Abstract Background Approximately 40% of patients with hormone receptor (HR)‐positive and human epidermal growth factor receptor 2 (HER2)‐negative advanced breast cancer (ABC) exhibit PIK3CA mutations. Aims This study aims to evaluate the safety, tolerability, pharmacokinetics, and preliminary antitumor activity of HS‐10352, a selective PI3Kα inhibitor, in this patient population. Materials and Methods Conducted as a phase 1 dose‐escalation trial, HS‐10352 was administered orally once‐daily (QD) at dose levels of 2, 4, 6, and 8 mg. The primary endpoints were dose‐limiting toxicity (DLT) and the maximum tolerated dose (MTD). This study is registered at ClinicalTrials.gov (NCT04631835). Results Between August 2020 and March 2022, a total of 18 female patients were enrolled. DLT, manifested as hyperglycemia, occurred in two patients in the 8 mg QD group, establishing an MTD of 6 mg QD. The most common treatment‐related adverse events were hyperglycemia (88.9%) and weight loss (61.3%). In the 6 mg QD group, four patients (66.7%) had a partial response (PR), and one (16.7%) had stable disease (SD). Among the four patients with PIK3CA mutated tumors in this dosage group, three (75.0%) had PR and one (25.0%) had SD. The median progression‐free survival was not reached (95% confidence interval, 11.1‐NA). Discussion and Conclusion HS‐10352 at 6 mg QD was well‐tolerated in patients with HR‐positive, HER2‐negative ABC, and showed preliminary antitumor activity in patients with PIK3CA mutated tumors. These findings support the further clinical development of HS‐10352.
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