Circulating tumor DNA dynamics and clinical outcome in metastatic colorectal cancer patients undergoing front-line chemotherapy.

Purpose: we tested whether ctDNA changes may be used to assess early response and clinical outcome in metastatic colorectal cancer (mCRC) patients undergoing front-line systemic anti-cancer therapy (SACT).

Experimental Design: 862 plasma samples were collected 4-weekly from baseline (BL) until disease progression in mCRC patients receiving front line SACT. ctDNA normalization was defined as ≥99% clearance after 1 month of therapy (Mo1) in the 3 variants with the highest allele frequency in BL ctDNA.

Results: 83 paired samples from 75 patients were available for analysis. 12 pairs (14.4%) showed no variants in either BL or Mo1. In the remaining 71 comparisons (65 patients), 37 (52.1%) showed ctDNA normalization at Mo1. Patients that cleared ctDNA had significantly longer overall (45.6 months) and progression-free survival (13.9 months) compared to non-normalized patients [OS= 22.6 months (Log-rank p = 0.01) and PFS= 10.7 months (Log-rank p = 0.036) respectively]. In addition, higher response rate was observed in patients with ctDNA clearance (72.9%) compared to non-normalized cases (38.2%). Longitudinal sequencing of at least four timepoints in pts with a PFS>10 months showed emerging variants in 47.8% of cases; in all these patients the trajectory of these new "outlier" variants appeared in stark contrast with the clinical-radiological course of disease and the trend in other mutations.

Conclusions: ctDNA clearance represents an early indicator of benefit from SACT in mCRC patients; serial tracking of multiple variants is warranted to improve specificity and to avoid misleading information due to the emergence of mutations of unknown clinical significance.