Abstract 2321: Ibrutinib enhances antitumor activity in solid tumors when combined with checkpoint inhibitors
Authors: Hsu Jeff , Huang Yujun , Chen Chun-Te , Chen Jun , Sirisawad Mint , Chang Betty Y. 
Published: July 2016

Introduction: Anti-CTLA-4 antibodies enhance antitumor immunity by activating antigen-specific cytotoxic T lymphocytes and depleting suppressive regulatory T cells (Treg) from the tumor microenvironment. Anti-PD-L1 or anti-PD-1 blocks negative regulators of T-cell activation and response, facilitating immune-mediated antitumor activity. These therapeutic antitumor approaches via immune checkpoint blockade may be enhanced by ibrutinib (ibr), a first-in-class oral inhibitor of Bruton's tyrosine kinase (BTK) and interleukin-2-inducible T-cell kinase (ITK). A previous study suggested ibr may alter the Th1/Th2 T-cell balance by inactivating ITK and potentially enhancing antitumor immune responses in solid tumor models, including those that do not express BTK (Sagiv-Barfi. PNAS. 2015). Here we report the combined effects of ibr and CTLA-4/PD-1/PD-L1 blockade in a panel of established syngeneic solid tumor models.

Methods: Immunocompetent mice with matched genetic background were inoculated subcutaneously in the right front flank with syngeneic tumor cells. Treatment began when mean tumor size reached approximately 70-100 mm3. Tumor growth inhibition (TGI) and tumor growth delay (TGD) were the study endpoints. The mean and the standard error of the mean for each group were calculated at each time point. Statistical analysis of independent sample t-tests and one-way ANOVA were used for comparisons between and among groups. Potential synergistic effects between treatments were analyzed by two-way ANOVA. Tumor and blood samples were collected for immunologic analysis at the tumor site or in peripheral compartments by flow cytometry.

Results: Treatment with ibr alone resulted in ∼15%-50% TGI across various syngeneic subcutaneous mice models, including MBT-2 (bladder cancer), MC38 (colorectal cancer), and Pan02 (pancreatic tumor), where BTK expression is not prevalent in the tumor lines and tumor cells are not sensitive to ibrutinib. Ibr in combination with checkpoint inhibitors demonstrated benefits in both TGI and TGD in the MC38 model. When ibr was combined with anti-CTLA-4 monoclonal antibody, complete regression of established MC38 tumors was observed.

Conclusion: The efficacy observed with ibr treatment alone in solid tumor models lacking BTK expression suggests ibr can lead to immunomodulation by altering the tumor microenvironment. Additionally, synergistic growth suppression was observed when ibr was combined with checkpoint inhibitors. To further understand the mechanism of action associated with combination ibr immunotherapy in syngeneic models, immunophenotypical analyses of tumor tissues are under investigation.

Citation Format: Jeff Hsu, Yujun Huang, Chun-Te Chen, Jun Chen, Mint Sirisawad, Betty Y. Chang. Ibrutinib enhances antitumor activity in solid tumors when combined with checkpoint inhibitors. [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 2321.