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 (CNS) metastases
* Cardiopulmonary dysfunction, including inadequate left ventricular ejection function at baseline, less than 50% by either echocardiogram (ECHO) or multiple-gated acquisition scan (MUGA)
* QT interval corrected through use of Fridericia's formula (QTcF) \> 470 milliseconds (ms)