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-positive metastatic or locally advanced inoperable breast cancer (MBC).

• Life expectancy of at least 6 months.

• ECOG performance status 0-2.

• Patients have adequate bone marrow, kidney, liver, heart, and lung function withoutclinically 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 priortherapy and/or surgery; any residual toxicities and toxicities related to currentanticancer treatment must be ≤ Grade 2, except for alopecia, neuropathy orlymphoedema.

• If female, non-pregnant, postmenopausal, or practicing reliable contraception.

• If male, sterilized or using reliable contraception.

Exclusion Criteria:

• Any planned intravenous chemotherapy regimens, antibody drug conjugates or checkpoint inhibitors, or previous therapy with those agents during the past 2 months, or 5 half-lives whichever is longer. For MBC maintenance therapy with a stable dose ofHER2-directed mAbs is allowed.

• Symptomatic CNS metastatic disease requiring treatment with high dose steroids (i.e., above 10 mg of prednisone or equivalent) within 14 days prior to firstadministration of ES2B-C001 (with or without adjuvant).

• Concurrent or recent (within 21 days or 5 half-lives) involvement in any otherclinical trial with an investigational drug, device, or other experimentalintervention.

• Concomitant severe or uncontrolled underlying medical and/or mental diseaseunrelated to the tumor, which in the opinion of the investigator is likely tocompromise 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 conditionssufficiently controlled on thyroid hormone therapy, and controlled insulin dependentdiabetes.

• Necessity for long-term immunosuppression (≤ 4 mg dexamethasone may be usedtransiently).

• Systemic infection requiring intravenous antibiotics within 14 days before dosing.

• Chronic use of anti-viral agents, except for human immunodeficiency virus (HIV) orhepatitis 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 firstdose of trial treatment. Note: Administration of inactivated or recombinantvaccines/killed vaccines are allowed.

• Birthmarks, tattoos, wounds, or skin conditions on deltoid region/buttocks that mayobscure 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 orintegrity.