Schéma : This is a randomized, 2-arm, Phase 3, open-label, multicenter study to compare the safety and efficacy of DS8201a versus the physician's choice (2:1) in HER2-low, unresectable and/or metastatic breast cancer subjects.

The study is expected to enroll ~360 DS-8201a subjects and ~180 physician's choice subjects:

60 hormone receptor (HR)-negative
240 HR-positive who have had prior therapy with a cyclin-dependent kinase (CDK) 4/6 inhibitor
240 HR-positive who are naive to treatment with CDK 4/6
There will be approximately 161 sites, including but not limited to, North America, Western Europe, and Asia.

There are planned follow-up visits after permanent discontinuation of study treatment to obtain information about subsequent treatment(s) and survival status.

The Sponsor proposes to define a new HER2-low population in this trial including tumors with IHC 1+ and IHC 2+/ISH- HER2 expression.

2 treatment arms:
- Experimental: DS-8201a
DS-8201a is administered as an intravenous (IV) infusion every 21 days (Q3W), initially for at least 90 minutes, then, if there is no infusion-related reaction, for a minimum of 30 minutes thereafter.
- Active Comparator: Physician's Choice
Physician's choice comparative therapy will be administered in accordance with the locally approved label. The physician's choice is predefined, prior to randomization, from the following options:
Capecitabine
Eribulin
Gemcitabine
Paclitaxel
Nab-paclitaxel