This is a study to compare the similarity in Pharmacokinetics (PK) profile of HLX319 vs. EU-Phesgo® in patients with HER2-positive early or locally advanced breast cancer .

This is a randomized, double-blind, parallel-controlled, multi-center Phase I equivalence study to compare the similarity in PK profile of HLX319 vs. EU-Phesgo® in patients with HER2-positive early or locally advanced breast cancer with a primary tumor > 2 cm or nodes-positive.

Inclusion Criteria:

• Voluntary participation in the clinical study and signed the Informed Consent Form (ICF).
• Male or female aged ≥ 18 years old at the time of signing the ICF;
• Histologically confirmed invasive breast cancer, stage II-IIIC, Human Epidermal Growth Factor Receptor 2 (HER2) positive confirmed by central laboratory.
• Participants agree to undergo surgery while meeting the criteria for surgery after neoadjuvant therapy.
• Left ventricular ejection fraction (LVEF) at baseline ≥ 55%.
• An Eastern Cooperative Oncology Group (ECOG) performance status (PS) score of 0-1.
• Adequate major organ functions.
• Women with child-bearing potential have a negative result of serum pregnancy test at screening period (within 7 days prior to the first dose) or if they are infertile, non- lactating, reproduction-age men and women following highly effective contraceptive measures until 7 months after last dose.

Exclusion Criteria:

• Stage IV breast cancer, bilateral breast cancer, or multicentric breast cancer.
• History of other malignancy within 5 years.
• Prior systemic therapy for breast cancer treatment or radiotherapy.
• Patients with a history of ductal carcinoma in situ (DCIS) or lobular carcinoma in situ (LCIS) who have received systemic therapy or radiotherapy to the ipsilateral breast.
• Patients who have undergone excision biopsy of the primary tumor and/or axillary lymph nodes or lymph node dissection.
• Have severe heart disease or medical conditions.
• Participants with viral hepatitis or those with autoimmune hepatitis, sclerosing cholangitis, or liver cirrhosis.
• Human Immunodeficiency Virus (HIV) infection, HIV antibody positive.
• Daily use of corticosteroid treatment is required.
• Sensitivity to any study medications or any of its ingredients or excipients.
• Participants who underwent any major surgery within 28 days prior to the first dose. Or participants who have received local radiotherapy, radiofrequency ablation, or interventional therapy within 2 weeks prior to the first dose.
• Received another interventional clinical trial therapy within 4 weeks prior to enrollment in the study, or intentionally participated in another interventional clinical trial during the entire study period.
• Severe, uncontrolled systemic diseases that may currently interfere with the therapeutic plan.
• Any other conditions which are inappropriate for the study in the opinion of the investigator.