Ultra-sensitive ctDNA mutation tracking to identify molecular residual disease and predict relapse in patients with early breast cancer.

1010 Background: Circulating tumor DNA (ctDNA) based detection of Molecular Residual Disease (MRD) in breast cancer patients presents a strategy to identify patients at high risk of relapse, although current assays have limited sensitivity for low level ctDNA detection. In this study we profile early breast cancer patients with an ultra-sensitive, whole genome sequencing-based, tumor-informed ctDNA platform, and correlate the findings with clinical outcomes. Methods: We analysed 598 plasma samples (median 8 samples/patient, range 2-14) from 76 patients with early breast cancer (23 TNBC, 33 HER2+, 16 HR+ and 4 unknown) enrolled in the ChemoNEAR study. Samples were collected at diagnosis before therapy, at Cycle 2 of neoadjuvant chemotherapy (NAC), post-surgery after neoadjuvant therapy if administered, and every 3 months during follow-up for the first year, and subsequently every 6 months for up to five years. Plasma cfDNA was analysed using the NeXT Personal ctDNA based MRD platform, a tumour-informed approach leveraging whole-genome sequencing of tumor and normal samples to produce personalised ctDNA sequencing panels, with each bespoke panel consisting of up to ~1,800 selected variants that enable ultra-sensitive MRD detection. MRD detection was correlated with clinical outcomes and histopathological data. Results: We detected a broad range of ctDNA levels ranging from 204,900 PPM to 2.2 PPM (median 296 PPM). 40% of all ctDNA detections were in the ultra-low range of <100 PPM. 97.8% (45/46) of patients had ctDNA detected at baseline prior to treatment, including 100% TNBC, 100% HER2+, and 83% of HR+ patients. At a median follow-up of 76 months (range 5-113) from study entry, detection of ctDNA associated with high risk of future relapse (HR undefined, p<0.0001; log-rank test) and shortened overall survival (p<0.0001; log-rank test) with median lead-time from ctDNA detection to clinical relapse of 12.5 months (range 1-60). MRD was identified in 100% (10/10) of patients who relapsed. Of the relapsed patients, the median level of ctDNA at first MRD detection was 18.5 PPM. No ctDNA undetected patients relapsed throughout follow up (58/58). Three patients had ctDNA detected in follow-up but had not relapsed at the time of data cut off. For patients that were ctDNA undetected at post-surgical landmark, 94% (31/33) patients did not relapse. Conclusions: NeXT Personal detected breast cancer relapse with a long lead-time over clinical relapse, and strongly associated with relapse free survival. The assay demonstrated high rates of ctDNA detection at diagnosis. The strong negative predictive value at landmark suggests potential use in de-escalation studies. Several prospective, interventional trials are being conducted to assess whether treatment based on MRD detection improves outcome.