Study of DS-8201a in Subjects With Advanced Solid Malignant Tumors

This study is one single group of participants in two periods. It is therefore not
randomized. The dosage strength will change during the study but all participants will
receive the same study drug DS-8201a. So the study is not a true 2 arm study, it is a 2 part
study.

Inclusion Criteria:

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

- Left Ventricular Ejection Fraction (LVEF) ≥ 50%

Part 1:

- Advanced/unresectable or metastatic breast cancer or gastric or gastroesophageal
junction adenocarcinoma that is refractory to or intolerable with standard treatment,
or for which no standard treatment is available.

Part 2a:

- Advanced breast cancer with HER2 overexpression that is refractory to or intolerable
with standard treatment, or for which no standard treatment is available.

- Treated with ado-trastuzumab emtansine (T-DM1)

Part 2b:

- Advanced gastric or gastroesophageal junction adenocarcinoma with HER2 overexpression
that is refractory to or intolerable with standard treatment, or for which no standard
treatment is available.

- Treated with trastuzumab

Part 2c:

- Advanced breast cancer with HER2 low expression that is refractory to or intolerable
with standard treatment, or for which no standard treatment is available.

Part 2d:

- Satisfy at least one of the following criteria

1. Advanced/unresectable or metastatic solid malignant tumor with HER2 expression
other than breast cancer and gastric or gastroesophageal junction adenocarcinoma
that is refractory to or intolerable with standard treatment, or for which no
standard treatment is available.

2. Advanced/unresectable or metastatic tumor with HER2 mutation that is refractory
to or intolerable with standard treatment, or for which no standard treatment is
available.

Part 2e:

- Advanced breast cancer with HER2 overexpression that is refractory to or intolerable
with standard treatment, or for which no standard treatment is available.

- Treated with ado-trastuzumab emtansine (T-DM1) (patients with HER2 overexpression
only)

Exclusion Criteria:

- Has a medical history of symptomatic Congestive Heart Failure (CHF) (NYHA classes
II-IV) or serious cardiac arrhythmia.

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

- Has a QTc prolongation to > 450 millisecond (ms) in males and > 470 ms in females.

- Has a medical history of clinically significant lung diseases