1. Fuerst, Mark L.

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A new large, real-world study suggests that levels of cell-free DNA in plasma do not appear to significantly affect circulating tumor DNA (ctDNA) detection among patients with resected colorectal cancer. This may lead to minimal residual disease (MRD) testing for colorectal cancer earlier after surgery, opening up patients' ability to receive adjuvant therapy in a more timely manner.

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ctDNA has shown to be a strong prognostic and predictive biomarker in colorectal cancer. Ultra-deep sequencing allows detection of small amounts of ctDNA in the background of non-tumor-derived cell-free DNA. However, after surgery or during systemic chemotherapy, an increase in cell-free DNA could decrease the ability to detect ctDNA.


"In our analysis, ctDNA can be reliably evaluated starting in Week 3 (Day 15) after surgery, regardless of underlying cell-free DNA concentration. Positive ctDNA is highly associated with poor recurrence-free survival. For patients seeking to include ctDNA in their medical decision-making, these results suggest that earlier testing of ctDNA is possible. Earlier testing may improve feasibility of ctDNA testing by minimizing time to receipt of results," said lead author Stacey A. Cohen, MD, Associate Professor at the University of Washington/Fred Hutchinson Cancer Center.


Study Details

At the 2023 ASCO Gastrointestinal Cancers Symposium (Abstract 5), Cohen reported on a study evaluating cell-free DNA kinetics and the impact on detecting MRD in a large group of colon cancer patients who had a commercially ordered tumor-informed cell-free or ctDNA assay. In this real-world setting, patients had ctDNA detected at the recommendation of their provider and at the time it was done by their provider. The study included a dataset of 14,425 Stage I-III colon cancer patients with at least one ctDNA result.


"We had an annotated cohort of 450 patients where providers submitted clinical information about those patients. From that, we could get the date of surgery, whether they got adjuvant chemotherapy, and if they recurred. We could look at more clinical outcomes in this annotated subset," Cohen said.


The bulk of these patients was tested early. "Importantly, about 60 percent, or over 8,000 patients, had their first ctDNA draw within the classic MRD time window of 0-12 weeks," said Cohen. "We saw an increase [in cell-free DNA] in 0-2 weeks, which then decreased at 2-4 weeks, and further decreased at 4-8 weeks." These differences were statistically significant. "Testing for MRD between Weeks 2 and 4 showed similar sensitivity as Weeks 4-8. And standard MRD testing time windows could potentially start as early as 2 weeks after surgery or Day 15 and beyond," she said.


Researchers projected that patients 8 weeks or up to 8 months from surgery might have been on chemotherapy. Again, patients in that time interval had a higher level of cell-free DNA compared to patients who are over 8 months from surgery, a difference that was statistically significant "but was a little bit more muted only because probably not all of those patients were actively on chemotherapy in that time interval," she said.


This prompted the researchers to ask: Does the higher cell-free DNA concentrations at these different time points affect our ability to detect ctDNA? Looking at the full cohort, they saw a significant amount of variability in Week 1, which was a little less in Week 2. "The cell-free DNA levels decreased by 8 weeks and plateaued, and then we see more variability, especially after 12 weeks, which is likely the effect of adjuvant chemotherapy," Cohen stated.


With ctDNA positivity, they saw a similar trend of high variability in Week 1, less so in Week 2, and then more consistency thereafter. "This suggests that ctDNA testing may be performed as early as 2 weeks after surgery," Cohen noted. The highest level of ctDNA positivity was seen in the first 2 weeks, and decreased to 18 percent, on average, and then decreased after 8 weeks, which again may be related to the chemotherapy effect.


In a multivariate analysis, the researchers looked at the full cohort to determine whether ctDNA positivity was related to any particular factor related to cell-free DNA. "Using the median level as our reference, we saw no difference in level of cell-free DNA concentration and our ability to detect ctDNA," Cohen said. "As with other series, we saw the highest frequency of ctDNA positivity in Stage III disease. And similar to some smaller series, we saw lower frequency in MSI, but a statistically significant higher frequency of BRAF-mutant patients having ctDNA positivity."


In the annotated cohort, ctDNA-negative patients had much lower recurrence rates compared to patients who were ctDNA-positive. "In the multivariate analysis, we saw no impact on cell-free DNA concentration, and MRD remained the strongest predictor of recurrence-free survival," Cohen said.


"Cell-free DNA concentration is significantly increased in the first 2 weeks after surgery. Higher levels of cell-free DNA do not seem to impact our ability to detect ctDNA. But high ctDNA positivity is seen in the first week after surgery, and we do see some decrease. We suspect that may relate to micrometastatic disease that's cleared by the immune system, and further investigation needs to be done in this area," she concluded.


Mark L. Fuerst is a contributing writer.