Main Inclusion Criteria:

          -  Histologically confirmed invasive adenocarcinoma of the breast

          -  Known HER2+ breast cancer

          -  Advanced or metastatic breast cancer harbouring PIK3CA mutation on tissue sample

          -  > 2 lines of anti-HER2 based regimens with at least 1 regimen with trastuzumab

          -  Radiological documented evidence of progressive disease

          -  Life expectancy ≥ 12 weeks

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

        Main Exclusion Criteria:

          -  Previous treatment with PI3K inhibitors

          -  Brain metastases untreated, unless treated > 4 weeks and only if clinically stable and
             not receiving corticosteroids

          -  History of clinically significant bowel disease

          -  ≥ grade 2 diarrhoea

          -  History of significant, uncontrolled, or active cardiovascular disease

          -  Any serious and/or unstable pre-existing psychiatric or neurologic illness or other
             conditions that could interfere with patient's safety

          -  Not controlled diabetes mellitus (glycated haemoglobin [HbA1c] >7%) and fasting plasma
             glucose >126 mg/dL

          -  Concurrent chronic treatment with steroids, as immunosuppressant, or another
             immunosuppressive agent