This clinical trial is evaluating tucatinib in combination with Doxil in participants with human epidermal growth factor 2 positive (HER2+) locally advanced or metastatic breast cancer. The main goals of this study are to:
* Learn how well the combination of tucatinib and Doxil works * Learn more about the side effects of the combination of tucatinib and Doxil
Written informed consent, according to local guidelines, signed and dated by the patient or by a legal guardian prior to the performance of any study-specific procedures, sampling, or analyses
At least 18 years-of-age at the time of signature of the informed consent form (ICF)
Eastern Cooperative Oncology Group (ECOG) Performance Status score of 0 or 1
Have a confirmed diagnosis of locally advanced/metastatic HER2+ breast cancer (based on local laboratory testing per American Society of Clinical Oncology (ASCO)/College of American Pathologists (CAP) guidelines immunohistochemistry 3+ (IHC3+) or fluorescence in situ hybridization + (FISH+))
Any systemic anti-cancer chemotherapy or small molecule, biologic, or hormonal agent from a previous treatment regimen or clinical study within 21 days or 5 half-lives (whichever is shorter) prior to the first dose of study drugs. At least 10 days must have elapsed between the last dose of such agent and the first dose of study drugs.
Prior treatment with anthracycline in any setting
Major surgery (excluding placement of vascular access) within 28 days of first dose of study drugs
Palliative radiation therapy within 14 days of first dose of study drugs