BACKGROUND: The phase III SANDPIPER study assessed taselisib (GDC-0032), a potent, selective PI3K inhibitor, plus fulvestrant in estrogen receptor-positive, HER2-negative, PIK3CA-mutant locally advanced or metastatic breast cancer. PATIENTS AND METHODS: Postmenopausal women with disease recurrence/progression during/after an aromatase inhibitor were randomized 2 : 1 to receive taselisib (4 mg; taselisib arm) or placebo (placebo arm) plus fulvestrant (500 mg). Stratification factors were visceral disease, endocrine sensitivity, and geographic region. Patients with PIK3CA-mutant tumors (central cobas® PIK3CA Mutation Test) were randomized separately from those without detectable mutations. The primary endpoint was investigator-assessed progression-free survival (INV-PFS) in patients with PIK3CA-mutant tumors. Secondary endpoints included objective response rate, overall survival, clinical benefit rate, duration of objective response, PFS by blinded independent central review (BICR-PFS), safety, and time to deterioration in health-related quality of life. RESULTS: The PIK3CA-mutant intention-to-treat population comprised 516 patients (placebo arm: n = 176; taselisib arm: n = 340). INV-PFS was significantly improved in the taselisib {7.4 months [95% confidence interval (CI), 7.26-9.07]} versus placebo arm (5.4 months [95% CI, 3.68-7.29]) (stratified hazard ratio [HR] 0.70; 95% CI, 0.56-0.89; P = 0.0037) and confirmed by BICR-PFS (HR 0.66). Secondary endpoints, including objective response rate, clinical benefit rate, and duration of objective response, showed consistent improvements in the taselisib arm. Safety was assessed in all randomized patients who received at least one dose of taselisib/placebo or fulvestrant regardless of PIK3CA-mutation status (n = 629). Serious adverse events were lower in the placebo versus taselisib arm (8.9% versus 32.0%). There were more discontinuations (placebo arm: 2.3%; taselisib arm: 16.8%) and dose reductions (placebo arm: 2.3%; taselisib arm: 36.5%) in the taselisib arm. CONCLUSION: SANDPIPER met its primary endpoint; however, the combination of taselisib plus fulvestrant has no clinical utility given its safety profile and modest clinical benefit.