CTIM-39. A PHASE 1B STUDY OF PEMBROLIZUMAB, IBRUTINIB AND RITUXIMAB IN RECURRENT/REFRACTORY (RR) PRIMARY CENTRAL NERVOUS SYSTEM LYMPHOMA (PCNSL)

Abstract

Abstract BACKGROUND PCNSL exhibit B-cell receptor (BCR) pathway activation due to CD79B and MYD88 mutations, concurrently with 9p24.1/PD-L1/PD-L2 copy number alterations. These genetic features suggest complementary approaches of targeting PD1, BCR/BTK (Bruton’s tyrosine kinase) and CD20. Pembrolizumab, ibrutinib and rituximab have each been used as monotherapy in RR PCNSL. We investigate these agents in combination to determine tolerability and synergistic efficacy. METHODS This is a phase 1b/2 study of pembrolizumab, ibrutinib and rituximab in RR PCNSL patients. In phase 1b, patients were accrued using a 3 + 3 design to determine the maximum tolerated dose/recommended phase 2 dose (MTD/RP2D). Patients were treated with pembrolizumab 200mg IV every 3 weeks, ibrutinib oral daily and rituximab 375mg/m2 IV weekly (for 4 doses). Two dose levels (DL) of ibrutinib (DL1: 560 mg, DL2: 840 mg) were evaluated. Primary endpoint is progression-free survival at six months (PFS6); secondary endpoints include safety, tolerability, objective response rate (ORR), duration of response (DOR), progression-free and overall survival. RESULTS We completed accrual on the phase 1b part and report results of 10 evaluable patients (50% women). The median age was 74 years (38-82) and median ECOG 1 (0-1). There was 1 DLT at DL1 (grade 4 transaminases) and 2 DLTs at DL2 (hypertension, flu-like symptoms); the MTD/R2PD of ibrutinib with pembrolizumab and rituximab is 560mg. The most common grade 3/4 adverse events were elevated transaminases and decrease in lymphocyte counts. There were 13 dermatologic events, all < grade 3. Reasons for discontinuing treatment include progression (6), consent withdrawal (1), toxicity (2). The ORR is 80% (6 complete responses, 2 partial responses). As of June 2023, the median DOR is 177 days (49-619); PFS6 is 50%. CONCLUSIONS The combination of pembrolizumab, ibrutinib and rituximab is well-tolerated with manageable side effects and encouraging efficacy. The phase 2 portion of the study is open.