The trial is a first-in-human, phase I, open-label, dose-escalating trial to assess the safety and tolerability of ES2B-C001 combined with or without [adjuvant], in patients with human epidermal growth factor receptor 2 (HER2) expressing metastatic breast cancer.

Inclusion Criteria:

• Patients aged ≥18 years at screening visit.
• Diagnosis of HER2-expressing locally advanced, unresectable, or metastatic BC, with HER2 IHC level 1+, 2+ (either FISH negative or positive), or IHC level 3+, after undergoing 2-3 lines of anticancer therapy.
• Life expectancy of at least 3 months.
• ECOG performance status 0-2.
• Patients have adequate bone marrow, kidney, liver, heart, and lung function without clinically significant laboratory parameters as judged by the investigator.
• 12-lead ECG without clinically significant abnormalities and no LBBB, QRS duration >140ms, or evidence of prior infarction.
• Recovered from side effects, adverse reactions, or adverse events due to prior therapy and/or surgery; any residual toxicities and toxicities related to current anticancer treatment must be ≤ Grade 2, except for alopecia, neuropathy or lymphoedema.
• If female, non-pregnant, postmenopausal, or practicing reliable contraception.
• If male, sterilized or using reliable contraception.

Exclusion Criteria:

• Any planned intravenous chemotherapy regimens or check point inhibitors, or previous therapy with those agents during the past 1 month and the patients are neutropenic. Maintenance therapy with a stable dose of HER2-directed mAbs or treatment with antibody drug conjugates (ADCs) for metastatic BC is allowed at the discretion of the treating physician.
• Symptomatic CNS metastatic disease requiring treatment with high dose steroids (i.e., above 10 mg of prednisone or equivalent) within 14 days prior to first administration of ES2B-C001 (with or without adjuvant).
• Concurrent or recent (within 21 days or 5 half-lives) involvement in any other clinical trial with an investigational drug, device, or other experimental intervention.
• Concomitant severe or uncontrolled underlying medical and/or mental disease unrelated to the tumor, which in the opinion of the investigator is likely to compromise patient safety and affect the trial's outcome.
• Previous documented coronary artery disease or congestive heart failure (>NYHA II).
• Echocardiography with LVEF <55%.
• Uncontrolled hypertension.
• Active, known, or suspected autoimmune disease, except thyroid conditions sufficiently controlled on thyroid hormone therapy, and controlled insulin dependent diabetes.
• Necessity for long-term immunosuppression (≤ 4 mg dexamethasone may be used transiently).
• Systemic infection requiring intravenous antibiotics within 14 days before dosing.
• Chronic use of anti-viral agents, except for human immunodeficiency virus (HIV) or hepatitis B or C virus (HBV, HCV) treatments.
• History of severe hypersensitivity reactions to any of the trial drug components.
• Has received a live or live-attenuated vaccine within 30 days prior to the first dose of trial treatment. Note: Administration of inactivated or recombinant vaccines/killed vaccines are allowed.
• Birthmarks, tattoos, wounds, or skin conditions on deltoid region/buttocks that may obscure the assessment of injection site reactions.
• Female patients who are pregnant, or lactating.
• Any infection (including SARS-CoV-2), that in the opinion of the investigator would, upon inclusion in the trial, lead to potentially harming patients' safety or integrity.

Within the study de-identified patient data will be shared with

• the operational study team, sponsor, auditor and for reporting of safety events to the regulatory agencies and EC
• Data Monitoring Committees/ or other independent committees during the study
• At the end - data listings are part of the Appendices of the CSR which will be shared to regulatory agencies and EC