Abstract LB007: Chemotherapy driven modulation of fibroblast activation protein in the pancreatic tumor microenvironment

Abstract Pancreatic ductal adenocarcinoma (PDAC) exhibits marked therapeutic resistance driven in part by stromal remodeling mediated by cancer-associated fibroblasts (CAFs). Fibroblast activation protein (FAP), a key CAF marker, is prominently expressed in the PDAC microenvironment, yet how chemoradiotherapy (CRT) modulates FAP across therapy-sensitive and therapy-resistant disease remains unclear. Here, we examine therapy-dependent regulation of FAP in human PDAC samples and preclinical models. Methods: FAP expression was assessed by immunofluorescence (IF) in human PDAC samples, including treatment-naïve tumors (n = 7), CRT-responsive tumors (n = 8), and adjacent pancreas tissues (n = 4), with transcriptomic validation by bulk RNA sequencing. External RNA-seq data from 137 PDAC samples obtained from a publicly available dataset. Combined, the analyses included treatment-naïve PDAC (n = 54), CRT responder PDAC (n = 9), partial CRT responder PDAC (n = 14), CRT-resistant PDAC (n = 56), and adjacent pancreas tissues uninvolved with cancer (n = 4). In parallel, chemotherapy-sensitive (PANC-1) and chemotherapy-resistant (SU8686) PDAC xenografts were treated with serial FOLFIRINOX, and tumors were collected at multiple time points for IF-based FAP analysis. Results: IF quantification demonstrated significantly increased FAP expression in CRT-sensitive PDAC compared with treatment-naïve tumors (p < 0.001) and adjacent pancreas (p < 0.0001), with predominant localization to stromal CAFs. Bulk RNA sequencing corroborated these findings, showing elevated FAP transcripts in CRT-treated PDAC relative to treatment-naïve tumors (p < 0.05) and in adjacent pancreas (p < 0.001), with minimal expression in adjacent pancreas. Integrated bulk and single-cell RNA sequencing revealed therapy-dependent modulation of FAP expression, with higher transcript levels in CRT-sensitive tumors compared with adjacent pancreas (p < 0.001) and treatment-naïve disease (p < 0.05), while no significant difference was observed between CRT-resistant and treatment-naïve tumors (p > 0.05). Single-cell analysis showed reduced inflammatory CAFs (iCAFs) in CRT-sensitive tumors and enrichment in CRT-resistant disease, whereas myofibroblastic CAFs remained remained consistently abundant across all treatment states with an overall increase in resistant tumors, indicating differential CAF plasticity underlying therapy-dependent FAP modulation. FAP expression was not influenced by chemotherapy regimen within response groups. In contrast to human samples, FAP expression remained stable in chemotherapy-sensitive PANC-1 xenografts but showed an early increase in chemotherapy-resistant SU8686 xenografts following FOLFIRINOX, with reduction at later time points. Conclusion: FAP expression is dynamically regulated by chemoradiotherapy in PDAC, with significant upregulation in CRT-responsive patients and persistent expression in treatment-naïve and CRT-resistant disease. These findings establish FAP as a robust stromal biomarker and highlight therapy-dependent CAF remodeling as a key feature of the PDAC tumor microenvironment. The discordance between human tissues and murine models underscores the importance of human tissue-based studies to understand stromal dynamics and guide translational research. Citation Format: Yousef Khazaei Monfared, Arvin Haj-Mirzaian, Danni L i, Shriya Krishna, Pedram Heidari, Shadi A. Esfahani. Chemotherapy driven modulation of fibroblast activation protein in the pancreatic tumor microenvironment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(8_Suppl):Abstract nr LB007.