Abstract C107: Preclinical response of murine syngeneic tumor models to immune checkpoint inhibitor antibodies

Tumor cell viability and proliferation is regulated by the balance between tumor self-survival and host elimination mechanisms. There are multiple mechanisms whereby tumors successfully evade host immune cell surveillance, permitting continued growth and proliferation. Immunotherapies are aimed at bolstering the host immune system. Delivery of these agents has been shown to actively eliminate disease in a number of patients to date. We evaluated several syngeneic mouse tumor lines for anti-tumor activity with anti-CTLA-4 and/or anti-PD-L1 antibodies, as well as the immunological changes occurring following treatment with these checkpoint inhibitors. The effectiveness of these inhibitors was assessed in the B16.F10 (melanoma), RIF-1 (sarcoma), 4T1-luc (mammary) and CT26 (colon) murine tumor models. Cells or tumor fragments (RIF-1) were implanted SC in the axilla. The anti-CTLA-4 9D9 antibody clone from Bio X Cell (West Lebanon, NH) was employed for responsiveness in each model. All treatments were initiated 3 days post implantation, prior to the appearance of established tumor, at inoculums known to have 100% take rates. The CT26 model was highly responsive with all animals showing tumor regressions and 70% tumor-free survivors at study end. The RIF-1 model was moderately responsive with several mice showing long term progression-free survival and a 56% increase in lifespan vs. the isotype control group. Both the B16.F10 and the 4T1-luc models were non-responsive. Tumor responsiveness to the anti-PD-L1 10F.9G2 antibody clone (Bio X Cell) was also compared in the RIF-1 and CT26 models. In contrast to the anti-CTLA-4 antibody studies, RIF-1 was found to be non-responsive and CT26 was moderately responsive to the anti-PD-L1 antibody. Evaluating immunological endpoints by flow cytometry is critical when assessing the effectiveness of immune checkpoint inhibitors. To this end, the immune profiles of both responsive (CT26) and non-responsive (4T1 and B16.F10) models were examined in the absence or presence of antibody treatments. Initial studies were focused on analysis of T cells (CD4 and CD8) as well as T regulatory cells (Tregs; CD4/CD25/FoxP3) and myeloid derived suppressive cells (MDSC; CD11b/Gr1). The 4T1 model was found to be highly MDSC driven and would be suitable to evaluate agents that target myeloid-derived cell populations. While this model was not very responsive to anti-CTLA-4 treatment, a trend toward decreased MDSCs following treatment was observed with anti-CTLA-4 and not the isotype control antibody. In contrast, the CT26 model was more highly mixed and contained populations of both Tregs and MDSCs. Treatment with anti-CTLA-4 increased the CD8+ T cell population and decreased the Treg cell population with minimal effect on the MSDC population. However, in this same model, anti-PD-L1 treatment increased CD8+ T cells and decreased both Tregs and MDSCs. A more complete mechanistic understanding of how the immune system responds to checkpoint inhibitors permits identification of appropriate models for monotherapy, as well as the rationale for designing drug combination studies. Data enhancing understanding of the above, along with tumor profiling on additional syngeneic models and more flow cytometry multiplexing, is expected from further ongoing research. Citation Format: Maryland Franklin, Matt Thayer, Chris Elders, Dan Saims, Scott Wise. Preclinical response of murine syngeneic tumor models to immune checkpoint inhibitor antibodies. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr C107.