Inclusion Criteria:

          -  Signed and dated written informed consent prior to beginning of protocol-specific
             procedures.

          -  Histologically or cytologically confirmed adenocarcinoma of the breast. Locally
             advanced and inoperable or metastatic disease.

          -  HER2-positive disease, defined as IHC status HER2+++ or CISH/FISH status positive.

          -  Female patients aged ≥ 18 years.

          -  In case of adjuvant treatment, disease-free interval of at least 12 months after
             completion of adjuvant treatment (excluding hormonal therapy).

          -  Eastern Cooperative Oncology Group (ECOG) performance status 0-1.

          -  Left Ventricular Ejection Fraction (LVEF) ≥ 50%.

          -  For women with childbearing potential, defined as physiologically capable of becoming
             pregnant:

               -  Negative pregnancy test.

               -  Agreement to use an effective form of contraception during study treatment and
                  for 7 months after the last dose of study treatment.

          -  Life expectancy of at least 12 weeks.

          -  Adequate organ and bone marrow function

          -  Fluent in spoken and written German and willing to answer the questionnaires

        Exclusion Criteria:

          -  Previous systemic treatment in palliative intention (chemotherapy, hormonal therapy
             and / or biological therapy)

          -  Persistent peripheral sensory or motor neuropathy grade 2 or higher (NCI CTCAE v5.0)

          -  Evidence of central nervous system metastases. CT or MRI of the brain is only
             mandatory in case of clinical suspicion of brain metastases

          -  Current uncontrolled hypertension (systolic > 150 mmHg and / or diastolic > 100 mmHg)
             or clinically significant cardiovascular disease

          -  History of LVEF < 50% during or after prior (neo)adjuvant therapy with trastuzumab

          -  Current severe, uncontrolled systemic disease (e.g. cardiovascular, pulmonary, or
             metabolic disease, wound healing disorder, ulcers, or bone fractures, or severe
             fungal, bacterial or viral infection)

          -  Major surgery within 28 days prior to start of study medication, or anticipation of
             the need for major surgery during the course of study treatment

          -  Current known infection with HIV, HBV, or HCV (testing not required)

          -  Dyspnea at rest due to complications of advanced malignancy, or other diseases
             requiring continuous oxygen therapy.

          -  Known hypersensitivity to any of the study medications or to excipients of recombinant
             human or humanized antibodies.

          -  Participation in investigational studies within 30 days or five half-lives of the
             respective IMP, whichever is longer, prior randomization.

          -  Pregnant or lactating women.