Abstract A098: Single nuclear profiling reveals type I IFN-mediated immune remodeling in platinum-refractory HRD-mutant PDAC responsive to checkpoint immunotherapy

Abstract INTRODUCTION: Although platinum chemotherapy improves survival in advanced PDAC patients with germline mutations in homologous recombination repair genes (mutHRD), treatment options are limited in platinum-refractory mutHRD PDAC. In a pilot study (n=12), we reported clinical benefit with dual immune checkpoint blockade (dICB; anti-PD1+CTLA4) in platinum-refractory mutHRD patients. Here, we validate these observations in a bi-institutional cohort and investigate potential mechanisms driving this acquired dICB sensitivity at single-cell resolution using prospectively accrued PDAC patient biopsies and avatar preclinical modeling. METHODS: We identified advanced mutHRD patients (n=21) who received dICB at 2 tertiary centers, and prospectively collected biopsies in a subset (n=6) prior to dICB receipt. We performed single-nucleus RNA sequencing (snRNAseq) from these samples, stratifying gene expression and pathway analysis by ultimate clinical benefit (complete/partial response or stable disease for at least 16 weeks) or resistance (progressive disease) to dICB (n=3 each). We validated these findings in an avatar murine model by performing scRNAseq in mice orthotopically injected with platinum-refractory vs. sensitive KPC-Brca2-shRNA knockdown tumors. RESULTS: In 21 platinum-refractory mutHRD PDAC patients treated with dICB, 11 (52%) demonstrated clinical benefit. snRNAseq in samples from dICB-benefit mutHRD platinum-refractory PDAC tumors demonstrated a marked decrease in myeloid cell abundance coupled with increased T-cell abundance vs. dICB-resistant samples. This correlated with a striking redistribution of tumor cell single-cell transcriptomes in dICB-benefit tumors, analysis of which revealed significant enrichment in antigen presentation, type I interferon (IFN) signaling, and dysregulation of apoptosis (all p-adj0. 001). Expression of key cGAS-STING pathway transcripts (CGAS, TBK1, IRF3) and direct IRF3 target genes—including T-cell chemoattractants CXCL9 and CXCL10, correlating with findings from whole-tumor NanoString analysis—were upregulated in dICB-benefit tumor cells. T-cell transcriptomes in dICB-benefit samples showed enrichment in IL2/STAT5 signaling, TCR activation, and memory/effector pathways. Using an avatar murine model, we phenocopied benefit of dICB in orthotopic cisplatin-resistant vs. sensitive KPC-shBrca2 PDAC tumors; scRNAseq analysis of tumors following gemcitabine/cisplatin treatment but before dICB initiation revealed enrichment of tumor cell-specific type I IFN activation, cytosolic DNA sensing, and T-cell chemotaxis pathway in cisplatin resistant vs. sensitive KPC-shBrca2 tumors, indicating STING pathway activation. CONCLUSIONS: Platinum-refractory mutHRD PDAC tumors that respond to dICB exhibit tumor cell-intrinsic activation of STING/type I IFN axis, invoking an immunotherapy-permissive microenvironment. Leveraging a unique biorepository and avatar modeling, these findings support a forthcoming Phase II clinical trial evaluating a bispecific anti-PD1/CTLA4 antibody in this biomarker-defined PDAC subset. Citation Format: Elena Shersher, Luis Nivelo, Clara J. Oh, Ifeanyichukwu C. Ogobuiro, Lucas Caeiro, Haleh Amirian, Karthik Rajkumar, Andrew M. Adams, Piyush Sharma, Nipun Merchant, Erkut Borazanci, Sarah D. Legrand, Ramiro E. Verdun, Anna Bianchi, Peter J. Hosein, Jashodeep Datta. Single nuclear profiling reveals type I IFN-mediated immune remodeling in platinum-refractory HRD-mutant PDAC responsive to checkpoint immunotherapy abstract. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Pancreatic Cancer Research—Emerging Science Driving Transformative Solutions; Boston, MA; 2025 Sep 28-Oct 1; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2025;85 (18Suppl₃): Abstract nr A098.