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 \>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 …