Taselisib is a potent and selective tumor growth inhibitor through PI3K pathway suppression. Thirty-four patients with locally advanced or metastatic solid tumors were treated (phase I study, modified 3+3 dose escalation; 5 cohorts; 3-16 mg taselisib once-daily capsule). Taselisib pharmacokinetics were dose-proportional; mean half-life was 40 hours. Frequent dose-dependent, treatment-related adverse events included diarrhea, hyperglycemia, decreased appetite, nausea, rash, stomatitis, and vomiting. At 12 and 16 mg dose levels, dose-limiting toxicities (DLT) were observed, with an accumulation of higher-grade adverse events after the cycle 1 DLT assessment window. Pharmacodynamic findings showed pathway inhibition at ≥3 mg in patient tumor samples, consistent with preclinical -mutant tumor xenograft models. Confirmed response rate was 36% for -mutant tumor patients with measurable disease 5/14: 4 breast cancer (3 patients at 12 mg); 1 non-small cell lung cancer, where responses started at 3 mg, and 0% in patients with tumors without known hotspot mutations (0/15). Preliminary data consistent with preclinical data indicate increased antitumor activity of taselisib in patients with -mutant tumors (in comparison with patients with tumors without known activating hotspot mutations) starting at the lowest dose tested of 3 mg, thereby supporting higher potency for taselisib against -mutant tumors. .