Circulating tumor DNA clearance of adjuvant chemotherapy in localized colorectal cancer using an ultrasensitive structural variant–based assay.

3625 Background: High-risk localized colorectal cancer (CRC) remains prognostically unfavorable. Adjuvant chemotherapy (ACT) is administered to patients with poor-risk features to eradicate molecular residual disease (MRD) and improve outcome. Despite ACT selection, some patients experience recurrence and others receive unnecessary treatment. Circulating tumor DNA (ctDNA) has emerged as a strong prognostic biomarker that could improve selection for ACT, but detection in the MRD setting is dependent on high sensitivity. In this study, we use an ultrasensitive assay to investigate ACT clearance in localized CRC patients included in the CITCCA trial and its impact on outcome. Methods: Patients with stage I–III CRC were enrolled in a prospective Swedish cohort study between 2020-2024. Standard of care treatment was given, and patients were followed with longitudinal ctDNA before and after surgery at 4-6 weeks, 3 and 6 months, 1 and 2 years. The clinical landmark (CLM) was defined as the 4-6 week postoperative test prior to initiation of ACT. The first sample after definitive treatment (FDT) was defined as the first test after treatment termination. ACT clearance was defined as a positive ctDNA test at either CLM or at CLM and 3 months, with subsequent negative ctDNA, in patients without recurrence. An ultrasensitive SV-based assay, Pathlight, was used for ctDNA analysis. The primary outcome was recurrence-free interval (RFI). Results: Of the 377 patients included, 135 patients (36%) received ACT with a median length of 5.2 months (range 0.1-7.4). Single agent fluoropyrimidine was given to 65 patients (48%) and 70 (52%) received an oxaliplatin-based doublet. Analyzable CLM samples were obtained from 106 patients (79%). Nineteen patients (18%) were ctDNA positive at CLM and received ACT. ctDNA positive patients who received ACT still showed a substantially higher risk of recurrence compared to ctDNA negative patients (HR, 38; 95% CI, 15–97). The same was seen in ctDNA-positive patients who did not receive ACT, i.e. clinically low-risk (HR, 35; 95% CI, 14–88). In the ctDNA positive cohort, patients who received ACT versus no ACT had a 2-year RFI of 58% (95% CI:39%-85%) versus 39% (95% CI:22%-69%). Persistent ACT clearance was seen in 8 patients, none of whom recurred. Transient clearance was seen in 6 patients, all of whom recurred. Five patients had persistently positive ctDNA despite ACT; all recurred. Eighty-seven patients (82%) were ctDNA negative at CLM but received ACT; 9 of these recurred (10%). Eighteen patients were ctDNA positive at CLM but did not receive ACT and 13 of these recurred. Conclusions: ctDNA is a strong prognostic biomarker and may be utilized to guide adjuvant treatment decisions in patients with localized CRC. Permanent clearance of ctDNA by administration of ACT is a favorable prognostic feature whereas ctDNA persistence after ACT predicts recurrence. Clinical trial information: NCT04726800 .