Inclusion criteria

Inclusion Criteria

• Have confirmed HER2+ breast cancer, as defined by the current American Society of Clinical Oncology - College of American Pathologists (ASCO/CAP) guidelines, previously determined at a Clinical Laboratory Improvements Amendments (CLIA)-certified or International Organization for Standardization (ISO)-accredited laboratory.
• History of prior treatment with a taxane and trastuzumab in the LA/M setting OR progressed within 6 months after neoadjuvant or adjuvant treatment, including a taxane and trastuzumab.
• Have progression of unresectable LA/M breast cancer after last systemic therapy (as confirmed by investigator), or be intolerant of last systemic therapy
• Have measurable disease assessable by RECIST v1.1
• Have Eastern Cooperative Oncology Group Performance Status (ECOG PS) 0 or 1
• Have a life expectancy of at least 6 months, in the opinion of the investigator
• CNS Inclusion - Based on medical history and screening contrast brain magnetic resonance imaging (MRI), participants with a history of brain metastases must have one of the following:
  • Untreated brain metastases not needing immediate local therapy. For participants with untreated central nervous system (CNS) lesions \>2.0 cm on screening contrast brain MRI, discussion with and approval from the medical monitor is required prior to enrollment
  • Previously treated brain metastases
• Have previously been treated with:
  • Lapatinib or neratinib within 12 months of starting study treatment (except in cases where lapatinib or neratinib was given for ≤21 days and was discontinued for reasons other than disease progression or severe toxicity)
  • Tucatinib or enrolled on a tucatinib clinical trial
  • Any investigational HER2/epidermal growth factor receptor (EGFR) or HER2 tyrosine kinase inhibitor (TKI) (eg, afatinib) at any time previously
  • Trastuzumab deruxtecan or another antibody-drug conjugate (ADC) consisting of an exatecan derivati
• Have received treatment with:
  • Any systemic anti-cancer therapy (including hormonal therapy) or experimental agent ≤21 days of first dose of study treatment or are currently participating in another interventional clinical trial. An exception for the washout of hormonal therapies is gonadotropin releasing hormone (GnRH) agonists used for ovarian suppression in premenopausal women, which are permitted concomitant medications
  • Treatment with non-CNS radiation ≤7 days prior to first dose of study treatment
  • Major surgery \<28 days of first dose of study treatme
• Have clinically significant cardiopulmonary disease (such as history of iterstitial lung disease (ILD)/pneumonitis that required systemic corticosteroids, or have current ILD/pneumonitis, or where suspected ILD /pneumonitis cannot be ruled out be imaging at screening)
• Have known myocardial infarction or unstable angina within 6 months prior to first dose of study treatment
• Known to be positive for hepatitis B by surface antigen expression. Known to be positive for hepatitis C infection. Participants who have been treated for hepatitis C infection are permitted if they have documented sustained virologic response of 12 weeks
• Presence of known chronic liver disease
• Active or uncontrolled clinically serious infection
• Have inability to swallow pills or significant gastrointestinal disease which would preclude the adequate oral absorption of medications