Neoadjuvant chemoradiotherapy with or without Pd-1 antibody sintilimab for pMMR/MSS/MSI-L locally advanced rectal cancer: A randomized controlled study (cohort B).

TPS210 Background: Neoadjuvant chemoradiotherapy (NACRT) could bring tumor downstaging and pathological response (pCR), and also survival benefit for locally advanced rectal cancer (LARC) patients. Several single arm prospective clinical trials have investigated combination effect of immunotherapy (PD-1 or PD-L1 antibody) and NACRT in LARC patients, such as the VOLTAGE clinical trial. A randomized trial is needed to confirm the benefit of immunotherapy in this setting and explore predictive biomarkers. This is a clinical trial with two cohorts according the MMR/MSI status (clinicalTrials.gov, NCT04304209). Methods: In this study, LARC patients with pMMR/MSS/MSI-L tumor will enter cohort B and be randomized into two arms. Main inclusion criteria include: cT3-4N0M0 or cTxN+M0 rectal adenocarcinoma, pMMR/MSS/MSI-L confirmed by immunohistochemistry or gene test, aged 18-75y; ECOG performance 0-1; no previous anti-tumor treatment for rectal adenocarcinoma. Main exclusion criteria include: active autoimmune diseases or a history of autoimmune diseases, and inadequate main organ functions. Patients in the experimental arm will receive four cycles of neoadjuvant PD1 antibody Sintilimab, Capeox chemotherapy and concurrent radiotherapy, followed by curative surgery or watch and wait, then four cycles of adjuvant Capeox chemotherapy. Patients in the control arm will receive four cycles of neoadjuvant Capeox chemotherapy and concurrent radiotherapy, followed by curative surgery or watch and wait, then four cycles of adjuvant Capeox chemotherapy Primary outcome measure is pCR rate. Secondary outcome measures include acute toxicity, tumor regression grade, R0 resection rate, local recurrence, distant metastasis. Sample size for this cohort is 134. Whole exome sequencing, RNA sequencing and immunohistochemistry of the rectal primary tumor are planned for biomarker searching and synergy effect mechanism investigation. The first patient has been enrolled in June, 2020. Clinical trial information: NCT04304209.