A Prospective, Biomarker-Stratified Phase 2 Study: β-Hydroxybutyrate as a Predictive Biomarker for Short-Course Radiotherapy Followed by Chemotherapy Plus PD-1 Inhibitor Versus Chemotherapy Alone in Locally Advanced Rectal Cancer

This is a prospective, single-arm, biomarker-stratified, multicenter phase 2 clinical trial. All enrolled patients first receive short-course radiotherapy (SCRT), followed by measurement of serum β-hydroxybutyrate (β-HB) levels to calculate the post-radiotherapy elevation rate. Patients are stratified into an β-HB-positive stratum (elevation significantly) and an β-HB-negative stratum (no elevation), then randomized 1:1 within each stratum to receive either perioperative chemotherapy plus PD-1 inhibitor or perioperative chemotherapy alone. The primary endpoint is the complete response (CR) rate, defined as the composite of pathological complete response (pCR) and clinical complete response (cCR). The study aims to evaluate the efficacy and safety of adding PD-1 inhibitor to perioperative chemotherapy in LARC, and to validate post-radiotherapy β-HB elevation as a predictive biomarker for immunotherapy benefit.

Inclusion Criteria: 1. Patients or their family members agree to participate in the study and sign the informed consent form; 2. Age 18-75 years, male or female; 3. Histologically confirmed Locally Advanced rectal adenocarcinoma; 4. inferior margin ≤ 10 cm from the anal verge; 5. ECOG performance status score is 0-1; 6. Untreated with anti-tumor therapy for rectal cancer, including radiotherapy, chemotherapy, surgery, etc; 7. There was no operative contraindication; 8. Laboratory tests were required to meet the following requirements: white blood cell (WBC) ≥ 4×109/L; Absolute neutrophil count (ANC) ≥ 1.5×109/L; Platelet count ≥ 100×109/L; Hemoglobin ≥90 g/L; Serum total bilirubin ≤ 1.5 × upper limit of normal (ULN); Serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤ 2.5 × ULN; Serum creatinine ≤1.5 times the upper limit of normal value or creatinine clearance rate ≥50 mL/min; International normalized ratio (INR) ≤ 1.5 × ULN; Activated partial thromboplastin time (APTT) ≤ 1.5 × ULN; 9. Urinary protein \< 2+ or 24-hour urinary protein excretion \< 1 g at baseline. Exclusion Criteria: 1. Patients with non-pMMR LARC； 2. Subjects who have previously received any form of immunotherapy, including but not limited to immune checkpoint inhibitors, immune checkpoint agonists, immune cell therapy, or any other treatment targeting tumor immunomodulatory mechanisms; 3. Presence of any concurrent disease, condition (including laboratory abnormality), history of substance abuse, or current evidence thereof, which, in the judgment of the Investigator, may compromise subject safety, interfere with the process of obtaining informed consent, affect subject compliance, or confound the safety assessment of the investigational product(s).