Clinical validation of plasma circulating-tumor DNA assay using highly sensitive Safe-SeqS technology for detecting <i>RAS</i> and <i>BRAF</i> V600E in metastatic colorectal cancer.

50 Background: We developed a circulating-tumor DNA (ctDNA) test assay for RAS/BRAF mutations using next-generation sequencing-based Safe-SeqS technology. The clinical performance of detecting RAS and BRAF mutations were evaluated versus approved liquid biopsy and tissue testing, respectively. Methods: Metastatic colorectal cancer (mCRC) patients (pts) were enrolled. The concordance of the RAS mutational status in ctDNA between Safe-SeqS assay and OncoBEAM RAS CRC kit, which is approved as a companion diagnostic in Japan, was evaluated using archival plasma samples. The primary endpoints were positive percent agreement (PPA) and negative percent agreement (NPA). The concordance of the BRAF V600E mutational status between plasma-Safe-SeqS assay and tissue-based testing using MEBGEN RASKET-B kit, which is approved as a companion diagnostic in Japan, was prospectively evaluated. The primary endpoints were sensitivity and specificity of Safe-SeqS assay compared to tissue-based testing. Multivariate analysis using LASSO regression model was performed to evaluate the discordance of BRAF V600E mutational status between Safe-SeqS assay and tissue-based testing. Results: In 125 eligible plasma samples, the PPA and NPA for RAS mutational status in ctDNA were 87.1% (95% confidence interval [CI]: 76.1–94.3) and 95.2% (95% CI: 86.7–99.0), respectively. On the other hand, in 103 eligible pts, the sensitivity and specificity of Safe-SeqS assay for BRAF V600E mutational status were 69.2% (95% CI: 48.2–85.7) and 98.7% (95% CI: 93.0–100.0), respectively. Multivariate analysis revealed the baseline longest diameter and the number of lesions in pts with peritoneal and/or lung metastasis were factors associated with discordance. The sensitivity for BRAF V600E mutations increased to 81.0% (95% CI: 58.1–94.6, fisher’s exact test p =0.02) when the 5 pts with lower tumor burden (peritoneal metastasis with the longest diameter &lt;20 mm, and lung metastasis with the longest diameter &lt;20 mm and &lt;10 lesions, according to the previous report [1] ) were excluded. Conclusions: The clinical validity of the ctDNA assay for detecting RAS/BRAF V600E mutations using Safe-SeqS technology was confirmed in pts with mCRC. Careful attention should be paid for mCRC pts with only peritoneal and/or lung metastases having fewer metastases or smaller diameter lesions. 1. Kagawa Y, et al. Clin Cancer Res 2021.