212 - Characterizing Radiation Induced Subclonal Kinetics in Pancreatic Ductal Adenocarcinoma

V. Bernard Pagan¹, D. Lin², K. Rajapakshe¹, D. Jiang², C. W. Tzeng³, M. S. Bhutani³, E. B. Ludmir⁴, A. C. Koong³, and C. M. Taniguchi³; ¹MD Anderson Cancer Center, Houston, TX, ²Department of Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX, ³The University of Texas MD Anderson Cancer Center, Houston, TX, ⁴Department of Gastrointestinal Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX

Purpose/Objective(s): Molecular pathways associated to therapeutic response in pancreatic ductal adenocarcinoma (PDAC) remain poorly understood. Although the evolutionary trajectory of PDAC has been well characterized throughout disease progression, the subclonal kinetics induced by radiotherapy has not been well elucidated. Exploring the evolutionary dynamics of subclones responding to stereotactic body radiation therapy (SBRT) could shed light on their adaptive processes and interactions with the tumor microenvironment (TME) to reveal novel therapeutic strategies. Materials/

Methods: Tumor biopsies were acquired before and after SBRT as part of a phase I/II dose escalation clinical trial of SBRT (50-60Gy in 5 fx) with a radiomodulating agent, avasopasem manganese. A total of 16 biopsies underwent single cell RNA sequencing (scRNA-seq) with 7 patients with matched samples before and after SBRT. Tumor subclones were identified based on copy number variations (CNV) inferred by scRNA-seq and confirmed by circulating tumor DNA (ctDNA).
Results: CNV inference revealed subclonal dynamics of cancer cells relating to SBRT response and resistance. Resistant subclones were characterized by augmented hypoxia, KRAS, epithelial-mesenchymal transition (EMT), and TNF-alpha signaling. These subclones were also seen to negatively regulate lymphocyte-mediated immunity and the adaptive immune response. In contrast, subclones responsive to SBRT exhibited an increase in oxidative phosphorylation signaling and pathway enrichment associated with cell proliferation, cell cycle progression, and RNA molecule modification and processing. Interestingly, SBRT induced a decrease in cancer cells characterized as harboring a chemoresistant basal-like molecular subtype, while increasing B-cell proliferation, immunoglobulin production, and inflammatory cancer associated fibroblasts. Patients with increased B cell and natural killer (NK)-like T cell infiltration demonstrated improved outcomes. CtDNA further allowed for identification of tissue resident subclones as well as their non-invasive monitoring during therapy.
Conclusion: Through longitudinal tumor biopsies, we were able to observe significant selection pressures on tumor subclones in PDAC following SBRT. This allowed us to identify molecular processes related to response to SBRT as well as TME modifications induced during adaptive pressures. Ultimately, identification of resistant and responsive populations may allow for characterization of novel targets for combinatory therapeutic intervention such as inhibition of KRAS related signaling.