ARX788 in Breast Cancer With Low Expression of HER2

Inclusion Criteria: * Willing and able to understand and sign an informed consent inform; * Age ≥18 years, and ≤75 years, male or female; * Diagnosed as HR arbitrary/HER2 low expression unresectable and/or metastatic breast cancer. HER2 low expression is defined as HER2 IHC 1+ (FISH negative or FISH not done) or HER2 IHC 2+ and FISH negative; * Received ≥2 lines of systemic chemotherapy regimens for recurrent or metastatic disease, and for HR positive subjects, also need to have received ≥2-line endocrine therapy ± targeted therapy (including neoadjuvant/adjuvant therapy); * Never had a HER2 positive result (IHC 3+ or FISH+) in previous pathological examinations; * Have archived or fresh tumor tissue samples for HER2 status confirmation; * According to the RECIST 1.1 standard, there is at least one measurable lesion; * Eastern Cooperative Oncology Group (ECOG) performance status of 0 or1; LVEF≥50%; * Adequate organ functions; * Acute toxicities from any prior therapy, surgery, or radiotherapy must have resolved to Grade ≤1; * Life expectancy ≥ 3 months. Exclusion Criteria: * History of allergic reactions to any component of ARX788, or with a history of protein drug allergy, a history of specific allergies (asthma, rheumatism, eczematous dermatitis), or a history of other severe allergic reactions, who are unsuitable for ARX788 treatment as per the investigator's judgments; * Previous treatment with T-DM1 or other HER2-ADC drugs; * Have other malignant tumors within 5 years before signing the informed consent form (except for non-melanoma skin cancer, cervical carcinoma in situ or other tumors that have been effectively treated, except malignant tumors that are considered cured); * Have primary central nervous system (CNS) tumors or CNS metastases; * Prior history of interstitial pulmonary disease requiring hormone therapy, drug-induced interstitial pulmonary disease, radiation pneumonia, or current clinically active interstitial pulmonary disease; * Suffering from keratitis, corneal diseases, retinal diseases or active eye infections that require intervention; * Unwilling or unable to stop wearing contact lenses for the duration of the study; * Cardiac insufficiency; * Uncontrolled hypertension; * Suffering severe or uncontrolled systemic diseases; * Had chemotherapy, radiotherapy, or immunotherapy within 4 weeks before the first dose; * Had breast cancer endocrine therapy within 2 weeks before the first dose; * Had palliative radiotherapy for bone metastasis within 2 weeks before the first dose; * Any uncontrolled infection, or other conditions that may limit trial compliance or interfere with evaluation; * Current known active infection with human immunodeficiency virus (HIV), hepatitis B virus, hepatitis C virus or syphilis; * Plan to receive major surgical treatment within 2 weeks before first dose or during the trial period, or have suffered severe traumatic injury; * Pregnancy or lactation; * Unwilling or unable to use acceptable methods for contraception during the entire treatment period of this trial and within 8 months after the last dose; * Participated in other clinical trials and used other experimental drugs within 4 weeks before the first dose; * Any mental or cognitive impairment may limit their understanding and implementation of the informed consent form; * Not suitable for participating in this trial, such as poor compliance.