Inclusion Criteria:

          -  Age ≥ 18 years and older

          -  Life expectancy > 3 months

          -  Eastern Cooperative Oncology Group Performance Status ≤ 1

          -  Metastatic breast cancer subjects previously treated with T DM1, and/or T-DXd, and/or
             tucatinib-containing regimens.

          -  Presence of at least one measurable lesion per RECIST v 1.1

          -  Subjects must have an adequate tumor sample available for confirmation of HER2 status

          -  Subjects with stable brain metastases

          -  Acute toxicities from any prior therapy, surgery, or radiotherapy must have resolved
             to Grade ≤1 as per the NCI-CTCAE v 5.0, except alopecia.

          -  Adequate organ functions

          -  Willing and able to understand and sign an informed consent inform and to comply with
             all aspects of the protocol.

          -  Female subjects must be surgically sterile, or have a monogamous partner who is
             surgically sterile, or at postmenopausal, or who commits to use an acceptable form of
             birth control; male subjects must be sterile (biologically or surgically) or commit to
             the use of a reliable method of birth control

        Exclusion Criteria:

        Any subject who meets any of the following criteria is excluded from the study:

          -  History of allergic reactions to any component of ARX788.

          -  Prior history of interstitial lung disease, pneumonitis, or other clinically
             significant lung disease within 12 months

          -  History of ocular events, or any current ongoing active ocular infections.

          -  History of congestive heart failure, unstable angina pectoris, unstable atrial
             fibrillation, or cardiac arrhythmia within 12 months prior to enrollment

          -  Grade 2 to 4 peripheral neuropathy (NCI CTCAE v 5.0)

          -  History of unstable central nervous system (CNS) metastases

          -  Current severe, uncontrolled systemic disease (eg, clinically significant
             cardiovascular, pulmonary, or metabolic diseases)

          -  Any uncontrollable intercurrent illness, infection (including subjects with active,
             symptomatic Covid-19 infections), or other conditions that could limit study
             compliance or interfere with assessments.

          -  Exposure to any other investigational or commercial anticancer agents or therapies
             administered with the intention to treat malignancy within 14 days before the first
             dose of ARX788.

          -  Clinically significant surgical intervention (excluding diagnostic biopsy) within 21
             days of the first dose of ARX788

          -  Radiotherapy administered less than 21 days prior to the first dose of ARX788, or
             localized palliative radiotherapy administered less than 7 days prior to the first
             dose of ARX788, or radiotherapy-induced toxicity of Grade 2 or greater based on
             NCI-CTCAE v 5.0.

          -  Pregnancy or breast feeding.

          -  Known active HCV, HBV, and/or HIV infection.