The clinical efficacy and safety of neoadjuvant chemoradiation therapy with immunotherapy for the organ preservation of ultra low rectal cancer: A single arm and open label exploratory study.

e15603 Background: Neoadjuvant chemoradiotherapy (nCRT) not only could bring tumor downsizing and downstaging but also could achieve clinical complete response (cCR) or pathological complete response (pCR). Although immunotherapy benefits MSI-H/dMMR rectal cancer patients, little response is observed in MSS/pMMR patients. Several clinical trials including VOLTAGE trial demonstrated that a combination of PD-1 inhibitor and nCRT could result in a promising pathological response even in MSS/pMMR patients. But the elucidation regarding organ preservation after treatment is lacking. Watch and Wait (W&W) is an effective strategy in rectal cancer patients showing cCR after nCRT to preserve organs. This study was designed to explore the efficacy, safety, and organ preservation rate of a combination of nCRT and PD-1 inhibitor(sintilimab) in MSS/pMMR patients with ultra-low rectal cancers. Methods: This was a single-arm, phase II trial. Patients with ultra-low rectal cancer that was confirmed as MSS/pMMR T 1- 3a N 0-1 M 0 and received 50 Gy (25×2 Gy) chemoradiotherapy and 2 cycles of sintilimab, followed by 6 cycles of capecitabine or CAPOX regimen plus 2 cycles of sintilimab as consolidation therapy. Patients with cCR were managed using the W&W strategy, patients with ncCR were given local excision. The primary outcome was cCR rate and secondary outcomes were anal preservation rate, organ preservation rate, tumor regression grade (TRG), and safety profile. Results: From 2021 January to 2021 December, a total of 23 patients were enrolled. The median age was 55 y (range; 38 to 75). Male patients were 61%. cT2 was 56.5% (13/23), cT3 was 43.5% (10/23), cN0 was 69.6% (16/23), and cN1 was 30.4% (7/23). All patients completed 50Gy chemoradiotherapy. Nineteen patients received 4 cycles of sintilimab treatment, 3 patients received 3 cycles of sintilimab, and 1 patient received 1 cycle of sintilimab. cCR was 43.5% (10/23), ncCR was 26.1% (6/23), and cPR was 30.4% (7/23). The anal preservation rate was not assessed in 1 patient due to cerebral ischemic stroke. The anal preservation rate was 95.5% (21/22), rectal preservation rate was 59.1% (13/22). Ten patients underwent surgery. Among them, two patients achieved pCR, and the major pathologic response rate (TRG0-1) was 60.0% (6/10). The overall CR(cCR+pCR) rate was 52.2% (12/23). Grade 3/4 treatment-related adverse events occurred in 17.4% (4/23) of patients. No unexpected adverse events or deaths were observed. Conclusions: Given the favorable tolerability and encouraging cCR rate, nCRT plus sintilimab could be a feasible and safe option for patients with pMMR/MSS, ultra-low rectal cancer who have a strong desire to preserve the anus. Based on these results, a prospective, randomized, controlled, multicenter, phase III study is currently in progress (NCT05215379). Clinical trial information: ChiCTR2100042785.