Inclusion Criteria:

- All participants must meet all of the following criteria to be eligible for
enrollment:

Informed Consent: The patient (and/or legal guardian, as age-appropriate) fully
understands the study, voluntarily agrees to participate, and signs a written informed
consent form (ICF). A separate biomarker consent form is required for EGFR testing prior
to screening.

Age: 2 to 18 years old at the time of consent. Life Expectancy: Estimated overall
survival of at least 3 months.

Histologically Confirmed Disease: Pathologically confirmed relapsed/refractory or
metastatic EGFR-positive solid tumor, belonging to one of the following subtypes:

Head and neck squamous cell carcinoma, nasopharyngeal carcinoma, or lymphoepithelial
carcinoma that progressed during or after at least one line of platinum-based
chemotherapy and PD-1/PD-L1 inhibitor therapy Rhabdomyosarcoma Neuroblastoma
Medulloblastoma Wilms tumor Atypical teratoid/rhabdoid tumors (AT/RTs) Diffuse intrinsic
pontine gliomas (DIPGs) Other EGFR-positive solid tumor subtypes deemed eligible by the
investigator Measurable Disease: At least one measurable tumor lesion by computed
tomography (CT) or magnetic resonance imaging (MRI) per RECIST v1.1 criteria (longest
diameter ≥10 mm; pathological lymph node short axis ≥15 mm).

Performance Status: Eastern Cooperative Oncology Group (ECOG) performance status score of
0, 1, or 2.

Adequate Bone Marrow Function:

Absolute neutrophil count (ANC) ≥1.5 × 10⁹/L Platelet count ≥75 × 10⁹/L Hemoglobin ≥80
g/L Exception for patients with bone marrow involvement: ANC ≥1.0 × 10⁹/L, platelets ≥50
× 10⁹/L, hemoglobin ≥75 g/L

Adequate Hepatic and Renal Function:

Serum creatinine ≤1.5 × upper limit of normal (ULN) Aspartate aminotransferase (AST) and
alanine aminotransferase (ALT) ≤2.5 × ULN Total bilirubin ≤1.5 × ULN Exception for
patients with liver involvement: AST/ALT ≤5 × ULN, total bilirubin ≤3 × ULN

Exclusion Criteria:

- Participants will be excluded from the study if they meet any of the following
criteria:

Hypersensitivity: Known hypersensitivity to any component of Becotatug Vedotin (MRG003)
or its excipients.

Symptomatic CNS Metastases: Presence of symptomatic central nervous system (CNS)
metastases.

Prior Malignancies: History of other primary malignant tumors, except for:

Locally excised basal cell or squamous cell carcinoma of the skin Cervical carcinoma in
situ Any prior malignancy that has been in complete remission for ≥3 years without
treatment Note: Melanoma (any stage) is explicitly excluded

Significant Liver Disease: Clinically significant liver disease, including:

Positive hepatitis C virus (HCV) antibody Chronic active hepatitis B (HBV DNA >20,000
IU/mL) HIV Infection: Known human immunodeficiency virus (HIV) infection. Severe Ocular
Abnormalities: History of severe ophthalmologic conditions, such as severe dry eye
syndrome or exposure keratitis.

Uncontrolled Systemic Diseases: Severe or uncontrolled medical conditions, including:

Interstitial lung disease or pneumonitis Active autoimmune diseases requiring systemic
immunosuppressive therapy

Cardiac Disease: Clinically significant cardiac dysfunction or cardiac disease,
including:

Congestive heart failure (New York Heart Association Class ≥II) Uncontrolled arrhythmias
QTc interval prolongation >450 ms (males) or >470 ms (females) Recent Antitumor Therapy:
Received any systemic antitumor therapy (chemotherapy, biological therapy, immunotherapy,
targeted therapy) within 3 weeks prior to the first dose of study drug, and have not
recovered to CTCAE v4.03 Grade ≤1 (except alopecia).

Recent Major Surgery: Underwent major surgical procedure within 3 weeks prior to the
first dose of study drug.

Planned Surgery: Planned surgical procedure during the study period, or any surgery
deemed necessary by the investigator.

Prior EGFR Therapy Toxicity: History of severe skin toxicity caused by prior
EGFR-targeted therapy, or chronic skin disease requiring ongoing oral or intravenous
treatment.

Other Significant Risks: Any other concurrent medical condition that, in the
investigator's judgment, would increase the risk of toxicity or compromise the patient's
ability to complete the study.