Inclusion Criteria:

- Female or male patients with histologically confirmed human epidermal growth factor
receptor 2-positive (HER2+) breast cancer who have completed chemotherapy in
combination with P+H IV and are currently receiving or will be receiving maintenance
P+H IV, PH FDC SC, or trastuzumab SC (regardless of remaining treatment cycles
[e.g., only 1 cycle remaining])

- HER2+ status must have been previously determined and is defined as 3+ by
immunohistochemistry (IHC) and/or positive by HER2 amplification by in situ
hybridization (ISH) with a ratio of ≥2 for the number of HER2 gene copies to the
number of chromosome 17 copies

- Eastern Cooperative Oncology Group (ECOG) performance status 0-2

- Intact skin at planned site of subcutaneous (SC) injections (thigh)

- Baseline and most recent (within 3 months) LVEF ≥ 50% measured by echocardiogram
(ECHO) or multiple-gated acquisition scan (MUGA)

- For women of childbearing potential: agreement to remain abstinent (refrain from
heterosexual intercourse) or use contraceptive measures, and agreement to refrain
from donating eggs, as defined in the protocol

- For men: agreement to remain abstinent (refrain from heterosexual intercourse) or
use a condom, and agreement to refrain from donating sperm, as defined in the
protocol

Exclusion Criteria:

- Current or prior history of active malignancy (other than current breast cancer)
within the last 5 years. Appropriately treated non-melanoma skin cancer; in situ
carcinomas, including cervix, colon, or skin; or Stage I uterine cancer within the
last 5 years are allowed

- Investigational treatment within 4 weeks of enrollment

- Patients with any severe infection within 4 weeks prior to initiation of study
treatment, including, but not limited to, hospitalization for complications of
infections should not be enrolled in the trial (in the current situation, this also
applies to patients with suspected or confirmed COVID-19 infection). Patients with
suspected or confirmed COVID-19 may be re-screened for eligibility following
physician-prescribed COVID-19 treatment and/or a quarantine period consistent with
current Centers for Disease Control and Prevention (CDC) guidelines.

- Patients who may have had a recent episode of thromboembolism and are still trying
to optimize the anticoagulation dose and/or have not normalized their INR

- Serious cardiac illness or medical conditions

- History of ventricular dysrhythmias or risk factors for ventricular dysrhythmias,
such as structural heart disease (e.g., severe left ventricular systolic dysfunction
[LVSD], left ventricular hypertrophy), coronary heart disease (symptomatic or with
ischemia demonstrated by diagnostic testing), clinically significant electrolyte
abnormalities (e.g., hypokalemia, hypomagnesemia, hypocalcemia), or family history
of sudden unexplained death or long QT syndrome

- Inadequate bone marrow function

- Impaired liver function

- Renal function with creatinine clearance <50 mL/min using the Cockroft-Gault formula

- Major surgical procedure unrelated to breast cancer within 28 days prior to study
entry or anticipation of the need for major surgery during the course of study
treatment

- Current severe, uncontrolled systemic disease that may interfere with planned
treatment (e.g., clinically significant cardiovascular, pulmonary, or metabolic
disease; wound-healing disorders)

- Pregnant or breastfeeding, or intending to become pregnant during the study or
within seven months after the last dose of study treatment

- Any serious medical condition or abnormality in clinical laboratory tests that, in
the investigator's judgment, precludes the patient's safe participation in, and
completion of, the study

- Known active liver disease, for example, active viral hepatitis infection (i.e.,
hepatitis B or hepatitis C), autoimmune hepatic disorders, or sclerosing cholangitis

- Concurrent, serious, uncontrolled infections, or known infection with human
immunodeficiency virus (HIV)

- Known hypersensitivity to any of the study drugs, excipients, and/or murine proteins
or a history of severe allergic or immunological reactions, e.g. difficult to
control asthma

- Previously experienced severe injection related reactions with P+H IV, PH FDC SC,
and/or trastuzumab SC

- Current chronic daily treatment with corticosteroids (dose >10 mg methylprednisolone
or equivalent excluding inhaled steroids)