MEN1611 With Trastuzumab (+/- Fulvestrant) in Metastatic Breast Cancer

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