Abstract CT130: Epacadostat with preoperative chemoradiation in locally advanced rectal cancer (LARC): Results of a translational phase I clinical trial

Abstract BACKGROUND: In preclinical studies, we found that the immuno-oncology target and tryptophan metabolizing enzyme indoleamine 2, 3-dioxygenase 1 (IDO1) promotes resistance to radiation in rectal cancer regardless of MSI status. Furthermore, IDO1 inhibition with epacadostat improved tumor radiosensitivity, abrogated immunosuppression, limited distal recurrence, and was radioprotective of the normal intestine. This trial aimed to evaluate the safety of IDO1 inhibition when combined with neoadjuvant short-course radiation (SCRT) and chemotherapy with capecitabine and oxaliplatin (CAPOX) for LARC. METHODS: The primary objective was to determine the recommended phase II dose (RP2D) of epacadostat for combination with SCRT and CAPOX. Secondary objectives included determining antitumor activity as measured by neoadjuvant rectal (NAR) score and pathological or clinical complete responses. During the dose escalation phase, 3 doses of epacadostat (300mg, 400mg, & 600mg BID) were explored. Epacadostat was combined with SCRT (5Gy x 5 fractions) followed by 6 cycles of CAPOX chemotherapy, until the day of surgery (~24 wks). Tissues and blood were collected for correlative studies. A control cohort was generated from patients receiving the same treatment without epacadostat. NCT03516708 RESULTS: Seventeen patients were enrolled from 4/2019 to 8/2023. RP2D of epacadostat in combination with SCRT and CAPOX chemotherapy was determined to be 400mg BID. Two patients experienced dose-limiting toxicities (DLT) at 600mg dose level, one with G2 elevated ALT and another with G3 maculopapular rash. No DLT was reported at other dose levels. Of six patients who underwent surgery, the mean NAR score was 7.46 (SD 7.11; range 0 - 20.4), indicating potentially better response compared to the mean NAR of 17.8 in an institutional 2:1 matched control cohort (p=0.03). Tumor tissue revealed increased IDO1 expression and kynurenine levels in all patient tumors after radiation. After epacadostat, serum levels of kynurenine were reduced while TNFα, IFNγ, and MIP1a were all increased. Global metabolomics and RNAseq revealed a distinct pattern of serum biomarkers and immunophenotype respectively in patients after treatment. Tumoroids were evaluated as ex vivo models of treatment response. CONCLUSION: Epacadostat in combination with SCRT and CAPOX was well-tolerated. An NCI supported Phase 2 trial is ongoing to further evaluate the promising disease responses reported in dose escalation phase. Citation Format: Haeseong Park, Katrina Pedersen, Katherine Huang, Matthew Mutch, Hyun Kim, Nikolaos Trikalinos, Olivia Aranha, Benjamin Tan, Rama Suresh, Lee Ratner, Moh'd Khushman, John Visconti, Kian-Huat Lim, Lauren Henke, Maddie Swiecicki, Jingxia Liu, Parag Parikh, Matthew Ciorba. Epacadostat with preoperative chemoradiation in locally advanced rectal cancer (LARC): Results of a translational phase I clinical trial [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(7_Suppl):Abstract nr CT130.