REVERSAL OF IMMUNE TOLERANCE AND INCREASED SURVIVAL AFTER XPO1 AND BTK INHIBITION IN MOUSE MODELS OF PRIMARY CNS LYMPHOMA (PCNSL)

Introduction: On the grounds of frequent deregulated chronic BCR signaling in PCNSLs, targeting the BCR pathway is emerging as a promising therapeutic avenue for these poor-prognostic patients. Unfortunately, responses to the inhibition of BCR signaling with ibrutinib are scarce and short-lasting. Of note, inhibition of the XPO-1 activity with selinexor has been shown to impede BCR and NF-kB signaling, while having excellent brain penetrance. Herein, we studied the activity of selinexor alone or in combination with ibrutinib in pre-clinical mouse models of PCNSL. Methods: Proliferation and survival analysis in DLBCL cell lines were performed by MTS and Annexin V-propidium iodide assays. Orthotopic xenograft models were established by the injection of ABC-DLBCL cells transfected with luciferase or patient-derived PCNSL cells into the brain parenchyma of athymic mice. Tumor growth was monitored by bioluminescence. Immunohistochemical (IHC) detection was performed using human anti-CD20, human anti-Ki-67 and mouse anti-F4/80 antibodies. Changes in the proportion of M1/M2 macrophages and in the expression of immune checkpoints were studied by flow cytometry. Results: Firstly, we observed that in vitro sensitivity to selinexor was independent of the cell of origin, but that a strong synergy with ibrutinib occurred mainly in ABC-DLBCL cells. Accordingly, after treating the OCI-Ly10 (ABC origin) bearing mice with 5mg/kg selinexor or vehicle three times a week, it was found that selinexor significantly blocked tumoral growth prolonging mice survival (median: 48 days vs. 34 days). Next, for the purpose of evaluating potential synergy between ibrutinib and selinexor, PCNSL mice were distributed in four groups: selinexor (5mg/kg twice a week), ibrutinib (25mg/kg daily), combination of both or vehicle. Although tumoral growth was blocked by all treatments, the combination of both drugs significantly increased survival compared to monotherapies. Further analysis of the innate immune microenvironment by flow cytometry and IHC showed that PCNSLs were infiltrated by macrophages, particularly by pro-tumoral M2 expressing PD1 and SIRPα. Notably, the treatment with the combination of drugs increased the proportion of inflammatory M1 macrophages, while the remaining M2 macrophages expressed lower levels of the immune checkpoints PD1 and SIRPα. Conclusions: Our results show that selinexor blocks PCNSL tumor growth and prolongs survival, while its combination with ibrutinib further increases survival. Additionally, we show that PCNSL in mice is infiltrated by tumor-promoting PD1+ and SIRPα+ M2 macrophages. Treatment with both drugs favors an anti-tumoral immune response by shifting polarization toward anti-tumoral M1. These observations provide pre-clinical evidence for the development of selinexor as a new therapeutic option for PCNSL. Keywords: B-cell receptor (BCR); macrophages; primary CNS lymphoma (PCNSL). Disclosures: Abrisqueta, P: Honoraria: Roche, Janssen and AbbVie. Bosch, F: Honoraria: Roche, Celgene, Takeda, Astra-Zeneca, Novartis, AbbVie, Janssen; Research Funding: Roche, Celgene, Takeda, Astra-Zeneca, Novartis, AbbVie, Janssen.