A First-in-human, Clinical Trial Assessing the Safety of ES2B-C001-S01 With or Without [Adjuvant] in Patients With 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 \>140 ms, 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 \<50%.
* 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.