Background: As one of the most frequent genomic alterations in BC, PIK3CA mutations occur in ~40% of ER-positive, HER2-negative breast tumors. PIK3CA mutations may mediate resistance to endocrine therapies and promote growth and proliferation of tumors in BC. Taselisib is a potent and selective PI3K inhibitor that preferentially degrades mutant versus wild-type PI3Kα via a unique mechanism not seen with alpelisib and pictilisib. In PIK3CA-mutant BC cell lines, taselisib had enhanced activity. Confirmed partial responses were reported in pts with PIK3CA-mutant BC treated with taselisib either as a single agent or in combination with fulvestrant. Methods: SANDPIPER is a double-blind, placebo-controlled, randomized, phase III study, designed to evaluate the efficacy and safety of taselisib plus fulvestrant in pts with ER-positive, HER2-negative, PIK3CA-mutant locally advanced or metastatic BC. Postmenopausal pts will be randomized 2:1 to receive either taselisib (4 mg qd) or placebo in combination with fulvestrant (500 mg intramuscular on Days 1 and 15 of Cycle 1, and on Day 1 of each subsequent 28-day cycle). Pts must have had disease recurrence or progression during or after aromatase inhibitor treatment. Randomization will be stratified by visceral disease, endocrine sensitivity, and geographic region. SANDPIPER enriches for pts with PIK3CA-mutant tumors and a centrally assessed, valid cobas PIK3CA Mutation Test result in tumor tissue is required prior to enrollment; pts with PIK3CA-mutant tumors are randomized separately from those with non-mutant tumors. The primary efficacy endpoint is investigator-assessed progression-free survival in pts with PIK3CA-mutant tumors (estimated by Kaplan–Meier methodology). Other endpoints include overall survival, objective response rate, clinical benefit rate, duration of objective response, safety, pharmacokinetics, and patient-reported outcomes. Enrollment is open for pts with PIK3CA-mutant tumors. Target enrollment is 600 pts and > 300 patients have been enrolled. Clinical trial information: NCT02340221