Inclusion Criteria:

          1. Is 18 Years and older in the United States or 20 Years and older in Japan

          2. Has a pathologically documented advanced/unresectable or metastatic breast cancer

          3. Documented HER3-positive disease measured by immunohistochemistry (IHC)

          4. Has disease that is refractory to or intolerable with standard treatment, or for which
             standard treatment no longer is available

          5. Has an Eastern Cooperative Oncology Group Performance Status 0-1

          6. Has Left Ventricular Ejection Fraction ≥ 50%

          7. Has measurable disease based on Response Evaluation Criteria in Solid Tumors (RECIST)
             version 1.1

             Additional Inclusion Criteria for Dose Finding Part and Dose Expansion Part:

          8. Has received 2-6 prior chemotherapy regimens for breast cancer, at least 2 of which
             were administered for treatment of advanced/unresectable or metastatic disease. At
             least 1 prior chemotherapeutic regimen must have included a taxane, administered in
             the neoadjuvant, adjuvant, or advanced setting. (With exception of Dose Expansion Part
             TNBC cohort. See additional inclusion criteria for Dose Expansion Part TNBC cohort.)

             Additional Inclusion Criteria for Dose Expansion Part Only:

          9. Is able to submit a fresh tumor biopsy sample prior to starting study treatment if not
             already submitted for HER3 expression

         10. Has documented hormone (estrogen and/or progesterone) receptor (HR)-positive and HER2
             negative expression according to American Society of Clinical Oncology - College of
             American Pathologists (ASCO-CAP) guidelines. (With exception of Dose Expansion Part
             TNBC cohort. See additional inclusion criteria for Dose Expansion Part TNBC cohort.)

             Additional Inclusion Criteria for Dose Expansion Part TNBC cohort Only:

         11. Has documented hormone (estrogen and progesterone) receptor (HR)-negative and HER2
             negative expression according to American Society of Clinical Oncology - College of
             American Pathologists (ASCO-CAP) guidelines

         12. Has progressed after receiving 1 to 2 prior chemotherapy regimens for
             advanced/unresectable or metastatic breast cancer.

        Exclusion Criteria:

          1. Prior treatment with a HER3 antibody

          2. Prior treatment with an antibody-drug conjugate (ADC) which consists of an exatecan
             derivative that is a topoisomerase I inhibitor (eg, DS-8201)

          3. Has a medical history of symptomatic congestive heart failure (New York Heart
             Association classes II-IV) or serious cardiac arrhythmia requiring treatment

          4. Has a medical history of myocardial infarction or unstable angina

          5. Has a corrected QT prolongation to > 450 millisecond (ms) in males and > 470 ms in
             females

          6. Has a medical history of clinically significant lung diseases (eg, interstitial
             pneumonia, pneumonitis, pulmonary fibrosis, and radiation pneumonitis) or who are
             suspected to have these diseases by imaging at screening period

          7. Has clinically significant corneal disease

             Additional Exclusion Criteria for Dose Expansion Part:

          8. Prior treatment with an govitecan derivative (eg, IMMU-132).