A phase II study of the PI3K inhibitor taselisib (GDC-0032) combined with fulvestrant (F) in patients (pts) with HER2-negative (HER2-), hormone receptor-positive (HR+) advanced breast cancer (BC).

520Background: The PI3K pathway is activated in HR+ BC, often via gain-of-function mutations in PIK3CA that occur in ~40% of HR+ BC. Taselisib is a potent and selective PI3K inhibitor, with greater selectivity against mutant PI3Kα isoforms than wild type (WT) PI3Kα. Phase Ib data demonstrated good tolerability and preliminary efficacy for taselisib + F in HR+ BC. Methods: This Phase II, open-label, single-arm study enrolled post-menopausal pts with HER2-, HR+ locally advanced or metastatic BC (mBC) who had progression or non-response to ≥ 1 prior endocrine therapy in adjuvant or mBC settings. Pts received taselisib (6 mg capsule PO qd) plus F (500 mg IM on Cycle 1 Day 1, Cycle 1 Day 15, the…