Different Doses of Dalpicicl Combined With Letrozole in the First-line Treatment of HR-positive, HER2-negative Advanced Breast Cancer

This study is a prospective, open-label, randomized, multicenter, two-cohort phase II clinical trial. Starting from December 1, 2024, it plans to enroll 120 patients with advanced first-line HR-positive, HER2-negative breast cancer. A centralized randomization system (IWRS) will be used for randomization, and stratification will be performed based on the following factors during randomization: 1) Visceral metastasis (yes vs no); 2) Disease-free interval (previously untreated vs 12 \< DFI ≤ 24 vs DFI \> 24). Cohort A: Dalpiciclib 125mg + Letrozole 2.5mg Cohort B: Dalpiciclib 150mg + Letrozole 2.5mg Imaging assessment will be conducted in accordance with the RECIST 1.1 criteria, and tumor imaging evaluation will be performed by investigators from the participating centers. Patients receiving dalpiciclib will undergo a safety visit 28 days after the last dose, followed by survival follow-up until the patient's death or trial termination (whichever comes first). Pharmacokinetic assessment: Blood samples will be collected once before dosing on Cycle 1 Day 15 (C1D15), 4 hours after dosing on C1D15, before dosing on Cycle 2 Day 1 (C2D1), and before dosing on Cycle 4 Day 1 (C4D1) to explore the population pharmacokinetic characteristics of dalpiciclib and the factors affecting its pharmacokinetics. The first dosing time of the subjects, each blood collection time, the dalpiciclib dosing time within three days before blood collection, and the dalpiciclib dosing time on the day of C1D15 blood collection must be accurately recorded. If dalpiciclib is not administered within 14 days before the planned PK blood collection, no PK blood collection will be performed on the day of that visit. If possible, PK samples should be collected simultaneously with samples for other laboratory tests.