Treatment of Breast Cancer

This is an open-label, multicenter, Phase I study designed to evaluate the safety, tolerability, and pharmacokinetics of GDC-0077 administered orally as a single agent in participants with locally advanced or metastatic Phosphatidylinositol-4,5-Bisphosphate 3-Kinase Catalytic Subunit Alpha (PIK3CA)-mutant solid tumors, including breast cancer, and in combination with standard-of-care endocrine and targeted therapies for the treatment of locally advanced or metastatic PIK3CA-mutant hormone receptor-positive (HR+)/human epidermal growth factor receptor 2 negative (HER2-) breast cancer. Participants will be enrolled in two stages: a dose-escalation stage (Stage I) and an expansion stage (Stage II). Participants will be assigned to one of six regimens: GDC-0077 as a single agent (Arm A), GDC-0077 in combination with palbociclib and letrozole (Arm B), GDC-0077 in combination with letrozole (Arm C), GDC-0077 in combination with fulvestrant (Arm D), GDC-0077 in combination with palbociclib and fulvestrant (Arm E), and GDC-0077 in combination with palbociclib, fulvestrant, and metformin (Arm F).