Ultrasensitive structural variant-based ctDNA detection of MRD in colorectal cancer: The CITCCA study.

208 Background: Colorectal cancer (CRC) is the second-leading cause of cancer death globally, with a significant risk of relapse after curative treatment. The incidence of CRC is rising, notably in younger patients. CRC treatment is increasingly individualized, with growing evidence supporting organ-sparing methods in rectal cancer. Sensitive biomarkers to detect molecular residual disease (MRD) and guide neoadjuvant (NAT) and adjuvant treatment (ACT) are needed. Circulating tumor DNA (ctDNA) has shown strong prognostic potential in previous studies and may identify patients with MRD and a high risk of relapse. Tumor-informed ctDNA assays typically target single-nucleotide variants, with variable clinical performance. In this study, we evaluate a novel ultrasensitive structural variant (SV)-based ctDNA assay (Pathlight) for the detection of MRD in CRC. Methods: Patients with CRC stage I-III undergoing curative intent surgery, and in some cases NAT and/or ACT, were enrolled. Participants were recruited from seven centres in Sweden (October 2020 - January 2024) and followed according to standard-of-care with computed tomography and CEA at 1 and 3 years. Longitudinal ctDNA testing was performed using Pathlight, for which a detection of 1 in 10 million (0.1 PPM) has been reported. Plasma was collected from patients before and after surgery at 4-6 weeks, 3 and 6 months, 1 and 2 years. The primary endpoint was the association between ctDNA status and recurrence-free interval; secondary endpoints evaluated Pathlight’s clinical performance and the association between ctDNA status and recurrence-free survival. Results: A total of 532 patients were enrolled in the study; of these, 494 had pathologic stage I–III CRC. Median age at surgery was 68 years (range 29-84) and 58% of patients were male. Tumor location was 44% rectal (n=218) and 56% colon (n=276). Pathological stage was: 3% stage 0, 26% stage I, 35% stage II and 35% stage III (with 18% T4). NAT was given to 50% of patients with rectal cancer (n=110) and 6% with colon cancer (n=16), while ACT was administered to 43% of patients with colon cancer (n=119) and 20% of rectal cancer (n=44). At the time of this analysis, 49 cases (27 colon and 22 rectal) have been processed, with a success rate of 98% (48/49). ctDNA fingerprints included a median of 11 SVs. Baseline detection in patients who were treatment-naive was 92% (24/26). For patients who received NAT, pre-surgery detection was 86% (19/22). Analysis of ctDNA during follow up (median 33 months) is ongoing. Conclusions: Preliminary results from this large observational longitudinal study, support the feasibility of ctDNA-based MRD analysis in localized CRC in the neoadjuvant, adjuvant and monitoring settings. Ultrasensitive SV-based ctDNA monitoring could not only inform NAT and ACT decisions, but may also guide organ-preserving strategies. Data from the full CITCCA cohort will be presented. Clinical trial information: NCT04726800 .