Abstract PO1-19-04: Dalpiciclib and tucidinostat in patients with HR+/HER2- advanced breast cancer and resistance to CDK4/6 inhibitors: A phase Ib trial

Abstract Background: Hormone receptor (HR)-positive/epidermal growth factor receptor 2 (HER2)-negative breast cancer accounts for approximately 70% of all breast cancers and is associated with a better prognosis. Estrogen plays an important role in the growth of HR-positive breast cancer, thus patients with this type of tumor are sensitive to hormone inhibitors. However, a quarter of patients will inevitably relapse. Several large clinical trials have demonstrated that the combination of CDK4/6 inhibitors and endocrine therapy (ET) as first or second-line therapy could improve the progression-free survival (PFS) and overall survival for patients with breast cancer. In China, dalpiciclib (a CDK4/6 inhibitor) in combination with fulvestrant has been approved for patients with recurrent or metastatic HR-positive/HER2-negative breast cancer who have failed ET. Moreover, the combination of dalpiciclib and aromatase inhibitors (AI) as an initial treatment for patients with locally advanced or metastatic HR-positive/HER2-negative breast cancer has also been accepted. However, the standard treatment for patients who have failed CDK4/6 inhibitors has not been established. Histone deacetylase (HDAC) inhibitors cause alterations in gene expression in many signaling pathways associated with oncogenesis, leading to the arrest of tumor cell growth and differentiation, as well as promoting tumor apoptosis. Tucidinostat (an HDAC inhibitor) plus AI has been approved for the treatment of locally advanced HR-positive/HER2-negative postmenopausal breast cancer patients who have failed ET. Theoretically, synergistic effects may occur when HDAC inhibitors are combined with CDK4/6 inhibitors. Currently, the clinical data on combination therapy with CDK4/6 inhibitors and HDAC inhibitors have not been reported. Herein, our study aims to explore the efficacy and safety of dalpiciclib and tucidinostat in patients with late-stage HR+/HER2- breast cancer who have failed CDK4/6 inhibitors. Trial Design: The present study is a single-arm, open-label, phase I, dose escalation clinical trial (NCT 05586841). Patients were assigned to four groups to receive either dalpiciclib 125mg/d and tucidinostat 25mg/BIW (group A) or dalpiciclib 125mg/d and tucidinostat 20mg/BIW (group B) or dalpiciclib 100 mg/d and tucidinostat 25mg/BIW (group C) or dalpiciclib 100 mg/d and tucidinostat 20mg/BIW (group D). The Bayesian Optimal Interval (BOIN) design was performed to determine dose-escalation and de-escalation to find the maximum tolerated dose (MTD) of dalpiciclib plus tucidinostat. Key Eligibility Criteria: The study is enrolling female patients who are older than 18 years, with HR-positive/HER2-negative locally recurrent or metastatic breast cancer (MBC), with Eastern Cooperative Oncology Group (ECOG) performance status score of 0 to 2. Before entering this trial, all patients must have received ≤1 previous line of chemotherapy for recurrent or MBC, and have failed CDK4/6 inhibitor therapy. Aims: The BOIN design is employed to explore the optimal dose combination of dalpiciclib and tucidinostat and determine recommended doses for a phase II dose expansion clinical trial, based on prespecified dose-limiting toxicity (DLT). Statistical Methods: Safety will be evaluated in the safety set including all patients who received at least one dose of study treatment and underwent at least one safety assessment. Efficacy will be assessed in the full analysis set, defined as all patients who received at least one dose of study treatment. Present and Planned Accrual: Up to 30 participants were planned to be enrolled in the present study. The first participant was enrolled on 18 January 2023. The recruitment is ongoing. Citation Format: Tao Wang, Jinmei Zhou, Xuexue Wu, Zefei Jiang. Dalpiciclib and tucidinostat in patients with HR+/HER2- advanced breast cancer and resistance to CDK4/6 inhibitors: A phase Ib trial 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 PO1-19-04.