Abstract 3209: BLZ945 and anti-PD-1 combination immunotherapy modulates the immune landscape in pancreatic ductal adenocarcinoma

Background: Immune therapies have shown great promise for some cancers. To date, pancreatic ductal adenocarcinomas (PDAC) have been largely resistant to immunotherapeutic approaches in part due to their exclusion of tumor infiltrating lymphocytes. The mechanisms by which this exclusion occurs are not well-characterized but are thought to involve both innate and adaptive immune responses. Methods: The primary objective of this study was to investigate how dual anti-PD-1/anti-CSF1R therapy changes the tumor immune cell niche and tumor growth in an allograft version of the KPC mouse model of PDAC (Pdx-1-Cre; LSL-KrasG12D; LSL-Trp53 R172H). Anti-PD1 therapy in the form of monoclonal antibody from BioXCell® was dosed twice weekly via intraperitoneal injections at 200µg/mouse. Anti-CSF1R in the form of the small-molecule inhibitor BLZ945 from Selleckchem® was dosed daily via oral gavage at 200 mg/kg. Human THP-1 cells differentiated and polarized to macrophages were used for viability and gene expression analyses. Results: In B6 recipient mice, the combination anti-PD-1 plus BLZ945 resulted in reduced percent tumor volume change (11% ±17) versus vehicle treated controls (121% ±174) (p=0.02). Flow cytometry of the spleens of tumor-bearing mice demonstrated an increase in conventional dendritic cells with combination treatment compared to controls (15.3 ±2.3% vs 8.3 ±0.6%, respectively; p=0.02). Mice lacking conventional dendritic cells (B6.BATF3-/-) had an increased percent tumor volume change (84% ±78) compared to B6 wild type mice (11% ±16) (p=0.01) when receiving combination treatment. IHC staining of allografted tumors revealed a trend towards increased infiltrating CD8+ T cells/hpf in B6 wild type mice treated with anti-PD-1 or combination therapy (21±24/hpf) compared to control treated mice (11±18/hpf). A similar trend was observed in B6.BATF3-/- mice (2.9±4.4/hpf vs 0.5±0.50/hpf); however, the magnitude was greatly reduced compared to B6 wild type mice. Interestingly, BLZ945 treated tumors did not show evidence of statistically significant reduced macrophage recruitment in vivo (268 vs 241, p=0.16). Macrophage viability in vitro was also unaffected by BLZ945 treatment (fold change in viability 0.03, p=0.6). Preferential changes in macrophage polarization in vitro were observed, with significant reductions in M2 polarization markers Arg-1, YM-1, and CD206 (p= 0.0005, 0.0029, and 0.012, respectively). Conclusions: The combination of an anti-PD-1 agent in combination with inhibition of the macrophage-predominant cytokine, anti-CSF1R, slows PDAC tumor growth in a conventional dendritic cell-dependent manner. Further characterization of the interplay between innate and adaptive immune cells upon immunotherapy treatment should be investigated to increase the efficacy of these therapeutics. Citation Format: Chelsie K. Sievers, Philip B. Emmerich, Hanna R. Rainiero, Connor Maloney, Rosabella Pitera, Cheri A. Pasch, Linda Clipson, Kristina A. Matkowskyj, Fotis Asimakopoulos, Dustin A. Deming. BLZ945 and anti-PD-1 combination immunotherapy modulates the immune landscape in pancreatic ductal adenocarcinoma [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 3209.