A multicentre, prospective clinical evaluation study for analyzing RAS mutational status utilizing plasma circulating tumor DNA in patients with metastatic colorectal cancer
Authors: Y. Kagawa, T. Kato, H. Bando, K. Akagi, T. Denda, T. Nishina, Y. Komatsu, E. Oki, T. Kudo, T. Yamanaka, T. Yoshino
Journal: Annals of Oncology (2018)

Introduction: Liquid biopsy has emerged as an excellent molecular diagnostic tool for assessing predominant spatial and temporal intratumoral heterogeneity with minimal invasiveness. The OncoBEAM RAS kit is a circulating tumor DNA (ctDNA) test intended for detecting plasma RAS mutational status in metastatic colorectal cancer (mCRC). We conducted multicentre, prospective clinical evaluation study investigating the concordance of RAS mutational status between plasma ctDNA and tumor tissue DNA in patients with mCRC. Methods: mCRC patients who have an adequate archived FFPE tumor tissue specimen obtained within 5 years and who have no prior treatment with anti-EGFR antibodies and regorafenib were enrolled. Plasma- and tissue-based RAS mutational status were determined by the OncoBEAM RAS kit and by the Tissue OncoBEAM RAS assay; the concordance of RAS mutational status was compared as the primary analysis. Plasma samples in discordant cases were transferred to RAS test using the plasma-NGS technology of Oncomine Colon cfDNA assay to cross-validate the accuracy of the OncoBEAM RAS kit. Results: A total of 280 mCRC out of 350 enrolled patients were eligible from eight hospitals in Japan. The overall agreement between plasma- and tissue-based analyses was 86.4% (242/280), with positive percent agreement of 82.1% (110/134) and negative percent agreement of 90.4% (132/146). Plasma RAS mutational status determined by OncoBEAM RAS showed 96% concordance with plasma-NGS, indicating the assay performance (accuracy) of plasma result of OncoBEAM RAS kit is highly reliable. Of 38 discordant cases, plasma-positive and tissue-negative results were observed in 14 cases, 6 out of which were determined as RAS-positive by plasma-NGS; the discordance was deemed due to tumor heterogeneity and a longer interval in sample collection from archived tissue to plasma. In contrast, plasma-negative and tissue-positive results were observed in 24 cases, 15 out of which were confirmed as negative by plasma-NGS, indicating ctDNA release into the blood stream was relatively low. Interestingly, 10 out of the 24 plasma-negative and tissue- positive cases (42%) had lung metastasis alone. The overall agreement between plasma- and tissue-based analyses in patients with lung metastasis alone (n = 31) was only 64.5%, while that in patients excluding lung metastasis alone (n = 249) was 89.2%. Conclusion: The accuracy of plasma RAS mutational status determined by OncoBEAM RAS kit was confirmed for Japanese mCRC patients. The concordance rate between plasma- and tissue-based analyses was 86.4% in overall, rising to 89.2% in patients excluding lung metastasis alone, with 13.6% of discordant cases being potentially attributed to variables of tissue heterogeneity, a longer interval in sample collection from archived tissue to plasma and a lower amount of ctDNA shed into plasma. Careful attention should be paid for mCRC patients with lung metastases alone when considering use of plasma ctDNA test instead of tissue-based test.