Main Inclusion Criteria:

1. Histologically confirmed invasive adenocarcinoma of the breast
2. Known HER2+ breast cancer
3. Advanced or metastatic breast cancer harbouring PIK3CA mutation on tissue sample
4. > 2 lines of anti-HER2 based regimens with at least 1 regimen with trastuzumab
5. Radiological documented evidence of progressive disease
6. Life expectancy ≥ 12 weeks
7. Eastern Cooperative Oncology Group (ECOG) performance status ≤ 2

Main Exclusion Criteria:

1. Previous treatment with PI3K inhibitors
2. Brain metastases untreated, unless treated > 4 weeks and only if clinically stable and not receiving corticosteroids
3. History of clinically significant bowel disease
4. ≥ grade 2 diarrhoea
5. History of significant, uncontrolled, or active cardiovascular disease
6. Any serious and/or unstable pre-existing psychiatric or neurologic illness or other conditions that could interfere with patient's safety
7. Not controlled diabetes mellitus (glycated haemoglobin [HbA1c] >7%) and fasting plasma glucose >126 mg/dL
8. Concurrent chronic treatment with steroids, as immunosuppressant, or another immunosuppressive agent