Inclusion Criteria:

- Diagnosis of metastatic breast cancer.

- Participants must have completed first line of metastatic chemotherapy and have
achieved complete or partial response or disease stability for at least 6 months from
the first confirmation of disease stabilization.

- No clinical sign of brain progression.

- At least one of the following 3 conditions: HER2 +++, Young age (< 50 years), and/or
estrogen receptor (ER)-/progesterone receptor (PgR)-

- Eastern Cooperative Oncology Group (ECOG) performance status ≤2.

- Life expectancy ≥3 months.

- Neutrophils ≥1.5 x 10^9/L, platelets ≥100 x 10^9/L, hemoglobin ≥9 g/dL, lymphocytes
≥1 x 10^9/L.

- Bilirubin level either normal or <1.5 x ULN (upper limit of normal).

- AST (aspartate aminotransferase) and ALT (alanine aminotransferase) ≤2.5 x ULN (≤5 x
ULN if liver metastases are present).

- Serum creatine <1.5 x ULN.

- Effective contraception if the risk of conception exists.

Exclusion Criteria:

- Concurrent chronic systemic immune therapy not indicated in the study protocol.

- Any investigational agent(s) within 4 weeks prior to entry.

- Participants that have completed 2nd, 3rd, or 4th line metastatic chemotherapy or
participant in active chemotherapy treatment.

- Clinically relevant coronary artery disease or a history of a myocardial infarction
within the last 12 months.

- Acute or sub acute intestinal occlusion or history of inflammatory bowel disease.

- Known Grade 3 or Grade 4 allergic reaction to any of the components of the treatment.

- Known drug abuse/alcohol abuse.

- Legal incapacity or limited legal capacity.

- Medical or psychological condition which in the opinion of the investigator would not
permit the participant to complete the study or sign meaningful informed consent.

- Women who are pregnant or breastfeeding.

- Any concurrent malignancy other than non-melanoma skin cancer, or carcinoma in situ
of the cervix (participants with a previous malignancy but without evidence of
disease for ≥5 years will be allowed to enter the trial).