Background: Clinical improvements for locally advanced rectal cancer have been relatively static over the past few decades. While immunotherapy shows no benefit in microsatellite stable (MSS) colorectal cancer, preclinical models suggest that radiotherapy (RT) can enhance neoantigen presentation, modulate the microenvironment, and improve the likelihood of anti-tumor activity with checkpoint inhibitor use. Using a “window-of-opportunity” study design, this prospective phase II trial will determine the safety and activity of this approach with the anti-PD-L1 agent durvalumab (MEDI4736). Methods: This multi-center phase II trial is currently enrolling patients (pts) with rectal cancer who are undergoing standard NCCN guideline-compliant neoadjuvant chemoradiotherapy (CRT). Eligibility includes pts with MSS stage II-IV rectal cancer with adequate organ function and pre-treatment diagnostic tumor available for profiling with intent to proceed to surgical resection after CRT. Stage IV disease must be limited such that the primary pelvic tumor requires definitive management. Standard ineligibility criteria include active infections, systemic steroid use, or other conditions making immunotherapy use unsafe. Treatment includes durvalumab (750mg IV infusion once every 2 wks) for 4 total doses beginning within 3-7 days after CRT completion. Surgery must be within 8-12 wks of the final CRT dose. Primary endpoint is a demonstrated improvement in Neoadjuvant Rectal Cancer (NAR) score compared to historical controls targeting a 20% relative risk reduction in DFS and 3-4% absolute OS improvement. Secondary endpoints include OS, DFS, toxicity, pCR, cCR, therapy completion, negative surgical margins, sphincter preservation, off-target “abscopal” effects for the subset of stage IV pts, and exploratory assessments of tumor infiltrating lymphocytes, tumor Immunoscore, circulating immunologic profiles, and molecular predictors of response. A safety run-in phase has completed as a precedent to full enrollment. Enrollment now continues to 47 total pts to achieve 41 surgically evaluable pts. Support: AstraZeneca-Medimmune, NSABP Foundation. Clinical trial information: NCT03102047