Abstract 4503: Polyamine blockade therapy: A strategy to block immunosuppression in pancreatic cancer

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Abstract Pancreatic ductal adenocarcinoma (PDAC) is characterized by dense desmoplasia which contributes to poor prognosis by hindering tumor entry of immune cells and therapeutics. This leads to PDAC being difficult to treat with gemcitabine, a current standard-of-care, with only a 22% 12-month survival rate. Therapeutic modalities are needed to tackle the PDAC tumor microenvironment (TME). An alternative target is polyamine metabolism which is required for tumor growth, survival, metastasis, and also for modulating generation, survival, and activity of immune cells. Myeloid-derived suppressor cells (MDSCs) are immune cells that have a key role in PDAC metastasis, invasion, progression, and in impeding anti-tumor immune response to therapeutics in PDAC. MDSCs include monocyte-like (M-MDSC) and neutrophil-like (PMN-MDSC) cells. Previous studies have shown that polyamine blockade therapy (PBT), consisting of difluoromethylornithine (DFMO) to block polyamine biosynthesis and a polyamine transport inhibitor (Trimer44NMe), elicits anti-tumor immune response, improved survival, and a decrease in MDSCs in other tumor types. This is of interest given our previous findings that PBT improved survival of PDAC-bearing mice. Here, we tested PBT in altering PDAC-stimulated MDSC survival and whether it shifts their immunosuppressive phenotype. Studies were conducted using CD11b+ bone marrow cells that were differentiated into MDSCs via GM-CSF and tumor-conditioned media from PDAC cells derived from a genetic mouse model for pancreatic cancer (KPC-mice). Bulk RNA-sequencing was conducted on MDSCs treated with PBT, gemcitabine or combination. MDSCs exhibited widespread downregulation of expression in the gemcitabine and combination treatment, but PBT alone exhibited more targeted downregulation. Specifically, Gene Ontology (GO) analysis revealed a decrease in genes associated with immune cell migration and chemotaxis, and KEGG analysis showed downregulation of inflammatory pathways. These findings indicate that PBT may modify MDSCs from their tumor-associated immunosuppressive and pro-inflammatory behavior. MDSC subtype-specific markers were investigated by flow cytometry. Results show subsets of immune cells, such as PMN-MDSCs, are reduced upon PBT treatment alone or in combination with gemcitabine, thereby highlighting the dependence of this subtype on polyamine metabolism. In conclusion, PBT alters the expression of MDSC immunomodulatory genes, decreases MDSC survival and supports the use of PBT for treatment of PDAC in combination with other immunomodulatory therapeutics. Citation Format: Joseph A. Goode, Sai Preethi Nakkina, Manav Gandhi, Otto Phanstiel, Deborah A. Altomare. Polyamine blockade therapy: A strategy to block immunosuppression in pancreatic cancer. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4503.

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