Phase 1, Multicenter, Open Label Study of DS-8201a to Assess Safety and Pharmacokinetics in Subjects With HER2 Positive Advanced and/or Refractory Gastric, Gastroesophageal Junction Adenocarcinoma, or Breast Cancer

HER2-positive cancer is a cancer that tests positive for a protein called human epidermal
growth factor receptor 2 (HER2). HER2 promotes the growth of certain cancer cells. This study
will test an experimental drug called DS-8201a that has not been approved by the health
authorities yet.

DS-8201a will be tested for safety in Chinese patients with certain types of stomach and
breast cancer that test positive for HER2. It also will test how DS-8201a moves within the
body (pharmacokinetics).