Abstract 57: Ibrutinib improves chimeric antigen receptor T cell control of leukemia by inhibiting myeloid-derived suppressor cells

Abstract Background: In patients with chronic lymphocytic leukemia (CLL), we and others have shown that the addition of the BTK inhibitor ibrutinib to CART cells increases the rates of durable complete responses. The mechanism for this remains unknown. Lymphodepletion (LD) is considered critical for CART cell therapy success, yet emergency myelopoiesis during recovery from LD may induce a surge of myeloid-derived suppressor cells (MDSC), particularly in patients with advanced cancer. BTK inhibition has been shown to inhibit MDSC function in vitro. We therefore hypothesized that administration of ibrutinib during and following LD could reduce MDSC-mediated suppression of CART cells and thereby improve CART cell outcomes. Methods: To explore the effects of ibrutinib on human MDSCs, we engineered suppressive monocytes through in vitro culture of CD14+ cells in the presence of IL-6 and GM-CSF. We added ibrutinib to the culture on Days 0-4 and compared the ability of the monocytes to inhibit proliferation of human CAR-T cells. To model CLL treatment with CART in immunocompetent mice, we used the Em-TCL1 adaptive transfer model of CLL. Mice were treated with ibrutinib (I), cyclophosphamide and fludarabine (FC), or ibrutinib + FC (FC+I). MDSCs were isolated from these mice for in vitro function studies. In subsequent experiments mice were also treated with syngeneic CART19 with or without FC or FC+I. Results: In vitro generated human MDSC suppressed CART19 function, and this was prevented by pre-treatment of MDSC with ibrutinib. Myeloid cells from the spleens of CLL-bearing mice inhibited syngeneic CART19 function ex vivo regardless of treatment with FC or I alone, but myeloid cells harvested from CLL-bearing mice treated with FC+I lost suppressive function. Finally, mice treated with CART19 after a LD regimen resembling that in our recently reported clinical trial (FC+I) showed a survival advantage over all other groups. Conclusions: BTK inhibition with ibrutinib prevents myeloid cell suppression of CART cells in vitro in human and murine systems. When combined with lymphodepleting chemotherapy (FC), ibrutinib enhances CART cell control of CLL in vivo, possibly by preventing MDSC-mediated suppression of CART cells. Citation Format: Benjamin F. Frost, Olga Shestova, Feng Shen, Chia Sharpe, John C. Byrd, Saar I. Gill. Ibrutinib improves chimeric antigen receptor T cell control of leukemia by inhibiting myeloid-derived suppressor cells abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 57.