods: Eligible patients had r/r PCNSL/SCNSL or newly diagnosed SCNSL, age≥18, ECOG≤2, normal end-organ function, and with any number and type of prior therapies. In patients with SCNSL disease, systemic disease needed to be absent. HD-MTX was given at 3.5g/m2 every 2 weeks for a total of 8 doses. To minimize adverse events, ibrutinib was stopped on days of HD-MTX infusion and was restarted 5 days after MTX infusion or after completion of MTX-clearance, if clearance of MTX required more than 5 days. Ibrutinib was continued daily after completion of 8 doses of MTX. Results: Six patients have been enrolled; 3 received 560mg and 3 received 840mg ibrutinib in combination with HD-MTX. Median age was 62 (range 43-74); median ECOG 1 (0:2; 1:3; 2:1). Two had r/r PCNSL and 4 SCNSL. Three had brain disease, one isolated cerebrospinal fluid (CSF) involvement and two parenchymal and CSF involvement. Three patients had recurrent (2 PCNSL; 1 SCNSL), two refractory (both SCNSL), and one newly diagnosed disease (SCNSL). There were no grade 4 adverse events. Grade 3 events were observed in 5 patients (lymphopenia in 3, ALT elevation in 2, diarrhea in 1, electrolyte changes in 1, hypertension in 1). The most common adverse events were hypokalemia, low WBC, hyperglycemia, ALT and AST elevation. There was no dose reduction of methotrexate or ibrutinib in any patient. After a median follow-up of 130 days, all patients were evaluated for response after 4 doses of HD-MTX, with 4/6 (67%) showing a response: 2 CR, 2 PR, and 1 SD, 1 PD; both non-responders were refractory SCNSL. Ibrutinib concentrations were measured in plasma and CSF. Conclusions: Patients with CNS lymphoma tolerate the combination of HD-MTX and Ibrutinib (at 560 and 840mg) well. Continued enrollment into a combination arm that includes rituximab, methotrexate and ibrutinib is ongoing. Clinical trial information: NCT02315326