Abstract P5-01-03: Ultrasensitive detection of minimal residual disease in patients treated for breast cancer

Background: Breast cancer (BC) may recur years to decades after initial treatment, leading to incurable, metastatic disease. Prior studies have shown blood biopsy can detect minimal residual disease (MRD), but not in all patients and often with very short lead time. Most efforts thus far have focused on tracking one or a few mutations via ctDNA, which may limit sensitivity. Here we sought to maximize the detection sensitivity of blood biopsies by tracking up to hundreds of individualized tumor mutations in cell-free DNA (cfDNA). Doing so enables us to break the detection ceiling imposed by the limited copies of each gene in the cell-free DNA in a blood draw. Methods: cfDNA was extracted from archival plasma samples (n=271) from 142 patients with stage 0-III BC enrolled prospectively onto two IRB-approved studies. We applied whole-exome sequencing (WES) to define up to several hundred mutations from each patient’s tumor. To limit potential errors, we employed strict criteria to select somatic SNVs to track using duplex sequencing in cfDNA. We required detection of ≥ 2 mutations for a cfDNA sample to be MRD+ and excluded any mutations also found in a patient’s own germline DNA. Results: We identified 142 patients treated for stage 0-III BC, who had postoperative blood and plasma samples available, and were monitored for distant recurrences for up to thirteen years. All patients had biopsy-proven BC, with 86 (61%) having HR+/HER2- BC, 31 (22%) having HR-/HER2-, or “triple negative” BC (TNBC), and 25 (18%) having HER2-positive disease. Three (2%) patients had stage 0 disease, 32 (23%) had stage I, 68 (42%) had stage II, and 39 (27%) had stage III BC at diagnosis. Archived plasma samples were collected post-operatively, at year 1, and at year 4. We created individualized assays targeting a median of 57 mutations (range 2 - 346) per patient. Reasoning that MRD status after completion of all local therapy and chemotherapy might best predict for distant recurrence, we conducted a landmark analysis at one year following surgery, and found all patients (n=6/6) with detectable MRD experienced recurrence (HR=21.2 (7.43-60.35)) in a median of 6.7 (range 3.4 - 15.8) months. Median lead time including all timepoints from first positive blood sample to recurrence was 18.9 (range: 3.4 - 39.2) months. Finally, in a multivariate model, MRD remained highly statistically significant independent of stage, subtype, and age at diagnosis. Conclusions: We present a novel approach to MRD tracking based on the premise that tracking many - rather than one or a handful of - mutations inherently increases sensitivity and that assay personalization maximizes sensitivity in a disease without multiple, recurrent mutations. To our knowledge, the lead time we show here is significantly longer than that seen in prior investigations, and if confirmed, could offer an opportunity to treat MRD long before the development of clinically apparent metastatic disease. Prospective studies are needed to determine whether earlier detection of MRD is clinically meaningful for patients. Citation Format: Heather A Parsons, Justin Rhoades, Sarah C. Reed, Greg Gydush, Priyanka Ram, Pedro Exman, Kan Xiong, Christopher Lo, Tianyu Li, Mark Fleharty, Greg Kirkner, Denisse Rotem, Ofir Cohen, Melissa Hughes, Shoshanna Rosenberg, Laura Collins, Kathy Miller, Brendan Blumenstiel, Carrie Cibulskis, Donna Neuberg, Samuel S. Freeman, Niall Lennon, Nikhil Wagle, Gavin Ha, Daniel G. Stover, Atish D. Choudhury, Gad Getz, Matthew Meyerson, Nancy U. Lin, Ian E. Krop, J. Christopher Love, G. Mike Makrigiorgos, Ann Partridge, Erika Mayer, Todd R. Golub, Viktor A. Adalsteinsson. Ultrasensitive detection of minimal residual disease in patients treated for breast cancer [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P5-01-03.