Abstract 1565: Targeting stromal-specific p38 MAPK signaling to stifle inflammatory reprogramming of cancer-associated fibroblasts in pancreatic cancer

Introduction: Major contributors to therapeutic resistance in pancreatic ductal adenocarcinoma (PDAC) are the desmoplastic stroma that acts as a barrier to drug delivery and effector immune cell infiltration. In our efforts to identify mediators of therapeutic resistance, we identified the pro-inflammatory cytokine interleukin-1α (IL-1) to be critical in the activation and reprogramming of neighboring cancer-associated fibroblasts (CAFs) within the stroma to a pro-inflammatory phenotype to promote myeloid cell chemotaxis. Our mechanistic studies have identified the p38 MAPK pathway as a novel mediator of IL-1-induced CAF activation from quiescent stellate cells into the inflammatory fibroblast. Our central hypothesis is that disruption of the IL-1/p38 MAPK signaling cascade in pancreatic stellate cells (PSCs) and CAFs can improve therapeutic resistance by remodeling the fibrotic stromal landscape and the overall immune microenvironment in PDAC tumors. Methods: Inhibition of phosphorylated p38 MAPK was achieved pharmacologically with Pexmetinib and genetically with an shRNA lentiviral system in CAF and PSC cell lines. Inflammatory PSC/CAF activation was determined by qPCR, immunofluorescence, and vitamin A assay. Ptf1acre/+;LSL-KrasG12D/+; Tgfbr2flox/flox (PKT) mice were treated with the p38 inhibitor, pexmetinib (30mg/kg, daily PO), or vehicle control for 2.5 weeks prior to sacrifice. For survival studies, PKT mice were treated with vehicle, gemcitabine (20 μg/twice weekly), pexmetinib, or combination until moribund. Results: Inhibition of p38 MAPK in PSCs prevented activation into an inflammatory fibroblast in vitro when stimulated with IL-1 or tumor cell cocultures. p38 MAPK inhibition in vivo resulted in signification reduction of PDGFR+ CAFs in a PDAC GEMM. Additionally, p38 MAPK inhibition resulted in a significant reduction of circulating myeloid cells (CD11b+) and intratumoral monocytic MDSCs (Ly6C+). p38 inhibition, in combination with chemotherapy, significantly improves overall survival in a PDAC GEMM. Conclusions: These findings provide important mechanistic data to explore p38 MAPK inhibition to target the fibrotic stroma and reduce immunosuppressive myeloid levels in tumors and provide compelling preclinical evidence to combine pexmetinib with chemotherapy to improve overall survival in PDAC. Citation Format: Samara Singh, Austin R. Dosch, Siddharth Mehra, Iago de Castro Silva, Anna Bianchi, Vanessa Tonin Garrido, Nilesh Deshpande, Zhiqun Zhou