Abstract 492: Ibrutinib inhibits Interleukin-2 inducible kinase, driving a Th1 selective pressure in human leukemia patients that serves to alleviate tumor-induced immunosupression. (2013)

Abstract Interleukin-2 Inducible Kinase (ITK) is an appealing therapeutic target given its critical role in polarizing T-cell immunity against various infectious, autoimmune, and neoplastic diseases. ITK activity promotes Th2 immunity, thereby subverting healthy Th1-based immune surveillance. Th2 CD4 T-cells depend on ITK for activation, whereas Th1 and CD8 T-cells have compensatory resting lymphocyte kinase (RLK) that conducts T-cell receptor activation in the absence of ITK. Similar to many other tumors, chronic lymphocytic leukemia (CLL) directs an aberrant Th2 bias that drives immune evasion, promotes a supportive microenvironment, and cripples adaptive immunity. In CLL, this immune suppression leads to severe infections which are a leading cause of death. Although ITK inhibitors have the promise of alleviating tumor-induced immune suppression, to date no clinically viable ITK inhibitor exists. Ibrutinib is an irreversible inhibitor of Bruton's tyrosine kinase (BTK) with outstanding activity and tolerability in B-cell malignancies. The striking homology between BTK and ITK combined with our in silico docking studies and in vitro kinase inhibition profiles led us to identify ibrutinib as the first clinically viable irreversible ITK inhibitor. We further confirmed both molecular and functional outcomes using Jurkat T-cells and in vitro polarized Th1 and Th2 CD4 T-cells. We found that mutation of the ITK-Cys442 covalent binding residue for ibrutinib alleviated molecular inhibition. We also demonstrated that Th1 and CD8 T-cell restricted expression of RLK provides a compensatory platform for T-cell activation, offering a molecular explanation for the selective outgrowth of cytotoxic Th1-biased immunity. This effect was confirmed using T-cells directly derived from CLL patients. To clearly demonstrate this effect in vivo, we utilized the Leishmania major model of Th2-mediated infection. Ibrutinib enhanced resistance to the parasite by eliciting robust Th1 immunity. To demonstrate a direct clinical relevance in the setting of CLL we utilized a novel listeriosis/leukemia mouse model. In this model, we observed complete recovery of functional immunity, with all ibrutinib-treated mice surviving a potentially lethal Listeria monocytogenes infection. We further confirmed irreversible binding of ITK in patients currently receiving ibrutinib as part of a clinical trial. Finally, we found that ibrutinib inhibited phosphorlyation of ITK's direct downstream target PLCγ1 in CD4 T-cells. Together, our results provide novel molecular insights into the mechanism of action of ibrutinib in the context of Th2-biased immunosuppressive leukemia. We also postulate that ibrutinib's irreversible ITK inhibitory effects may prove beneficial in a number of other autoimmune, inflammatory, parasitic, and viral diseases. Citation Format: Jason A. Dubovsky, Kyle A. Beckwith, Gayathri Natarajan, Jennifer A. Woyach, Samantha Jaglowski, Joshua Hessler, Ta-Ming Liu, Betty Y. Chang, Karilyn M. Larkin, Matthew R. Stefanovski, Frank W. Frissora, Lisa L. Smith, Kelly A. Smucker, Joseph M. Flynn, Jeffrey A. Jones, Leslie A. Andritsos, Kami Maddocks, Amy M. Lehman, Richard Furman, Jeff Sharman, Anjali Mishra, Michael A. Caligiuri, Abhay R. Satoskar, Joseph J. Buggy, Natarajan Muthusamy, Amy J. Johnson, John C. Byrd. Ibrutinib inhibits Interleukin-2 inducible kinase, driving a Th1 selective pressure in human leukemia patients that serves to alleviate tumor-induced immunosupression. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 492. doi:10.1158/1538-7445.AM2013-492