Abstract PS11-11: Targeted safety events from a phase I/Ib study evaluating GDC-0077 alone and in combination with endocrine therapy (ET) ± palbociclib (palbo) in patients (pts) with PIK3CA-mutant (mut), hormone receptor-positive/HER2-negative metastatic breast cancer (HR+/HER2- mBC)

Abstract Background Activating mutations in PIK3CA, encoding the p110α subunit of phosphatidylinositol 3-kinase (PI3K) occur in ~40% of HR+/HER2- BCs. GDC-0077, a PI3Kα-selective inhibitor and mutant PI3Kα degrader, is being developed as an anticancer agent. A phase I/Ib study of GDC-0077 alone and combined with other therapies is ongoing in pts with locally advanced or metastatic PIK3CAmut, HR+/HER2- mBC (NCT03006172). Targeted safety events are presented here. Methods Safety was assessed via NCI-CTCAE v4 for GDC-0077 administered alone (Arm A), with letrozole and palbo (Arm B), with letrozole (Arm C), with fulvestrant (Arm D), or with fulvestrant and palbo (Arm E, plus metformin in Arm F …