Abstract PO-122: Combined CDK and BET inhibition reprograms the tumor and stromal compartments to enhance anti-tumor immunity in immunologically-cold CDKN2A-deficient pancreatic cancer

Background: Pancreatic ductal adenocarcinoma (PDAC) has a highly immunosuppressive tumor microenvironment (TME). The immune landscape of these tumors is further influenced by cross-talk with fibroblasts present in stroma. Importantly, genomic features also influence immune and stromal composition within PDAC and may provide opportunity for personalized immunotherapy. Given the central role of CKDN2A in PDAC (being lost in up to half of patients, and the second most commonly alteration across all metastatic disease), we hypothesize that the CDKN2A-deficient PDAC TME can be adapted for therapeutic targeting to sensitize PDAC to immunotherapy. Methods: Bioinformatic analysis of PDAC samples from the TCGA database evaluated how CDKN2A loss alters expression of T cell-inflamed gene signatures. Preclinical in vitro and in vivo studies were conducted with CDKN2A-targeted CRISPR-Cas9-edited luciferase-expressing KPC cells (KPC-luc) to generate isogenic KPC-lucΔCKDN2A cells. The BET inhibitor JQ-1 and CDK inhibitor palbociclib were used to evaluate impact on apoptosis and immune-related signaling using isogenic KPC-luc cells and patient-derived PDAC organoids. Spheroids were used to evaluate impact of CDK/BET inhibition on cross-talk with pancreatic cancer-associated fibroblast (CAF) spheroids. Finally, in vivo studies tested the ability of CDK/BET inhibition to enhance T cell migration, and delay tumor growth when combined with PD-L1 blockade. Results: CDKN2A loss was enriched in tumors lacking a T cell-inflamed gene signature. Similarly, CDKN2A loss decreased T cell migration in vitro and into the KPC-lucΔCDKN2A TME in vivo. Loss of CDKN2A rendered KPC-luc cells susceptible to direct apoptosis from dual CDK/BET inhibition. Combined CDK/BET inhibition at sublethal doses also modulated immune phenotype of tumors, inducing DNA damage and cGAS/STING to increase type I interferon and T cell migration in vitro. Additionally, treatment with BETi alone or combined with CDKi also modulated checkpoint ligands on KPC-luc cells and patient-derived organoids. In stroma, treatment of PDAC CAFs with JQ-1 polarized CAFs towards a myofibroblast CAF (myCAF) phenotype. This could also be mediated via cross-talk between tumor and CAF cells, as JQ-1-treated KPC-lucΔCDKN2A cells or supernatants cultured with CAF spheroids also induced myCAFs. This cross-talk was bidirectional, as supernatants from BETi-treated CAFs modulated expression of checkpoint ligands on KPC-luc cells. In vivo, combined BET/CDK inhibition increased T cell infiltration into PDAC tumors, and when combined with PD-L1 blockade diminished tumor growth. Conclusion: While CDKN2A loss promotes an immunosuppressive PDAC TME, it also renders tumors susceptible to BET/CDK inhibition. This approach polarizes CAF cells towards a myCAF phenotype, directing cross-talk between tumors and stroma to promote a more immunogenic phenotype. This suggests combined CDK/BET inhibition may be a personalized therapeutic approach for patients with CKDN2A-deficient PDAC, and a means to sensitize PDAC to immunotherapy. Citation Format: Brian M. Olson, Alison J. Thomas, Michael B. Ware, Gregory B. Lesinski. Combined CDK and BET inhibition reprograms the tumor and stromal compartments to enhance anti-tumor immunity in immunologically-cold CDKN2A-deficient pancreatic cancer [abstract]. In: Proceedings of the AACR Virtual Special Conference on Pancreatic Cancer; 2021 Sep 29-30. Philadelphia (PA): AACR; Cancer Res 2021;81(22 Suppl):Abstract nr PO-122.