Abstract 1080: JAK inhibition as a therapeutic approach to enhance radiation response in rectal cancer

Background: Rectal cancer (RC) remains a leading cause of cancer-related mortality worldwide. Despite existing therapeutics such as neoadjuvant radiotherapy, 5-year survival outcomes remain low at ~60%. Rectal tumors are heterogeneous and underpinned by dysregulated cellular signaling. This study aimed to investigate the IL6/JAK/STAT3 pathway as a prognostic biomarker in RC and explore the therapeutic potential of JAK inhibitors using in vitro/ex vivo models. Methods: A retrospective RC cohort (n=206) was stained via immunohistochemistry for IL6/JAK/STAT3 pathway members. Staining intensity was quantified and scores were analyzed for association with survival outcomes and clinicopathological characteristics. In a prospective RC cohort, the effect of radiation on IL6/JAK/STAT3 signaling was investigated in serially collected pre- and post-treatment tumor biopsies at the transcriptomic and protein level. Drug screening of repurposed JAK inhibitors (alone and in combination with radiation) was performed in cell lines (DLD-1, HCT116, HT29, SW620), mouse organoids (AKPT and KPN) and patient-derived organoids (PDOs). Responses were measured using WST-1 assays, clonogenics, and immunofluorescent staining for markers of proliferation. Results: High tumoral expression of pSTAT3tyr705 was associated with reduced cancer-specific survival (CSS) in RC patients with stroma-rich tumors (HR=1.781, 95%CI; 1.419-2.236, p=0.001). Similarly, high tumoral expression of JAK2 in stroma-rich cases was associated with reduced CSS (HR=1.854, 95%CI; 1.287-2.671, p=0.001). A significant increase in IL6/JAK/STAT3 gene signatures was observed at 2- and 6- weeks post treatment induction compared to baseline in rectal biopsy specimens (n=3). In vitro, JAK inhibition with 10µM AZD1480 or Ruxolitinib significantly reduced viability of cell lines HCT116 (p=0.018) and SW620 (p=0.050). When AZD1480 was combined with 0-8Gy of radiation, colony formation was significantly reduced upon combination treatment compared to vehicle controls in a dose-dependent manner and linear quadratic modelling showed a synergistic effect of combined treatment in HT29 (p=0.004) and SW620 lines (p=0.014). AKPT, KPN and a subset of PDOs showed a significant reduction in cell viability, proliferation and increased apoptosis following JAK inhibition. Conclusion: This study represented a step towards establishing the role of JAK inhibition as a therapeutic approach for RC. Ongoing research aims to validate the synergistic effect of combining JAK inhibition with radiation. Citation Format: Kathryn A. Pennel, Phimmada Hathakarnkul, Sara S. Al-Badran, Umar Hashmi, Lily Hillson, Jean A. Quinn, Leia Jones, Colin W. Steele, Donald C. McMillan, James H. Park, Owen J. Sansom, Joanna Birch, Campbell S. Roxburgh, Joanne Edwards. JAK inhibition as a therapeutic approach to enhance radiation response in rectal cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1080.