The tumor microenvironment can provide a strong limitation to the antitumor activity of immunotherapy. Established tumors can escape immune responses by creating an inflammatory milieu that supports and recruits tumor-infiltrating myeloid cells (TIMs) with immune suppressive functions. Colony stimulating factor-1 (CSF-1) is a cytokine that is secreted by various cancer types including melanoma. It binds to colony stimulating factor-1 receptor (CSF-1R) on the myeloid cells. It stimulates the proliferation and recruitment of immunosuppressive M2-macrophages and myeloid derived suppressor cells (MDSC) into the tumor microenvironement. In addition, neurotrophins are a family of growth factors that includes nerve growth factor (NGF). NGF is the ligand for a family of tyrosine kinase receptors (Trks) that includes Trk-A, -B, and -C. Signal transduction through the Trk receptors regulates tumor growth. Trk receptors have been shown to be overexpressed in different cancer types. For example, Trk-C is overexpressed in 60% of patients with melanoma at various stages.

PLX7486 is a novel orally bioavailable small molecule Trk and CSF-1R dual-inhibitor that is now being studied in Phase I clinical trials. PLX7486 can target Trks directly on cancer cells and block myeloid cell recruitment to the tumor microenvironement through CSF-1R. We hypothesize that PLX7486 would synergize with immunotherapies to result in a greater antitumor effect by inhibiting the recruitment of immunosuppressive macrophages.

In order to block the Trk receptors on cancer cells with PLX7486, we first confirmed the expression of Trk receptors in murine cancer cell lines, such as MC38, SA1N, MB49, 4T1, B16F10, and 3LL. These cancer cell lines have also been exposed to PLX7486 with a broad range of concentrations (0.1 to 10 μM). We have shown that PLX7486 has a direct cytotoxic effect with IC50 of 5-8μM on most of the cancer cell lines at a 72 hour time point. Futher, the exposure of MC38 to PLX7486 resulted in the expected effects of inhibiting downstream AKT pathway signaling. In addition, PLX7486 has also been shown to have cytotoxic direct effect on bone marrow-derived macrophages and murine macrophage cell line RAW264.7 with IC50<1μM, including inhibition of the MAPK pathway. In order to test whether PLX7486 is toxic to T-cells, pmel-1/Thy1.1+ T-cells were adoptively transferred into mice, which then received daily oral gavage with PLX7486 (20mg/kg) for 5 days. Interestingly, the amount of pmel-1/Thy1.1+ T-cells increased in the spleen (vehicle: 1.76±0.2%, PLX7486: 3.52±0.4%). We then tested the combination of PLX7486 and immunotherapy reagents such as anti-CTLA-4 and anti-PD-1 in vivo in MC38 and B16F10 tumor models. We have observed a superior antitumor effect in the combined therapy group compared with either single treatment group alone.

In conclusion, our promising data derived from in vivo models combining Trk/CSF-1R blockade and immunotherapy supports the rationale for further testing of such combination in patients with cancer.

This abstract is also being presented as Poster A06.

Citation Format: Stephen Mok, Colm Duffy, Reginald Du, James P. Allison. Blocking Colony Stimulating Factor 1 Receptor (CSF-1R) and Tropomyosin Receptor Kinase (Trk) improves antitumor efficacy of immunotherapy. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2016 Oct 20-23; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2017;5(3 Suppl):Abstract nr PR01.