A Study of T-DXd in Participants With or Without Brain Metastasis Who Have Previously Treated Advanced or Metastatic HER2 Positive Breast Cancer

Inclusion:

Participants should have pathologically documented breast cancer that is: unresectable/advanced or metastatic; confirmed HER2-positive status expression as determined according to American Society of Clinical Oncology/College of American Pathologists guidelines
Participant must have either: no evidence of BM, or untreated BM on screening contrast brain magnetic resonance imaging/ computed tomography (MRI/CT) scan, not needing immediate local therapy or previously-treated stable or progressing BM
Participants with BMs must be neurologically stable
For participants requiring radiotherapy due to BMs, there should be an adequate washout period before day of first dosing:
≥ 7 days since stereotactic radiosurgery or gamma knife
≥ 21 days since whole brain radiotherapy
Eastern Cooperative Oncology Group performance status 0-1
Previous breast cancer treatment: radiologic or objective evidence of disease progression on or after HER2 targeted therapies and no more than 2 lines/regimens of therapy in the metastatic setting
Participant with the following measurable: at least 1 lesion that can be accurately measured at baseline as ≥ 10 mm in the longest diameter with CT or MRI and is suitable for accurate repeated measurements; or following Non-measurable diseases: Non-measurable, bone-only disease that can be assessed by CT or MRI or X-Ray. Lytic or mixed lytic bone lesions that can be assessed by CT or MRI or X-ray in the absence of measurable disease as defined above is acceptable; Participants with sclerotic/osteoblastic bone lesions only in the absence of measurable disease are not eligible; and Non-measurable CNS disease (Cohort 2 only)
Adequate organ and bone marrow function within 14 days before the day of first dosing as defined in the protocol
Left ventricular ejection fraction ≥ 50% within 28 days before enrollment
Negative pregnancy test (serum) for women of childbearing potential

Exclusion Criteria

Known or suspected leptomeningeal disease
Prior exposure to tucatinib treatment
Refractory nausea and vomiting, chronic gastrointestinal disease, or previous significant bowel resection that would preclude adequate absorption, distribution, metabolism, or excretion of T-DXd
History of another primary malignancy except for malignancy treated with curative intent with no known active disease within 3 years before the first dose of study intervention and of low potential risk for recurrence
Based on screening contrast brain MRI/CT scan, participants must not have any of the following: any untreated brain lesions > 2.0 cm in size; ongoing use of systemic corticosteroids for control of symptoms of BMs; any brain lesion thought to require immediate local therapy; have poorly controlled (> 1/week) generalized or complex partial seizures, or manifest neurologic progression due to BMs not withstanding CNS-directed therapy
Has spinal cord compression
Known active hepatitis B or C infection, such as those with serologic evidence of viral infection within 28 days of Cycle 1 Day 1. Participants with past or resolved hepatitis B virus infection are eligible, if negative for hepatitis B surface antigen and positive for anti-hepatitis B core antigen
Participants positive for hepatitis C virus (HCV) antibody are eligible only if polymerase chain reaction is negative for HCV RNA
Uncontrolled infection requiring IV antibiotics, antivirals, or antifungals
Receipt of live, attenuated vaccine within 30 days prior to the first dose of T-DXd
Participants with a medical history of myocardial infarction within 6 months before screening, symptomatic congestive heart failure (New York Heart Association Class II to IV)
History of (non-infectious) ILD/pneumonitis that required steroids, has current ILD/pneumonitis, or where suspected ILD/pneumonitis cannot be ruled out by imaging at screening
Lung-specific intercurrent clinically significant illnesses and any autoimmune, connective tissue or inflammatory disorders
Prior exposure, without adequate treatment washout period before the day of first dosing, to chloroquine/hydroxychloroquine: < 14 days
Anticancer chemotherapy: immunotherapy (non-antibody-based therapy), retinoid therapy, hormonal therapy: < 3 weeks
< 6 weeks for nitrosoureas or mitomycin
Antibody-based anticancer therapy: < 4 weeks
Any concurrent anticancer treatment. Concurrent use of hormonal therapy for noncancer- related conditions is allowed
Unresolved toxicities from previous anticancer therapy, defined as toxicities (other than alopecia) not yet resolved to Grade ≤ 1 or baseline
Palliative radiotherapy with a limited field of radiation within 2 weeks or with wide field of radiation, radiation to the chest, or to more than 30% of the bone marrow within 4 weeks before the first dose of study intervention
Participants with prior exposure to immunosuppressive medication within 14 days prior to first study dose
Participants with a known hypersensitivity to study intervention or any of the excipients of the product or other monoclonal antibodies