This is a phase 2, multicentre, open-label, randomised, controlled trial with a parallel-group design. The study aims to evaluate the efficacy and safety of single-agent Becotatug Vedotin as adjuvant therapy in patients with high-risk locoregionally advanced nasopharyngeal carcinoma (NPC).

Eligible patients with high-risk locoregionally advanced NPC, defined as T4N1M0 disease, any T with N2-3M0 disease, stable disease (SD) or progressive disease (PD) after induction chemotherapy, or detectable plasma EBV DNA following induction chemotherapy, who have completed curative-intent chemoradiotherapy, will be randomly assigned to either the adjuvant Becotatug Vedotin group or the observation group. Participants assigned to the experimental group will receive Becotatug Vedotin at a dose of 2.3 mg/kg administered intravenously on Day 1 of each 21-day cycle for a total of three cycles. Participants assigned to the control group will undergo observation alone.

The primary endpoint is event-free survival (EFS). Secondary endpoints include overall survival (OS), distant metastasis-free survival (DMFS), locoregional failure-free survival (LRFFS), and safety. All efficacy analyses will be performed in the intention-to-treat (ITT) population. Safety analyses will be conducted in the safety population, defined as all randomized patients.

Inclusion Criteria:

1. Histologically confirmed non-keratinizing nasopharyngeal carcinoma(WHO II/III).
2. Eligible patients with high-risk locoregionally advanced NPC, defined as AJCC (9th edition) stage T4N1M0 or Tany N2-3M0 disease, or persistent high-risk features after induction chemotherapy (SD, PD, or detectable plasma EBV DNA).
3. ECOG performance status ≤1.
4. Induction and concurrent chemoradiotherapy with the recommended regimen have been completed.
5. No later than 6 weeks after the completion of the last radiotherapy treatment.
6. Adequate hematologic (neutrophil count > 1.5×10^9/L, hemoglobin > 90g/L and platelet count > 100×10^9/L), hepatic (alanine aminotransferase, aspartate aminotransferase ≤ 1.5×ULN, bilirubin ≤ 1.5×ULN, alkaline phosphatase < 2.5×ULN) and renal function (creatinine clearance > 50 ml/min)
7. Patients must be informed of the investigational nature of this study and give written informed consent.
8. Women of childbearing potential (WOCBP) who are sexually active must be willing to adhere to effective contraception during treatment and for 1 year after the last dose of the study drug. Men who are sexually active with WOCBP must be willing to adhere to effective contraception during treatment and for 1 year after the last dose of the study drug.

Exclusion Criteria:

1. Patients who could not tolerate or were allergic to Becotatug Vedotin.
2. Patients with severe chronic or active infection that must be treated with systemic antibacterial, antifungal, or antiviral therapy before randomization, including but not limited to tuberculosis infection.
3. Prior malignancy except adequately treated basal cell or squamous cell skin cancer, in situ cervical cancer.
4. Pregnancy or lactation (consider pregnancy test in women of child-bearing age and emphasize effective contraception during the treatment period).
5. Interstitial lung disease or pneumonia requiring oral or intravenous steroid therapy within 1 year.
6. Patients who are known to be intolerant or sensitive to any therapeutic agents.
7. Any severe intercurrent disease, which may bring unacceptable risk or affect the compliance of the trial, for example, unstable cardiac disease requiring treatment, renal disease, chronic hepatitis, diabetes with poor control (fasting plasma glucose > 1.5×ULN), and emotional disturbance.