Abstract OT1-03-01: Phase 1/1b study of novel oral selective estrogen receptor degrader (SERD) LSZ102 in combination with alpelisib (BYL719) in estrogen receptor-positive (ER+), human epidermal growth factor receptor-2–negative (HER2–) advanced breast cancer (ABC) with progression on endocrine thera

Background: Although ET remains the basis of therapy for ER+, HER2– ABC, treatment resistance frequently occurs. Novel strategies to target the receptor and/or alternative pathways to overcome therapeutic resistance are under investigation. LSZ102 is a novel, orally bioavailable, nonsteroidal SERD. Preclinically, LSZ102 inhibits ER gene transcription, induces receptor degradation, blocks ER-dependent cell growth, and has synergistic activity with the phosphoinositide 3-kinase (PI3K)-alpha inhibitor alpelisib (BYL719). The present study is evaluating the safety and tolerability of LSZ102 plus alpelisib in patients with ER+, HER2– ABC with progression on ET. Trial Design: This phase 1/1b, open-label study is enrolling ˜18-30 patients (men and women of any menopausal status) in Arm C of the dose-escalation part of the study, which investigates the combination of LSZ102 and alpelisib; additional study arms will investigate LSZ102 as a single agent or in combination with ribociclib. Enrollment in Arm C started after identification of a safe and tolerable single-agent dose for LSZ102. Alpelisib dosing began at 200 mg/day and will not be escalated beyond the maximum tolerated dose (MTD) determined in the alpelisib single-agent arm of study CBYL719X2101 (400 mg/day). Dose escalation of alpelisib in combination with LSZ102 is guided by BLRM and integrates Cycle 1 DLT rates, lower grade and later cycle AE, PK, PD and preliminary activity to identify a recommended dose for expansion (RDE). Patients will receive treatment until disease progression, unacceptable toxicity, or withdrawal of consent. For inclusion in the study, patients must have histologically confirmed ER+, HER2– ABC and disease progression after ET for ABC or recurrence on/within 12 months of completion of adjuvant ET. In the escalation part of the study, patients are eligible regardless of PIK3CA status. Premenopausal women must receive concomitant treatment with a gonadotropin-releasing hormone agonist. Eligible patients must have adequate bone marrow and organ function, Eastern Cooperative Oncology Group performance status of 0 or 1, and have completed and recovered from acute toxicities of radiotherapy and/or prior anticancer therapy. Exclusion criteria include symptomatic central nervous system metastases, clinically significant cardiac disease or impaired cardiac function (including a QT interval corrected for heart rate using Fridericia9s formula [QTcF] >460 ms in women or >450 ms in men), uncontrolled diabetes mellitus type II (or type I), and prior treatment with a PI3K inhibitor. The primary objectives are characterization of safety and tolerability for the combination and identification of a recommended dose. Secondary objectives include characterization of pharmacokinetic properties and pharmacodynamic effects. Recruitment for Arm C is ongoing. NCT02734615 Citation Format: Curigliano G, Cresta S, Yap Y-S, Juric D, Duhoux FP, Terret C, Takahashi S, Layman RM, Kundamal N, Baldoni D, Liao S, Crystal A, Jhaveri K. Phase 1/1b study of novel oral selective estrogen receptor degrader (SERD) LSZ102 in combination with alpelisib (BYL719) in estrogen receptor-positive (ER+), human epidermal growth factor receptor-2–negative (HER2–) advanced breast cancer (ABC) with progression on endocrine therapy (ET) [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr OT1-03-01.