Abstract 529: A PI3Kαδ inhibitor combined with irradiation enhance the effect of PD-1 blockade in syngenic triple negative breast cancer model

Background:Breast cancer has been showing relatively poor response to immune checkpoint blockades potentially due to low immunogenicity of tumor and immunosuppressive tumor microenvironment. Radiation (RT) has been showing immune stimulatory effect and PI3Kδhas immune suppressive effect via modulation of regulatory T cell (Treg). Therefore, we hypothesize that in situ tumor vaccination via RT and suppression of immune tolerance via PI3Kαδ inhibition could enhance the clinical efficacy of immune checkpoint blockade. As PI3Kα controls a key oncogenic signaling pathway, we evaluated whether RT or novel PI3Kαδ inhibitor or both could enhance the efficacy of antitumor effect of a PD-1 blockade in immune competent syngenic triple negative breast cancer (TNBC) model. Materials & Methods:4T1 murine breast cancer cells were grown subcutaneously in the hind limb of BALB/c mice. The mice were grouped as following seven groups: control, RT, PI3Kαδ inhibitor, PI3Kαδ inhibitor + RT, PD-1 blockade, PD-1 blockade + RT, and triple combination group. Tumors were irradiated using 24 Gy/3 fractions. PD-1 blockade (10 mg/kg) and PI3Kαδ inhibitor (4 mg/kg) were administered every other day for two weeks, respectively. Tumor size was measured periodically using a Vernier caliper and bioluminescence imaging to evaluate efficacy of each modality and combination therapy. Immune-modulatory function was evaluated using FACS and immunohistochemical (IHC) study. Results:Triple combination of RT, PD-1 blockade, and PI3Kαδ inhibitor significantly delayed tumor regrowth whereas PD-1 inhibitor alone showed only modest effect in 4T1 syngenic TNBC model. FACS and IHC study for immune repertoire using tumor samples showed that RT and PD-1 blockade modestly increased the proportion of cytotoxic CD8+ T cells and PI3Kαδ inhibitor led to decrease the proportion of Treg. Triple combination showed remarkable increase of cytotoxic CD8+ T cells suggesting synergistic immune modulatory effect of RT, PD-1 blockade and PI3Kαδ inhibitor. Triple combination led to significant upregulation of c-GAS/STING pathway in the tumor and increased IFN-γ level in blood as well compared to each modality alone. Conclusions:Taken together, combination of RT and PI3Kαδ inhibitors maximized immune stimulatory effect in immune competent syngenic TNBC model and enhanced the response of PD-1 blockade via non-redundant synergistic immune modulatory effect. This study provides a preclinical rationale for the combination of PI3Kαδ inhibitor and RT with PD-1 blockade to overcome the immune tolerance of breast cancer. (Work supported by the grants from NRF #2017R1A2B4002710 & #2017M2A2A7A01018438 to In Ah Kim) Citation Format: Ji Min Park, Joo Ho Lee, Danhyo Kim, Eric Eunsik Kim, Jun Hyung Bae, In Ah Kim. A PI3Kαδ inhibitor combined with irradiation enhance the effect of PD-1 blockade in syngenic triple negative breast cancer model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 529.