The goal of this interventional clinical trial is to compare the efficacy of neoadjuvant chemotherapy versus upfront surgery in adults aged 18-70 years with stage II (high-risk)-III, non-MSI-H colon adenocarcinoma and elevated baseline CEA (>5 ng/mL) undergoing curative-intent treatment. This single-center, open-label, randomized controlled study will evaluate 2-year disease-free survival (2y-DFS) as the primary endpoint, with all study-related procedures-including longitudinal ctDNA-based molecular residual disease (MRD) monitoring, Immunoscore assessment, tumor tissue sequencing, and surveillance imaging-provided at no cost to participants. The main questions it aims to answer are:

• Does a treatment strategy involving neoadjuvant CAPOX followed by surgery improve 2y-DFS compared with upfront surgery followed by standard adjuvant chemotherapy?
• Do postoperative ctDNA-MRD status and its longitudinal dynamics predict 2y-DFS?
• Does combining ctDNA-MRD with Immunoscore enhance prognostic risk stratification for recurrence beyond either biomarker alone?

Participants will:

• Be randomized 1:1 (N=100) to one of two treatment pathways:

  • Arm A: Neoadjuvant CAPOX × 4 cycles → curative surgery (R0 planned) → postoperative management per standard practice
  • Arm B: Upfront curative surgery → postoperative standard adjuvant chemotherapy per guideline → routine surveillance

• Undergo baseline assessments prior to treatment initiation, including blood draw, colonoscopy, primary tumor next-generation sequencing (for personalized ctDNA-MRD assay development), and Immunoscore testing-all provided free of charge as part of the study.

• Provide postoperative blood samples for ctDNA-MRD testing at approximately postoperative day ~7 and day ~30 (before adjuvant therapy start, if applicable).

• During follow-up, provide serial blood samples every 3 months, aligned with routine surveillance visits, for repeat ctDNA-MRD analysis.

• Receive standard-of-care postoperative surveillance (including imaging and clinical evaluations) through 2 years, with all study-mandated assessments covered by the trial.

This trial integrates clinical intervention with comprehensive biomarker profiling to determine whether early systemic therapy alters MRD dynamics and improves outcomes in high-risk, CEA-elevated colon cancer.

Colorectal cancer (CRC) is one of the most common malignancies worldwide and remains a leading cause of cancer-related mortality. In China, CRC incidence ranks among the top cancers, and CRC-related death constitutes a major public health burden. Approximately 80% of CRC patients present with non-metastatic disease (stages I-III), representing the primary population for whom recurrence prevention is critical. Despite guideline-recommended curative-intent surgery and perioperative systemic therapy, at least 30% of patients with resectable stage II-III colon cancer experience recurrence or distant metastasis after initial treatment. This underscores an urgent need for more effective therapeutic strategies and better tools for risk-adapted management.

Current adjuvant chemotherapy decisions for stage II (high-risk) and stage III colon cancer rely largely on TNM staging and clinicopathologic risk factors, which do not directly measure residual tumor burden. Consequently, some patients may be overtreated with unnecessary toxicity, while others at high risk of relapse may receive insufficient therapy. Elevated baseline carcinoembryonic antigen (CEA >5 ng/mL) is a well-established adverse prognostic factor in colon cancer and identifies a subgroup with higher recurrence risk. Emerging data also suggest that patients with proficient mismatch repair (pMMR)/non-MSI-H tumors-particularly those with elevated CEA-may benefit from early systemic intervention before surgery. Therefore, evaluating whether neoadjuvant chemotherapy improves outcomes compared with standard upfront surgery represents a clinically meaningful question in this high-risk population.

This trial is designed as a single-center, open-label, randomized controlled study to directly compare two perioperative treatment strategies in adults aged 18-70 years with stage II (high-risk)-III, non-MSI-H colon adenocarcinoma and baseline CEA >5 ng/mL. All participants will be randomly assigned (1:1) to either:

Arm A (Neoadjuvant CAPOX Pathway): 4 cycles of neoadjuvant CAPOX chemotherapy followed by curative-intent surgery (R0 planned), with postoperative management per standard guidelines; or Arm B (Upfront Surgery Pathway): immediate curative-intent surgery followed by standard adjuvant chemotherapy and routine surveillance.

The primary objective of the study is to determine whether the neoadjuvant approach improves 2-year disease-free survival (2y-DFS) compared with upfront surgery. Importantly, all study-mandated procedures-including tumor tissue sequencing, Immunoscore assessment, personalized ctDNA-MRD assay development, and serial blood-based MRD monitoring-are provided at no cost to participants as part of the research protocol.

In parallel, the study incorporates a comprehensive biomarker program to explore mechanisms of treatment response and refine recurrence prediction. Circulating tumor DNA (ctDNA)-defined molecular residual disease (MRD) reflects microscopic residual tumor burden at the molecular level. Postoperative ctDNA positivity has been consistently associated with high recurrence risk in CRC, and serial monitoring can detect molecular relapse weeks to months before radiologic evidence. Immunoscore (IS), which quantifies CD3+ and CD8+ T-cell densities in the tumor core and invasive margin, provides a robust measure of anti-tumor immunity and independently predicts outcomes in colon cancer. We hypothesize that integrating these complementary biomarkers may enhance risk stratification and reveal biological insights into how neoadjuvant therapy modulates MRD dynamics and immune contexture.

Study Objectives Primary Objective

• To compare 2-year disease-free survival (2y-DFS) between patients receiving neoadjuvant CAPOX followed by surgery versus those undergoing upfront surgery followed by standard adjuvant chemotherapy.

Secondary/Exploratory Objectives To evaluate the prognostic value of postoperative ctDNA-MRD status (at ~day 7 and ~day 30) and its longitudinal changes for predicting 2y-DFS.

To assess whether combining ctDNA-MRD with Immunoscore improves recurrence risk stratification beyond either biomarker alone.

To estimate 2-year overall survival (2y-OS), local recurrence rate, and distant metastasis rate in each treatment arm and across biomarker-defined subgroups.

To characterize the impact of neoadjuvant chemotherapy on ctDNA-MRD clearance rates and immune microenvironment features compared with upfront surgery.

Study Design This is a single-center, prospective, randomized (1:1), open-label clinical trial enrolling approximately 100 eligible participants. Randomization ensures balanced comparison of two clinically relevant pathways within a controlled framework, with treatment efficacy (2y-DFS) as the primary endpoint. While biomarker analyses are hypothesis-generating and secondary, they are prospectively integrated to support precision oncology applications.

Study Procedures and Assessments Baseline (prior to allocated treatment)

All participants will undergo standardized baseline evaluations, including:

Clinical assessment and eligibility confirmation (ECOG 0-1); Colonoscopy (as clinically indicated); Blood draw for baseline ctDNA analysis;

Tumor tissue collection (from diagnostic biopsy or surgical specimen) for:

• Whole-exome or targeted NGS to enable development of a personalized ctDNA-MRD assay;
• Immunoscore testing using a validated institutional platform. All biomarker-related assays are funded by the study. Treatment Phase Arm A: Neoadjuvant CAPOX × 4 cycles → curative surgery (R0 planned). Arm B: Upfront curative surgery → adjuvant chemotherapy per NCCN/ESMO guidelines.

All treatments follow institutional standards; adverse events are managed per routine care.

Postoperative ctDNA-MRD Sampling

Blood will be collected at key timepoints:

• Postoperative day 7
• Postoperative day 30 These samples enable early MRD assessment before adjuvant therapy initiation (when applicable).

Follow-up Phase Participants will undergo standard postoperative surveillance (imaging, labs, clinical visits) every 3 months through 2 years. Serial blood samples for ctDNA-MRD testing will be collected at each visit, synchronized with routine care. All imaging and study-related biospecimen analyses are covered by the trial.

Biomarker Strategy ctDNA-MRD will be assessed using a patient-specific NGS-informed panel, with results reported as positive/negative at each timepoint.

Immunoscore will be categorized as High, Intermediate, or Low per validated criteria.

Combined risk groups (e.g., ctDNA-/Immunoscore-High = lowest risk; ctDNA+/Immunoscore-Low = highest risk) will be evaluated for association with 2y-DFS and treatment response.

While biomarker results may inform clinical discussions, treatment decisions remain within standard-of-care discretion unless future protocol amendments specify otherwise.

Outcomes Primary Outcome 2-year Disease-Free Survival (2y-DFS): time from surgery to first recurrence (local/distant), second primary CRC, or death from any cause.

Secondary Outcomes 2-year Overall Survival (2y-OS) 2-year local recurrence and distant metastasis rates ctDNA-MRD detection rates, clearance kinetics, and prognostic performance alone and combined with Immunoscore Comparative analysis of MRD dynamics and immune profiles between treatment arms Statistical Considerations The primary analysis will compare 2y-DFS between arms using Kaplan-Meier estimates and log-rank tests. Cox regression models will adjust for key covariates (e.g., stage, lymphovascular invasion, treatment arm). Biomarker analyses will be exploratory but pre-specified, with model performance metrics (C-index, net reclassification improvement) used to assess added value of combined ctDNA/Immunoscore stratification.

Eligibility Criteria (Summary) Key Inclusion: Age 18-70; ECOG 0-1; histologically confirmed stage II (high-risk) or III, non-MSI-H colon adenocarcinoma; CEA >5 ng/mL; no metastasis; R0 resection planned; ability to provide tumor tissue and serial blood samples.

Key Exclusion: MSI-H/dMMR; metastatic/peritoneal disease; prior malignancy; major comorbidity; pregnancy; inability to undergo contrast imaging or provide biospecimens; MRD panel customization failure.

Participant Support A dedicated research team will coordinate follow-up visits, sample collection, and communication of biomarker results. All study-related diagnostics-including NGS, Immunoscore, and serial ctDNA-MRD tests-are provided free of charge, and reports will be shared with participants and treating clinicians to support informed decision-making alongside standard care.

Inclusion Criteria:

Participants must meet all of the following criteria:

Age 18 to 70 years (inclusive) at the time of written informed consent. ECOG performance status 0-1, without deterioration within 2 weeks prior to enrollment; anticipated life expectancy ≥12 weeks.

Histologically or cytologically confirmed colon adenocarcinoma, non-MSI-H/dMMR, with pathologic stage (AJCC/UICC TNM 8th edition) of:

High-risk stage II, or Stage III. High-risk stage II features include: T4, poor/undifferentiated histology (high grade; excluding MSI-H), lymphovascular invasion, perineural invasion, preoperative bowel obstruction or tumor perforation, positive/unknown margin status, insufficient margin clearance, <12 lymph nodes examined, or high-grade tumor budding.

Tumor location consistent with colon cancer: distal tumor margin ≥12 cm from the anal verge on preoperative endoscopy.

Baseline serum CEA >5 ng/mL prior to treatment. No evidence of distant metastasis (distant organ and/or distant lymph node metastasis) based on comprehensive clinical evaluation.

Ability to provide required clinical data for study collection. Ability to provide adequate fresh tumor tissue from endoscopy and/or surgery for WES/NGS to develop an individualized ctDNA MRD panel, and ability to provide required blood samples for ctDNA testing (baseline, postoperative ~day 7, postoperative ~day 30).

Candidate for curative-intent R0 resection. Willing and able to comply with the protocol schedule, including regular follow-up visits and necessary treatments, and provides written informed consent.

Exclusion Criteria:

Participants will be excluded if any of the following apply:

Prior or concurrent other malignant tumor. Any severe comorbidity that, in the investigator's judgment, may significantly affect follow-up or short-term survival.

Any other medical condition, or social/psychological circumstance, that in the investigator's judgment makes the participant unsuitable for the study.

MSI-H/dMMR tumor. Evidence of metastatic disease by pathology, clinical assessment, or imaging, including isolated distant lesions, distant disease, or non-contiguous intraperitoneal metastasis.

Multiple primary colon cancers. Underwent open surgery at a non-colon site within 14 days prior to enrollment. Unable to provide required tumor tissue for WES/NGS or personalized MRD panel development, personalized MRD panel customization failure, or unable to provide required blood samples (baseline, postoperative ~day 7, postoperative ~day 30).

History of blood transfusion within 2 weeks prior to surgery or intraoperatively.

Unable to undergo contrast-enhanced CT or MRI for routine clinical follow-up. Fever ≥38°C within the past 7 days, or clinically significant active infection (including active tuberculosis), or active fungal/bacterial/viral infection requiring systemic therapy.

Inadequate bone marrow reserve or organ function meeting any of the following laboratory abnormalities (within 1 week prior to testing without corrective treatment):

ANC < 1.5 × 10⁹/L Platelets < 90 × 10⁹/L Hemoglobin < 90 g/L (<9 g/dL) ALT > 3 × ULN AST > 3 × ULN or total bilirubin > 1.5 × ULN Creatinine > 1.5 × ULN or creatinine clearance < 45 mL/min (Cockcroft-Gault) Albumin < 28 g/L Pregnant or breastfeeding, or planning pregnancy during the study period. Any other condition that, in the investigator's judgment, indicates the participant should not participate.