Study to Assess the Safety of Alpelisib Plus Fulvestrant, in Men and Post-menopausal Women With HR-positive, HER2-negative, Advanced Breast Cancer (aBC) With PIK3CA Mutation, Whose Disease Progressed on or After Endocrine Treatment

Key Inclusion Criteria: * Participants with confirmed PIK3CA mutant advanced or metastatic breast cancer * Postmenopausal females and males ≥ 18 years old with confirmed HR-positive, HER2-negative advanced or metastatic breast cancer. * Adequate liver function * Adequate renal function * Fasting plasma glucose (FPG) ≤140 mg/dL (7.7 mmol/L) and glycosylated hemoglobin (HbA1c) ≤ 6.4% * ECOG (Eastern Cooperative Oncology Group) Performance Status \< 2 * Fasting Serum amylase ≤ 2 × ULN and Fasting Serum lipase ≤ ULN * Potassium within normal limits, or corrected with supplements * Calcium (corrected for serum albumin) and magnesium within normal limits or ≤ grade 1 if judged clinically not significant by the investigator Key Exclusion Criteria: * Known hypersensitivity to alpelisib or fulvestrant, or to any of the excipients of alpelisib or fulvestrant * Participant ineligible for endocrine therapy per the investigator's judgment * Participant has received prior treatment with any PI3K inhibitors and / or mTOR inhibitor * Participant with type I diabetes or not controlled type II (based on FPG and HbA1c, see inclusion criterion 6) * Participant has a concurrent malignancy or malignancy within 3 years of study screening period, with the exception of adequately treated, basal or squamous cell carcinoma, non-melanomatous skin cancer or curatively resected cervical cancer * Participant has not recovered to grade 1 or better from related side effects of prior anti cancer therapy (w…