Low-Dose Radiotherapy to Sensitize Pucotenlimab Plus CAPEOX for pMMR Locally Advanced Rectal Cancer

Overview

This is a prospective, open-label, randomized, parallel-group phase II trial evaluating the efficacy and safety of a low-dose radiotherapy sensitization strategy combined with a PD-1 antibody (pucotenlimab) and CAPEOX as neoadjuvant therapy in patients with pMMR/MSS locally advanced rectal adenocarcinoma. Participants will be randomized 1:1 to receive either 2 Gy or 5 Gy low-dose radiotherapy. Low-dose radiotherapy is delivered as a single fraction of 2 Gy (Arm A) or 5 Gy (Arm B). On the day after radiotherapy, participants will start pucotenlimab 200 mg IV Q3W (administered on Day 2 of each 21-day cycle) plus CAPEOX chemotherapy. Early response will be assessed after 2 cycles using endoscopy and pelvic MRI to guide subsequent treatment: participants with partial response may discontinue radiotherapy and continue neoadjuvant systemic therapy; participants with stable disease may switch to standard chemoradiotherapy; participants with progressive disease will receive multidisciplinary-team-guided salvage therapy. After 4 cycles, participants with clinical complete response may adopt a watch-and-wait strategy; otherwise, they will undergo radical surgery 2-4 weeks after completion of neoadjuvant therapy. Long-term follow-up will include recurrence and survival outcomes and quality of life.

Rectal cancer remains a major global health burden, with a higher risk of local recurrence and worse prognosis compared with colon cancer. The introduction of total mesorectal excision (TME) has substantially reduced local recurrence rates, and the incorporation of peri-operative radiotherapy and chemotherapy has further improved oncologic outcomes. Current National Comprehensive Cancer Network (NCCN) guidelines recommend pre-operative chemoradiotherapy followed by radical surgery for patients with stage II-III locally advanced rectal cancer.In recent years, total neoadjuvant therapy (TNT) has emerged as an optimized treatment paradigm, increasing pathologic complete response (pCR) rates to approximately 20-40%, and even higher when combined with immunotherapy. For patients achieving a clinical complete response (cCR) after neoadjuvant therapy, a non-operative "watch-and-wait" strategy has been shown to be safe and effective, allowing organ preservation without compromising long-term oncologic outcomes.However, most patients with proficient mismatch repair (pMMR) or microsatellite-stable (MSS) rectal cancer derive limited benefit from immune checkpoint blockade alone. Preclinical and clinical evidence suggests that radiotherapy, particularly low-dose radiotherapy (LDRT), may enhance antitumor immune responses by modulating the tumor microenvironment and promoting immune sensitization. Whether different low-dose radiotherapy regimens can differentially enhance the efficacy of immunotherapy combined with chemotherapy in pMMR/MSS locally advanced rectal cancer remains unknown.To address this question, the present study is designed as a prospective, open-label, randomized, parallel-group phase II clinical trial evaluating a low-dose radiotherapy sensitization strategy combined with a PD-1 antibody (pucotenlimab) and CAPEOX chemotherapy as neoadjuvant treatment.Eligible participants with pMMR/MSS locally advanced rectal adenocarcinoma will be randomized in a 1:1 ratio to receive either 2 Gy or 5 Gy low-dose radiotherapy. On the day following radiotherapy, all participants will initiate systemic treatment with pucotenlimab 200 mg administered intravenously every 3 weeks (Q3W) on Day 2, in combination with CAPEOX chemotherapy in 21-day cycles.Treatment response will be assessed early after 2 cycles of neoadjuvant therapy using endoscopy and pelvic magnetic resonance imaging (MRI). Based on response evaluation, subsequent treatment will be adapted: participants demonstrating partial response may discontinue radiotherapy and continue systemic neoadjuvant therapy; those with stable disease may transition to standard chemoradiotherapy; and those with progressive disease will receive multidisciplinary team-guided salvage treatment.After completion of 4 cycles of neoadjuvant therapy, response will be reassessed. Participants achieving a clinical complete response may adopt a watch-and-wait strategy with close surveillance, whereas those without cCR will proceed to radical surgery, typically performed 2-4 weeks after completion of neoadjuvant treatment.The primary objective of this study is to compare the complete response rate, defined as the sum of clinical complete response and pathological complete response, between the two low-dose radiotherapy regimens. Secondary objectives include long-term disease control, survival outcomes, surgical quality metrics, postoperative morbidity, stoma rates, and patient-reported quality of life. This study aims to determine whether optimization of low-dose radiotherapy can enhance immune-chemotherapy efficacy and increase organ preservation opportunities in patients with pMMR/MSS locally advanced rectal cancer.