Serine/Glycine-Restricted Diet With Chemoradiotherapy for Rectal Cancer

This is a Phase Ib/II seamless, open-label, randomized controlled trial evaluating the safety and efficacy of a serine/glycine-restricted diet combined with neoadjuvant chemoradiotherapy and immunotherapy in patients with locally advanced rectal cancer (LARC). The study consists of two phases. Phase Ib is a non-randomized, single-arm safety run-in phase enrolling 6 patients, all of whom will receive the experimental regimen (serine/glycine-restricted diet plus CAPOX chemotherapy, PD-1 inhibitor, and short-course radiotherapy). The primary objective of Phase Ib is to assess safety, tolerability, and dietary compliance. If the safety criteria are met (≥3 grade diet-related adverse event rate ≤20% and compliance rate ≥70%), the study will proceed to Phase II. Phase II is a randomized, open-label, parallel-controlled phase in which 134 additional patients will be randomized in a 1:1 ratio to either the experimental group (serine/glycine-restricted diet plus standard neoadjuvant chemoradiotherapy and immunotherapy) or the control group (standard neoadjuvant chemoradiotherapy and immunotherapy alone). The total enrollment is 140 patients (6 in Phase Ib + 134 in Phase II). The primary endpoint is the complete response rate (pCR + cCR). Secondary endpoints include major pathological response (MPR) rate, R0 resection rate, mrTRG regression grade, event-free survival (EFS), progression-free survival (PFS), overall survival (OS), adverse event profile, changes in serum amino acid levels, quality of life (EORTC QLQ-C30), and nutritional status. Exploratory endpoints include gut microbiome diversity and tumor immune microenvironment changes.