Early, On-Treatment Levels and Dynamic Changes of Genomic Instability in Circulating Tumor DNA Predict Response to Treatment and Outcome in Metastatic Breast Cancer Patients.

Background: Circulating tumor DNA (ctDNA) offers high sensitivity and specificity in metastatic cancer. However, many ctDNA assays rely on specific mutations in recurrent genes or require the sequencing of tumor tissue, difficult to do in a metastatic disease. The purpose of this study was to define the predictive and prognostic values of the whole-genome sequencing (WGS) of ctDNA in metastatic breast cancer (MBC).

Methods: Plasma from 25 patients with MBC were taken at the baseline, prior to treatment (T0), one week (T1) and two weeks (T2) after treatment initiation and subjected to low-pass WGS. DNA copy number changes were used to calculate a Genomic Instability Number (GIN). A minimum predefined GIN value of 170 indicated detectable ctDNA. GIN values were correlated with the treatment response at three and six months by Response Evaluation Criteria in Solid Tumours assessed by imaging (RECIST) criteria and with overall survival (OS).

Results: GIN values were detectable (>170) in 64% of patients at the baseline and were significantly prognostic (41 vs. 18 months OS for nondetectable vs. detectable GIN). Detectable GIN values at T1 and T2 were significantly associated with poor OS. Declines in GIN at T1 and T2 of > 50% compared to the baseline were associated with three-month response and, in the case of T1, with OS. On the other hand, a rise in GIN at T2 was associated with a poor response at three months.

Conclusions: Very early measurements using WGS of cell-free DNA (cfDNA) from the plasma of MBC patients provided a tumor biopsy-free approach to ctDNA measurement that was both predictive of the early tumor response at three months and prognostic.