Safety, Tolerability, and Pharmacokinetic (PK) Study of DHES0815A in Participants With Human Epidermal Growth Factor Receptor (HER)2-Positive Breast Cancer

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