Abstract 3143: Monitoring circulating tumor cell (CTC) and circulating tumor DNA (ctDNA) genomic alterations in ER positive (POS)/HER2 negative (NEG) advanced breast cancer during endocrine therapy: correlative study of AZD9496 oral SERD phase I trial

Purpose: The vast majority of advanced ER POS breast cancers eventually cease responding to endocrine (ET) and other therapies leading to evolution of lethal disease. However, timely monitoring of the molecular events associated with response/progression in tissue biopsies is logistically difficult. The use of liquid biopsies, such as CTC and ctDNA, in this context has been of recent interest. Patients and Methods: Individual CTC and ctDNA were obtained at different time points from patients with advanced ER POS/HER2 NEG breast cancer enrolled in a Phase I trial of AZD9496, an oral selective estrogen receptor degrader (SERD) ET. The CTC, purified using tandem CellSearch®/DepArray™ technologies, were genomically profiled by DNA single cell next generation sequencing (scNGS). Plasma ctDNA was isolated from blood collected in Streck BCT tubes. Genomic profiling was performed by targeted gene panel scNGS for CTC and ddPCR for ERα gene (ESR1) mutations in ctDNA. Results: 123 high-quality CTCs from 12 patients profiled by scNGS showed 100% concordance with ctDNA in detection of driver ESR1 somatic mutations. CTC scNGS additionally revealed extensive intra-patient heterogeneity of driver alterations, that would have been unresolvable by bulk ctDNA profiling, including separate subclonal CTC populations emerging within the same patient. ScNGS revealed potential opportunities for targeted therapies in 73% of patients, directed at alterations in PIK3CA, FGFR2, KIT and BRAF, at times present as 2 or more targets in the same or different cell populations. In one patient, an emergent, distinct, BRAF p.V600E targetable alteration was detected in a subpopulation of CTCs collected at the progression time point but not at baseline. Conclusion: Serial monitoring of CTC and ctDNA genomic alterations is feasible and should enable real-time tracking of response/progression, tumor evolution and opportunities for precision medicine interventions. Citation Format: Andi K. Cani, Emily M. Dolce, Elizabeth P. Darga, Kevin Hu, Martha Brown, Chia-Jen Liu, Jackie Pierce, Kieran Bradbury, Kimberly Aung, Gaia Schiavon, Danielle Carroll, T. H. Carr, Teresa Klinowska, Justin Lindemann, Gayle Marshall, Vicky Rowlands, Elizabeth A. Harrington, J. Barrett, Anne Armstrong, Richard Baird, Erika Hamilton, Seock-Ah Im, Komal Jhaveri, Manish R. Patel, Caroline Dive, Scott A. Tomlins, Aaron M. Udager, Daniel F. Hayes, Costanza Paoletti. Monitoring circulating tumor cell (CTC) and circulating tumor DNA (ctDNA) genomic alterations in ER positive (POS)/HER2 negative (NEG) advanced breast cancer during endocrine therapy: correlative study of AZD9496 oral SERD phase I trial [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 3143.