Study Comparing DS-8201a and T-DM1 for HER2-Positive Breast Cancer That Cannot Be Removed or Has Spread

Inclusion Criteria

• Must be able to understand and sign a consent form approved by an ethics committee before any study procedures.
• Adults aged 18 years or older.
• Have breast cancer that cannot be surgically removed or has spread to other parts of the body.
• Cancer must be confirmed as HER2-positive by a central laboratory.
• Previously treated with trastuzumab and taxane, either in advanced/metastatic settings or within 6 months after completing treatment that included these drugs.
• Must show documented progression of cancer on scans during or after the most recent treatment, or within 6 months after completing adjuvant therapy.
• Must have at least one measurable tumor according to specific criteria.

Exclusion Criteria

• Previously treated with an anti-HER2 ADC like T-DM1 in the metastatic setting, unless it was in the adjuvant/neoadjuvant setting and the disease did not progress within 12 months after treatment.
• Have uncontrolled or significant heart disease, including recent heart attack, heart failure, or abnormal heart rhythms.
• Have a history of lung conditions like ILD/pneumonitis that required steroids, or current lung issues that cannot be ruled out by imaging.
• Have untreated or active brain metastases, or require medication to control symptoms from brain metastases.
• Have severe allergic reactions to the study drugs or similar medications.