Abstract 1554: PI3Kγδ inhibitor combined with radiation enhances the antitumor immune effect of PD-1 blockade in syngenic murine breast cancer model and humanized patient-derived xenograft model
Authors: Han Min Guk , Jang Bum-Sup , Kang Mi Hyun , Kim In Ah 
Published: July 2021

Abstract
Purpose: Breast cancer is generally viewed as immunologically ‘cold', imposing an immune-suppressive tumor microenvironment (TME) and responding poorly to lone immune checkpoint blockade (ICB). As an adjunct to ICB, radiation therapy (RT) holds promise in terms of in situ tumor vaccination effect, although it is known to promote immune suppression, increasing regulatory T cells (Treg), myeloid-derived suppressor cells (MDSCs), and M2 tumor-associated macrophages (TAMs). It was our contention that combined use of RT and a PI3Kγδ inhibitor to combat immune suppression might enhance the efficacy of ICB.
Methods: Murine breast cancer cells (4T1) were grown in both immune-competent and -deficient BALB/c mice, and tumors were irradiated by 3 fractions of 24 Gy. A PD-1 blockade (10 mg/kg) and a PI3Kγδ inhibitor (IPI145; 15 mg/kg) were then administered every other day for 2 weeks. We analyzed mRNA-sequencing data from humanized patient-derived xenograft (PDX) model to identify differentially activated immune-related pathways. Transcriptomic and clinical data were acquired from The Cancer Genome Atlas (TCGA) pan-cancer cohort, and the ‘xCell' deconvolution algorithm was used to profile immune cellular distributions at certain levels of PIK3C and PIK3CD expression.
Results: In the immune-competent syngenic 4T1 murine tumor model, PD-1 blockade alone led to tumor hyperprogression, whereas a three-pronged strategy of PI3Kγδ inhibitor, RT, and PD-1 blockade significantly delayed primary tumor growth, boosted abscopal effect, and improved animal survival by comparison. The immune-deficient syngeneic 4T1 murine tumor model failed to show this synergism in delaying tumor growth and the abscopal effect. RT significantly increased not only CD8+cytotoxic T-cell fractions but also immune-suppressive Tregcells, MDSCs, and M2 TAMs. However, PI3Kγδ inhibitor significantly lowered proportions of Treg, MDSCs, and M2 TAMs, achieving dramatic gains in splenic, nodal, and tumor CD8+ T-cell populations after triple combination therapy. Triple combination therapy significantly delayed tumor growth in humanized PDX model as well. Analyses of RNA sequencing data of humanized PDX samples showed triple combination increased immune response pathways and increased CD8+ T-cell and decreased Treg and M2 TAM. In the TCGA pan-cancer cohort, higher tumor purity-adjusted Treg/CD8+T-cell and M2/M1 TAM ratios and worse overall patient survival were associated with high PIK3CG (PI3Kγ) or PIK3CD (PI3Kδ) gene expression.
Conclusion: These findings collectively indicate that PI3Kγ and PI3Kδ are clinically relevant targets in an immunosuppressive TME. Combining PI3Kγδ inhibitor, RT, and PD-1 blockade may thus be a viable approach, helping to overcome the therapeutic resistance of immunologically cold tumors such as breast cancer.
Citation Format: Min Guk Han, Bum-Sup Jang, Mi Hyun Kang, In Ah Kim. PI3Kγδ inhibitor combined with radiation enhances the antitumor immune effect of PD-1 blockade in syngenic murine breast cancer model and humanized patient-derived xenograft model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1554.