Abstract 553: Ibrutinib, a BTK inhibitor, impairs the generation and function of myeloid derived suppressor cells
Authors: Stiff Andrew R. , Trikha Prashant , Wesolowski Robert , Kendra Kari , Uppati Sarvani , Abood David , McMichael Elizabeth , Duggan Megan , Campbell Amanda , Muthusamy Natarajan , Tridandapani Susheela , Caliguiri Michael , Byrd John C. , Carson William E. 
Published: July 2016

Abstract
Myeloid derived suppressor cells (MDSC) interfere with anti tumor immune responses. MDSC have also been shown to antagonize the effectiveness of immune based therapies including immune checkpoint blockade. As a result, MDSC have received attention as potential targets for immune based combination therapies. There has been limited success in the identification of clinically active agents with the ability to inhibit the function or generation of MDSC. Ibrutinib is an orally available irreversible inhibitor of Bruton's tyrosine kinase (BTK) that is FDA approved for the treatment of B cell malignancies. In addition to B cells, cells of the myeloid lineage including monocytes and macrophages express BTK, and treatment with ibrutinib has been shown to alter their function and differentiation. As a result, it was hypothesized that ibrutinib would interfere with the function or generation of MDSC in the setting of cancer. MDSC isolated from the spleens of multiple murine tumor models (EMT6, 4T1, and C26) as well as MDSC from patients with metastatic melanoma expressed BTK. Treatment with ibrutinib at doses ranging from 0.1-5 μM inhibited the phosphorylation of BTK in both murine and human MDSC. Ibrutinib treatment of murine and human MDSC resulted in a significant reduction in nitric oxide (NO) production (p&lt; 0.05), but had only modest effects on MDSC levels of IDO and arginase. Ibrutinib was also able to inhibit murine MDSC migration in response to EMT6 cell line conditioned media and the chemokine CXCL12 (p&lt; 0.05). In addition, ibrutinib inhibited human MDSC migration in response to GM CSF (p&lt; 0.05). Ibrutinib reduced the expression of the myeloid adhesion molecules CD11a (p&lt; 0.05) and CD49D (p&lt; 0.01) by MDSC, which could explain the reduction in migration. Importantly, ibrutinib significantly reduced the ability of MDSC to suppress CD8+ T cell proliferation compared to DMSO (21.98% vs. 12.49% proliferation, p&lt; 0.05). Daily treatment with ibrutinib effectively inhibited the in vitro generation of human MDSC from monocytes by promoting HLA DR expression (p&lt; 0.05). Using the EMT6 mammary carcinoma model in vivo, ibrutinib treatment resulted in a significant reduction of MDSC in both the spleen and tumor (p&lt; 0.05). Ibrutinib also reduced the frequency of splenic MDSC in wild type B16F10 tumor bearing mice, but not in BTK mutant XID mice. In addition, both murine and human MDSC did not express significant levels of alternative ibrutinib targets including ITK, Bmx, and Blk. These results suggest that inhibition of BTK is the primary driver behind the observed effects of ibrutinib on MDSC function and generation. Finally, the combination of ibrutinib and anti PDL1 therapy was significantly more effective than either agent alone (p&lt; 0.01 and p&lt; 0.05) producing complete tumor regression in 50% of EMT6 tumor being mice. The results support further investigation of ibrutinib in combination with immune based therapies for solid tumors.
Citation Format: Andrew R. Stiff, Prashant Trikha, Robert Wesolowski, Kari Kendra, Sarvani Uppati, David Abood, Elizabeth McMichael, Megan Duggan, Amanda Campbell, Natarajan Muthusamy, Susheela Tridandapani, Michael Caliguiri, John C. Byrd, William E. Carson. Ibrutinib, a BTK inhibitor, impairs the generation and function of myeloid derived suppressor cells. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 553.