Abstract PO1-20-02: MiRaDor: A proof-of-concept study of treatment efficacy by monitoring Minimal Residual Disease (MRD) using circulating tumor DNA (ctDNA) in hormone receptor-positive/HER2-negative (HR+/HER2-) early breast cancer (EBC)

Abstract Background: Detection of MRD through ctDNA has been consistently associated with high risk of relapse. Longitudinal evaluation of ctDNA to monitor MRD is a minimally invasive tool that may predict disease recurrence and treatment response. MiRaDor aims to improve the clinical utility of ctDNA and understand its applicability in high-risk, HR+/HER2- EBC patients (pts). Trial Design: MiRaDor (NCT05708235) is a multicenter, open-label, non-comparative, phase II trial. Selection criteria include pts: a) with HR+/HER2- EBC at high risk of relapse, b) on adjuvant treatment with endocrine therapy (ET) for at least 2 years with 3 additional years of planned ET, c) no prior treatment with cyclin-dependent kinases 4/6 inhibitors or fulvestrant, and d) had surgery for their primary BC in the last 5 years. The trial has two phases, ctDNA surveillance (n=1260) and treatment (n=40-60). The surveillance phase will analyze ctDNA, with the FoundationOne® Tracker, every 3 months for the 1st year and every 6 months thereafter until ctDNA detection, end of the adjuvant ET, or completion of treatment phase accrual. Pts with positive ctDNA without radiological disease progression (PD) will be allocated to 1 of the treatment arms (n=10): A) standard ET; B) giredestrant; C) giredestrant + abemaciclib; or D) giredestrant + inavolisib (if PIK3CA mutation). Treatment will continue until evidence of PD, physician’s and/or patient’s decision, or until the end of study treatment (abemaciclib/inavolisib for up to 2 years, 5 years for ET). Tumor assessments will be periodically performed to rule out radiological PD. The primary objective is to evaluate the rate of pts with at least 90% decrease or clearance in ctDNA after 3 months of treatment. Key secondary objectives include the proportion of pts with 90% decrease in baseline ctDNA at 6, 9, and 12 months, 70%, and 50% decrease in ctDNA at 3, 6, 9, and 12 months, and treatment safety and tolerability. A cohort expansion up to 20 pts in 1 or 2 arms will occur if at 3 months a 90% ctDNA decrease is observed in at least 30% of pts and if, after 3 additional months, a 90% ctDNA decrease is maintained in at least 20% of pts. Citation Format: Antonio Llombart-Cussac, José Manuel Pérez-García, María Gion, Daniel Alcalá-López, Janet Fazal-Salom, Alexandros Lazaris, Mario Mancino, Eileen Shimizu, Susana Victorino, Rui Rui Zhang, Zuzana Machackova, Frauke Schimmoller, Matthew Wongchenko, Joseph Gligorov, Peter Schmid, Javier Cortés. MiRaDor: A proof-of-concept study of treatment efficacy by monitoring Minimal Residual Disease (MRD) using circulating tumor DNA (ctDNA) in hormone receptor-positive/HER2-negative (HR+/HER2-) early breast cancer (EBC) [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 PO1-20-02.