Abstract LB-123: Pulsatile inhibition of PI3K converts immune suppression by Tregs and M2-TAM to anti-tumor immune response in animal models insensitive or resistant to the monotherapies of PI3K and checkpoint inhibitors

Despite remarkable results in treating certain cancers, immune checkpoint inhibitors (ICIs) are lack of activity in ‘cold9 tumors and not always effective in inflamed ‘hot9 tumors. In addition to aberrant activation in cancer cells, the PI3K pathway plays both positive and negative roles in immune response. Therefore, the overall outcome of PI3K inhibition on anti-tumor immunity and the combination strategy with ICIs should be carefully investigated. In this study, PI3K inhibition alone and in combination with anti-PD-1/L1 were evaluated in a set of 8 representative syngeneic tumor models non-responding or insensitive to ICIs. We first compared an oral pan-PI3K inhibitor BAY 1082439 dosed continuously (QD) versus intermittently (2On/5Off), and performed a comprehensive evaluation of an approved intravenously dosed pan-PI3K inhibitor copanlisib (2On/5Off). We found that intermittent treatment, regardless oral or iv, produced better anti-tumor efficacy, increased intratumoral CD8 + /T reg ratio and a better safety profile, while continuous inhibition of PI3K led to a significant reduction in splenic B cells and lymphoid organ weight. In addition, intermittent treatment of PI3K inhibitor with subsequent combination of anti-PD-1 induced complete tumor regression in 50-100% of animals bearing T reg high A20 tumor compared to 0% response in the monotherapy groups. Synergistic combination was also demonstrated in M2-TAM high CT26 and MC38 CRC models. Of note, no tumor growth was observed in a re-challenge study conducted 3 months post complete tumor regression in the combination group of the CT26 model, indicating that tumor specific memory T cells were generated which prevented tumor recurrence. Analysis of tumor infiltrating leukocytes revealed significant reductions in T reg and/or M2-TAM but increases in CD8 + T cell, Granzyme B + cell, M1-TAM and activated DC. The findings in mice were also confirmed in human PBMC derived T reg (induced by TGF-β) and M2 (induced by M-CSF/IL-4) differentiation assays. The pan-PI3K inhibitor copanlisib, but not a PI3Kδ selective inhibitor, effectively blocked TGF-β-induced T reg (CD25 + /FoxP3 + ) differentiation, despite both inhibitors suppressing CD3-stimulated T reg proliferation. Furthermore, copanlisib not only blocked M2 differentiation, but also could redirect differentiated M2 to immune stimulating M1 even in the presence of M-CSF/IL-4. Taken together, pulsatile pan-PI3K inhibition could effectively convert an immune suppressive effect observed with continuous treatment to a favorable anti-tumor immune response. Combination of intermittently dosed PI3K inhibitor copanlisib with ICIs therefore might be a promising strategy to overcome the resistance induced by intratumoral oncogenic signaling and an immune suppressive tumor microenvironment. Citation Format: Sarah Glaeske, Franziska Huebner, Anna Anurin, Andreas Janzer, Sabine Zitzmann-Kolbe, Juliane Paul, Katja Glaeske, Sandra Berndt, Dominik Mumberg, Matyas Gorjanacz, Karl Ziegelbauer, Bertolt Kreft, Ningshu Liu. Pulsatile inhibition of PI3K converts immune suppression by T reg s and M2-TAM to anti-tumor immune response in animal models insensitive or resistant to the monotherapies of PI3K and checkpoint inhibitors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr LB-123.