Key Inclusion Criteria:

          -  Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1

          -  Measurable disease by RECIST v1.1 with at least one measurable target lesion

          -  Locally advanced or metastatic HER2-positive breast cancer that has relapsed or is
             refractory to established therapies

          -  Adequate hematologic and end-organ function

          -  For dose-expansion cohort only: no more than two prior systemic
             chemotherapy-containing regimens in the advanced/metastatic setting (excluding
             trastuzumab emtansine, which is considered a targeted cytotoxic agent)

        Key Exclusion Criteria:

          -  Treatment with chemotherapy, hormonal therapy (except hormone replacement therapy,
             oral contraceptives), immunotherapy, biologic therapy, radiation therapy (except
             palliative radiation to bony metastases), or herbal therapy as cancer therapy within 4
             weeks prior to initiation of DHES0815A

          -  History of exposure to the protocol specified doses of anthracyclines

          -  Pregnancy, lactation, or breastfeeding

          -  Major surgical procedure within 4 weeks prior to Day 1

          -  Evidence of a significant uncontrolled concomitant disease of the nervous system,
             pulmonary, autoimmune, renal, hepatic, endocrine, or gastrointestinal disorders; or a
             serious non-healing wound or fracture

          -  Known active bacterial, viral, fungal, mycobacterial, or other infection

          -  Clinically significant history of liver disease, including active viral or other
             hepatitis, current alcohol abuse, or cirrhosis

          -  Untreated or active central nervous system metastases

          -  Cardiopulmonary dysfunction, including inadequate left ventricular ejection function
             at baseline, less than 50% by either echocardiogram or multiple-gated acquisition scan

          -  QT interval corrected through use of Fridericia's formula (QTcF) > 470 milliseconds