Abstract PO3-02-12: Dalpiciclib combination with letrozole as neoadjuvant therapy for HR-positive HER2-negative breast cancer: a single-arm, prospective exploratory clinical study

Abstract Background: CDK4/6 inhibitors effectively block tumor cells from progressing from the G1 phase to the S phase, thereby interrupting the cell cycle progression and inhibiting tumor cell proliferation. Dalpiciclib is a novel CDK4/6 inhibitor developed independently in China, which has been approved for use in combination with fulvestrant for the treatment of HR-positive/HER2-negative recurrent or metastatic breast cancer in patients who have experienced disease progression after prior endocrine therapy, or in combination with aromatase inhibitors as initial treatment for HR-positive/HER2-negative locally advanced or metastatic breast cancer patients. This study aims to explore the efficacy and safety of dalpiciclib in combination with letrozole as neoadjuvant therapy for HR-positive, HER2-negative breast cancer. Methods: This is a single-arm, open-label study that enrolled female breast cancer patients with early or locally advanced HR-positive, HER2-negative tumors and an ECOG performance status of 0 to 1. After enrollment, patients first received 4 cycles of dalpiciclib (150mg po qd, d1-21, q4w) in combination with letrozole as neoadjuvant therapy. Breast MRI was performed every 2 cycles, and treatment response was evaluated according to the RECIST 1.1 criteria. If patients achieved a confirmed complete response (CR) or partial response (PR) after 4 cycles, they would continue to receive an additional 4 cycles of dalpiciclib in combination with letrozole treatment. If the treatment response was stable disease (SD) or disease progression (PD), the investigator might modify other treatment regimens based on the individual circumstances of the subjects. The primary endpoint was residual cancer burden (RCB 0/I), and the secondary study endpoints were objective response rate (ORR), complete cell cycle arrest (CCCA, C1D15 Ki67≤2.7%) rate and safety. This study is registered at the Chinese Clinical Trial Registry Centre (registration No. ChiCTR2200057104). Results: From April 2022 to June 2023, a total of 38 patients were screened and enrolled in the study. All patients had ER ≥ 50%, and 84% (32/38) had Ki67 ≥ 88%. Among the 21 patients who completed the assessment after 4 cycles, the overall response rate (ORR) was 81% (17/21). Additionally, among the 22 patients who underwent C1D15 biopsy and Ki67 analysis, 55% (12/22) had C1D15 Ki67 ≤ 2.7%. After completing 8 cycles of neoadjuvant treatment with the combination of dalpiciclib and letrozole,12 patients underwent surgery. The postoperative RCB scores were as follows: 1 patient had RCB (0-I), and 11 patients had RCB (II-III). The most common adverse event was decreased neutrophil count (25/38 66%). Among the 38 patients, 16 (42%) experienced grade 3 or worse treatment-related adverse events. There were no occurrences of grade 4 or worse adverse events. Conclusions: The combination of dalpiciclib and letrozole has shown efficacy in downstaging and shrinking tumors in patients with early or locally advanced HR-positive/HER2-negative breast cancer. Patients who exhibit a positive response in the assessment after 4 cycles may consider it as a viable option for chemo-free neoadjuvant treatment. Citation Format: Jingruo Li, Yuanting Gu, Chuang Du, Jianhua Zhang, Yingying Zhang, Linfeng Zhang, Nan Wang, Lin Li, Fang Wang, Pengwei Lv, Hong Zong, Xinhong Pei, Bingjian Xue, Yan Wang, Dongcheng Gao. Dalpiciclib combination with letrozole as neoadjuvant therapy for HR-positive HER2-negative breast cancer: a single-arm, prospective exploratory clinical study abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO3-02-12.