A Phase Ii Study To Evaluate Safety And Efficacy Of Neoadjuvant Pembrolizumab And Radiotherapy In Localized Mss Rectal Cancer

TPS272 Background: Locally advanced rectal cancer remains a clinical challenge with few improvements noted over the past few decades. Although immunotherapy has no current clinical role 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 inhibitors use. This prospective phase II trial will test that hypothesis in addition to confirming safety of this approach using a “window-of-opportunity” study design with the anti-PD-1 agent Pembrolizumab. Methods: This monocentric phase II trial, will enroll patients (pts) with rectal cancer who are undergoing neoadjuvant short course RT (scRT) (25 Gy in 5 fractions). according to the standard of care. Eligible includes pts with MSS stage II-III rectal cancer with adequate organ function and availability of pre-treatment tumor, who are undergoing scRT with intention to proceed to surgical resection. Standard ineligibility criteria include active infections, systemic steroid use, or other conditions making immunotherapy use unsafe. Treatment includes 4 doses of Pembrolizumab (200mg IV, once every 3 wks), the first dose being given before the first scRT fraction. Surgery will be performed within 12-16 weeks of the final scRT dose. Primary endpoint is tumor regression grade (TRG) using the Mandard regression grade score targeting a 30% pathological complete response (pCR) compared to 10% in historical controls. Secondary endpoints include OS, DFS, toxicity, local and distant relapse-free survival, negative surgical margins, QoL, quality of surgery and exploratory assessments of tumor infiltrating lymphocytes, profiling of circulating immune cell populations, and molecular predictors of response. A safety stopping rule is planned based on Wald’s sequential probability ratio test for the occurrence of the safety outcome. Enrollment target is 25 pts. Support: MSD. Clinical trial information: NCT04109755 .