Introduction: In normal tissue homeostasis, interaction of phosphatidylserine externalized on apoptotic cells (ACs) with the TAM (Tyro3, Axl MerTK) family RTK MerTK, via its ligands Gas6 and Protein S, leads to AC phagocytosis (efferocytosis). The resulting clearance of AC antigens, immunosuppressive M2 macrophage polarization, and suppression of pro-inflammatory cytokine production promotes tolerance to AC-derived self-antigens. This homeostatic response is coopted in tumors, which are abundant in both ACs and TAM ligands, leading to a blunted anti-tumor immune response. Syngeneic tumors implanted in MerTK -/- mice exhibit poor growth and metastasis, correlating with enhanced production of pro-inflammatory cytokines, splenocyte proliferation, and decreased IL-10 compared with those implanted in WT mice. Moreover, MerTK aberrantly expressed on hematological and epithelial malignancies promotes survival and chemoresistance. Thus, pharmacological inhibition of MerTK may have clinical benefit by increasing availability of dead tumor cell antigens and promoting an anti-tumor immune response, or by directly blocking tumor cell survival. We have therefore developed small molecule inhibitors of MerTK.
Methods: MerTK kinase activity was assayed using ADP-Glo. Cellular MerTK activity was stimulated in HUVEC or H1299 cells using anti-MerTK crosslinking and measured by immunoprecipitation followed by anti-phospho-MerTK blot, or by downstream phospho-Akt Ser 473 using HTRF. A high content assay was used to measure cell number, apoptosis and proliferation. Effects on immune function were tested in human primary dendritic cells (LPS-induced IL-23 production) and human primary T cells (anti-CD3/CD28-induced IL-2 production or IL-2 induced phospho-STAT5). Efferocytosis of apoptotic Jurkat cells by human primary macrophages was detected by flow cytometry. For the PD assay, MerTK expressing tumors were grown in nude mice. 30-60 minutes post-compound dosing, MerTK was stimulated in vivo for 1hr. Tumors were snap frozen and tumor lysates were blotted with anti-phospho-MerTK antibodies. Anti-tumor efficacy was studied in syngeneic models.
Results: Here we describe novel MerTK-selective and Mer-Axl small molecule inhibitors that potently block MerTK in biochemical assays. These compounds exhibit selectivity for TAM family members in an in vitro kinase panel. In cells, antibody-induced MerTK phosphorylation as well as downstream phosphorylation of Akt was inhibited by both classes of compounds with EC50 <100nM. Moreover, MerTK kinase inhibition blocked efferocytosis. Overt cytotoxic or antiproliferative effects were not observed and, importantly, compounds did not block the activity of TLR and T cell immune effector pathways. Compounds inhibited MerTK phosphorylation in tumor tissue in vivo. Anti-tumor activity of these novel MerTK inhibitors is under investigation in syngeneic mouse models both as single agents and in combinations.
Conclusions: We have discovered potent and novel small molecule inhibitors of MerTK that may have clinical benefit by both direct anti-tumor effects and by enhancing the anti-tumor immune response.
Note: This abstract was not presented at the conference.
Citation Format: Sacha J. Holland, Alexander M. Owyang, Sylvia Braselmann, Chrystelle Lamagna, Sothy Yi, Chi Young, Roy Frances, Arthur Bagos, Meagan Chan, Ernest Tai, Stacey Siu, Gary Park, David Lau, Matt Duan, Rao Kolluri, Jiaxin Yu, Ihab Darwish, Somasekhar Bhamidipati, Donald G. Payan, Esteban Masuda. Small molecule inhibitors of the anti-inflammatory TAM receptor MerTK [abstract]. In: Proceedings of the Second CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; 2016 Sept 25-28; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(11 Suppl):Abstract nr B060.
- ©2016 American Association for Cancer Research.