Background: The combination of capecitabine plus long course radiotherapy (RT) is the standard preoperative therapy in cT3-4 cN+ rectal cancer. Pathologic Complete remission (pCR) can be considered as surrogate end point of efficacy of treatment in terms of disease free survival (DFS). Preclinical data points heavily toward a strong synergy between RT and immune treatments. Methods: This is a prospective phase II, open label, single arm, multi-centre study, conducted with support from AstraZeneca, in patient with locally advanced rectal cancer who receive concomitant CT/RT therapy (825 mg/m2 twice daily capecitabine every day and 5040 cGy radiotherapy for 5 days per week for 5 weeks) followed by durvalumab (1500 mg Q4W for 3 administrations). Surgery is performed after 10-12 weeks from neoadjuvant therapy. The primary endpoint is pCR rate after at least 1 cycle of durvalumab. The sample size has been estimated by using the optimal Simon’s two-stage design. If more than 4 complete responses are observed in the first 19 enrolled patients, 36 additional patients will be accrued for a total of 55 evaluable patients. Results: Between November 2019 and July 2020, 20 patients were accrued and 19 were evaluable for study objectives, concluding the first stage of the trial. Baseline characteristics of the first 19 evaluable patients enrolled are listed in the table. All patients received 3 infusions of durvalumab; 18 patients underwent surgery after a median of 13 weeks from CHT/RT end. Five complete pathological responses (ypT0N0) were observed, allowing to proceed to the second stage. About toxicity, four patients had Grade 3-4 adverse events (AE); the most frequent G3-4 AE related to the neoadjuvant therapy were anemia (n=1), diarrhea (n=2) and neuthropenia (n=2). No grade 3 and 4 adverse events related to Durvalumab treatment were observed. Eight patients had G1-2 AE related to durvalumab, the most common being asthenia (n=2) and nausea (n=2). Conclusions: At the end of study’s first stage the preoperative treatment with radiotherapy plus capecitabine followed by durvalumab showed a safe toxicity profile and promising activity in terms of pCR rate. The second part of the trial is ongoing, and the accrual is under completion (44 patients enrolled as of 10 February 2021). Clinical trial information: NCT04083365

ECOG PS: Eastern Cooperative Oncology Group performance status; DRE: digital rectal exploration; cT: clinical tumor; cN: clinical node; T stage: tumor stage; N stage: nodal stage; EMVI: extramural vascular invasion.