e14595 Background: The endocannabinoid system is widely expressed in the human body, including the innate and adaptive immune system, where endocannabinoids, Δ9-tetrahydrocannabinol and synthetic ligands regulate immune suppression. The effects of endocannabinoids on immune regulation are primarily mediated by G-protein coupled cannabinoid CB2 receptors (CB2R) via several mechanisms, including immune cell development, migration, proliferation and T and NK cell effector functions. The upregulated expression of CB2R and elevated levels of endocannabinoids have been observed in a variety of tumor microenvironments and are associated with the aggressiveness of cancer. Methods: NK cell function was determined by co-culturing TT-816 pretreated NK cells with K562 cancer cells for 24 hours. The mixed lymphocyte reaction assay was conducted by co-culturing human CD4+ T cells with monocyte-derived dendritic cells. Cell viability was measured by FACS and IFN-g by MSD. The in vivo efficacy of TT-816 was determined in the murine syngeneic B16F10 melanoma and MC38 colorectal cancer models. Tumor-bearing C57BL/6 mice were randomized 6 days after inoculation and treated with vehicle (0.5% CMC, QD) or with TT-816 at 1, 4 or 10 mg/kg (QD) orally, or anti-PD1 (RMP1 14) at 5 mg/kg (Q3D), intraperitoneally. Tumor size and body weight were determined every 2 to 3 days. Data represent mean±SEM. Results: TT-816 concentration-dependently enhanced the functions of NK cells, dendritic cells and T cells. It increased NK cell killing of the human cancer cells and IFN-g production, significantly stimulated the expression of CD86, HLA-DR, IL-12 and TNF-a in monocyte-derived dendritic cells, and enhanced CD4+ T cell proliferation and IFN-g production in a mixed lymphocyte reaction assay. In B16F10 melanoma and MC38 colorectal cancer models, TT-816 dose-dependently inhibited the tumor growth. Furthermore, combination of TT-816 with anti-PD1 synergistically inhibited both tumor growth. In B16F10 model, treatment with TT 816 (4 mg/kg, QD) alone significantly inhibited the tumor growth by 45%. By contrast, anti-PD1 had little effect (TGI 5%). However, the combination of TT-816 with anti-PD1 resulted in a TGI of 72%. In the MC38 model, treatment with TT-816 (4 mg/kg, QD) and anti-PD1 alone, achieved TGI of 50% and 36%, respectively. When combined, TGI was increased to 88%. Conclusions: TT-816 is a novel, oral small molecule immune response modifier that selectively blocks CB2R on immune cells and cancer cells. Preclinical data have demonstrated that it stimulates antitumor immune response and inhibits cancer growth in both hot and cold tumor models. TT-816 is currently undergoing phase 1 clinical trials for the treatment of a broad range of solid tumors.