Pancreatic ductal adenocarcinoma exists in a complex desmoplastic microenvironment that provides stromal support for tumor growth and conceals the tumor from immune surveillance. Tumor-associated stroma comprises a mix of fibroblasts, immunosuppressive T regulatory cells (Tregs), myeloid suppressive monocytes (MDSCs) and tumor-associated macrophages (TAMs) that promote tumor growth and restrain immune-mediated tumor cell killing. The targeting of immune infiltrates may impair stromal support and enhance immune-mediated killing of pancreatic cancer cells. Bruton's tyrosine kinase (Btk) is a nonreceptor enzyme in the Tec kinase family expressed among cells of hematopoietic origin including B cells, myeloid cells, mast cells and platelets, but not T cells, where it regulates multiple cellular processes. Here we describe an unexpected finding of ACP-196, a potent, novel, second generation Btk inhibitor with improved selectivity and target coverage that binds covalently to a cysteine residue (Cys481) in the front position of the ATP-binding pocket.

In an orthotopic mouse model of pancreatic cancer, KPC derived pancreatic cancer cells (KrasG12D; Trp53R172H; Pdx1-Cre) were injected into the pancreases. Vehicle, single agent ACP-196 (15 mg/kg/BID, gavage), single agent gemcitabine (50 mg/kg, IV) and combination ACP-196 with gemcitabine were evaluated for efficacy. By 4 weeks of treatment, mice in the vehicle group showed signs of health deterioration and all mice were euthanized, tumors were collected and measured. Relative to the vehicle treatment, ACP-196 monotherapy resulted in a >2-fold reduction in tumor growth compared with less than a 2-fold reduction with gemcitabine alone. The combination of ACP-196 and gemcitabine resulted in a further reduction in tumor growth when compared to each single agent. Interestingly, analysis of tumor tissues showed that single agent ACP-196 inhibited immunosuppressive populations of TAMs and MDSCs. Surprisingly, Treg populations were also reduced with a robust expansion of CD8+ T cells in the tumors. None of these effects were observed with gemcitabine alone. Although Btk is not expressed in T cells, this finding maybe the result of inhibiting the MDSC and TAM populations within the tumor microenvironment, a mechanism of action which is currently under investigation. Taken together, these data identify Btk as a novel target for modulating tumor immune escape and suggest that pharmacologic targeting of suppressive myeloid cells by ACP-196 induces therapeutic benefit. ACP-196 is currently being evaluated in clinical trials including frontline and salvage pancreatic cancer.

Citation Format: Brian J. Lannutti, Michael Gulrajani, Fanny Krantz, Elena Bibikova, Todd Covey, Katti Jessen, Wayne Rothbaum, David M. Johnson, Roger Ulrich. ACP-196, an orally bioavailable covalent selective inhibitor of Btk, modulates the innate tumor microenvironment, exhibits antitumor efficacy and enhances gemcitabine activity in pancreatic cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 408. doi:10.1158/1538-7445.AM2015-408