Circulating tumor DNA as an early marker of therapeutic response in patients with metastatic colorectal cancer

Abstract

Background: Early indicators of treatment response in metastatic colorectal cancer (mCRC) could conceivably be used to optimize treatment. We explored early changes in circulating tumor DNA (ctDNA) levels as a marker of therapeutic efficacy.

Patients and methods: This prospective study involved 53 mCRC patients receiving standard first-line chemotherapy. Both ctDNA and CEA were assessed in plasma collected before treatment, 3 days after treatment and before cycle 2. Computed tomography (CT) scans were carried out at baseline and 8-10 weeks and were centrally assessed using RECIST v1.1 criteria. Tumors were sequenced using a panel of 15 genes frequently mutated in mCRC to identify candidate mutations for ctDNA analysis. For each patient, one tumor mutation was selected to assess the presence and the level of ctDNA in plasma samples using a digital genomic assay termed Safe-SeqS.

Results: Candidate mutations for ctDNA analysis were identified in 52 (98.1%) of the tumors. These patient-specific candidate tissue mutations were detectable in the cell-free DNA from the plasma of 48 of these 52 patients (concordance 92.3%). Significant reductions in ctDNA (median 5.7-fold; P < 0.001) levels were observed before cycle 2, which correlated with CT responses at 8-10 weeks (odds ratio = 5.25 with a 10-fold ctDNA reduction; P = 0.016). Major reductions (≥10-fold) versus lesser reductions in ctDNA precycle 2 were associated with a trend for increased progression-free survival (median 14.7 versus 8.1 months; HR = 1.87; P = 0.266).

Conclusions: ctDNA is detectable in a high proportion of treatment naïve mCRC patients. Early changes in ctDNA during first-line chemotherapy predict the later radiologic response.

Keywords: biomarker; circulating tumor DNA; metastatic colorectal cancer; treatment response.

Figure 1.

CONSORT diagram showing the flow of patients through the study, including the number of patients included in each of the analysis end points and reasons for exclusion.

Figure 2.

(A) Correlation between pretreatment ctDNA levels and the sum of longest tumor diameters by RECIST. Outliers emphasized in the text (LCR 108 and LCR 081) are indicated by arrows, with corresponding images shown in (C) and (D); (B) Correlation between pretreatment CEA levels and the sum of longest tumor diameters by RECIST; (C) CT images of a patient (LCR 108) with innumerable small lung metastases (majority are nonmeasurable by RECIST) and a high pretreatment ctDNA level (41); (D) CT images of a patient (LCR 081) with a large cystic pelvic mass but relatively low pretreatment ctDNA level (13.6); (E) Changes in ctDNA levels during cycle one of chemotherapy; (F) Changes in CEA levels during cycle one of chemotherapy. Day 3 = 3 days post-treatment, precycle 2 = before cycle 2 treatment.

Figure 3.

(A) Correlation plot of percentage change in RECIST and fold change in ctDNA; (B–D) Receiver operating characteristic (ROC) analysis of fold change in ctDNA, ctDNA level after one cycle of chemotherapy and fold change in CEA, to predict tumor response; the circle in (B) indicates the location on the curve corresponding to a 10-fold change threshold (sensitivity 0.75, specificity 0.64); (E) Kaplan–Meier estimate for PFS in metastatic colorectal cancer patients with <10- and ≥10-fold reduction in ctDNA after one cycle of chemotherapy.