Association between ctDNA levels, timing of blood collection, and overall survival in metastatic colorectal cancer.

245 Background: While circulating tumor DNA (ctDNA) detection is influenced by various factors, the impact of blood collection timing remains understudied. This study investigated the relationship between ctDNA detection, and blood collection timing and prognosis in patients with metastatic colorectal cancer (mCRC) from the GOZILA study. Methods: GOZILA is a nationwide plasma genomic profiling study using Guardant360 CDx for advanced solid tumors. This study evaluated the association of ctDNA levels with blood collection timing and prognosis in patients with mCRC. ctDNA levels were determined by the maximum variant allele frequency. Blood collection timing was categorized into five groups: naive (before initiation of 1st line treatment), after treatment initiation (days 1-14 of treatment), during treatment (between “after treatment initiation” and “before progression disease [PD]”), before PD (from 28 days before PD to PD), and after PD (after disease progression but before the next line of treatment). Overall survival (OS) was compared across treatment lines using ctDNA level cutoff of 10. Results: We analyzed 2,398 blood samples from 1,994 patients with mCRC enrolled in the GOZILA between February 2018 and July 2023. ctDNA levels were significantly lower during treatment compared to the naive (median, 13.2 vs. 5.7, p < 0.001). No significant differences were observed between naive and other blood collection timing. Compared to blood collection before 1st line treatment, ctDNA levels significantly decreased before 2nd line (median, 13.5 vs. 8.3, p = 0.02) and 3rd line treatments (median, 13.5 vs. 7.6, p = 0.01). Patients with ctDNA levels<10 demonstrated significantly better OS across all treatment lines, including 1st line (median, NA vs. 23.9 months, HR 0.49, 95% CI, 0.34-0.72), 2nd line (median, 22.1 vs. 10.3 months, HR 0.42, 95% CI, 0.33-0.55), and later line treatments (FTD/TPI: median, 12.7 vs. 5.7 months, HR 0.51, 95% CI, 0.26-0.99; FTD/TPI+Bev: median, 9.3 vs. 6.5 months, HR 0.55, 95% CI, 0.37-0.82; Regorafenib: median, 13.5 vs. 7.4 months, HR 0.45, 95% CI, 0.26-0.80). Conclusions: In mCRC, blood collection during treatment is suboptimal due to decreased ctDNA levels. Patients with ctDNA levels <10 showed significantly better OS across all treatment lines. Optimal timing of blood collection can enhance ctDNA levels, enabling more accurate gene mutation assessment and stratifying outcomes.