Abstract 5610: Targeting Rb1 in experimental pancreatic cancer remodels immune cell landscape and suppresses tumor progression
Authors: Tcyganov Evgenii N. , Kwak Taekyoung , Poli Adi Narayana Reddy , Donthireddy Laxminarasimha , Cassel Joel , Kossenkov Andrew , Shinde Rahul , Stanger Ben Z. , Salvino Joseph , Montaner Luis J. 
Published: June 2022

Abstract
Background: Pancreatic ductal adenocarcinoma (PDA) remains among the cancers with the poorest prognosis. Currently, no effective treatment available for PDA. Immunotherapy that has been shown effective for several cancer types is not successful for PDA treatment overall. The possible reasons of this PDA immunotherapy resistance include lack of neoantigen mutations, low T cell responses and overall immunosuppression state in the tumor microenvironment mediated by myeloid cells and tumor stroma. Recent studies suggest that targeting tumor microenvironment can overcome this immunotherapy resistance and significantly improve the disease outcome. Retinoblastoma (Rb1) protein is a known tumor suppressor protein. In addition, there is accumulating evidence for its role in immune cell response as well, including regulation of apoptosis in myeloid cells. However, the mechanisms of this phenomenon remain not clear. Here we show that targeting Rb1 with a novel molecule AP-3-84 developed in our lab causes apoptosis of tumor-associated macrophages (TAMs) in the model of PDA, induces T cell infiltration of the tumor and decreases tumor burden.
Results: Recently our lab has developed the new small molecule AP-3-84 that is able to bind Rb1 and modulate its activity. Originally, we found that this Rb1 modulator can induce cell death of TAMs and thioglycolate-induced macrophages, but not of the other cell types (neither tumor nor T cells). Gene expression changes upon AP-3-84 treatment showed the induction of oxidative and ER stress programs with a clear activation of p53-dependent genes and caspases. We tested the use of AP-3-84 as immunotherapy against PDA cancer growth using 2 PDA cell lines intrinsically characterized by low T cell and high T cell infiltration (T-low-PDA and T-high-PDA, respectively, originally generated and characterized by J. Li et al., Immunity 2018). In accordance with our in vitro observation, AP-3-84 was able to reshape immune cell landscape in mice with T-high-PDA by depleting macrophages and inducing T cell infiltration. Importantly, those changes were accompanied by significant reduction of tumor growth. Addition of AP-3-84 to immunotherapy (anti-CD40/PD1/CTL4) was also able to decrease tumor burden in T-low-PDA model, which is otherwise resistant to this immunotherapy. Thus, AP-3-84 treatment was able to significantly improve the outcome of experimental PDA by altering immune cell subsets and represent a prospective clinical approach for the clinic.
Statistical analysis of the data was conducted using unpaired Student t-test with additional false discovery rate adjustment for multiple comparisons for gene expression analysis.
Conclusions: Modulation of Rb1 in myeloid cells is a novel promising target for PDA treatment. This approach reduces PDA tumor burden by shifting the immune cell landscape towards greater T cell infiltration upon tumor macrophage depletion.
Citation Format: Evgenii N. Tcyganov, Taekyoung Kwak, Adi Narayana Reddy Poli, Laxminarasimha Donthireddy, Joel Cassel, Andrew Kossenkov, Rahul Shinde, Ben Z. Stanger, Joseph Salvino, Luis J. Montaner. Targeting Rb1 in experimental pancreatic cancer remodels immune cell landscape and suppresses tumor progression [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5610.