Introduction

Rat sarcoma viral oncogene homolog (RAS) and v-raf murine sarcoma viral oncogene homolog B1 (BRAF) V600E mutations (MT) stimulate the mitogen-activated protein kinase (MAPK) pathway and promote carcinogenesis and cancer growth in multiple cancers.1 According to the Catalog of Somatic Mutations in Cancer and The Cancer Genome Atlas databases, which compile information on somatic MT associated with cancer, the frequency of RAS MT in colorectal cancer (CRC) is 33–42% for KRAS MT, 2–9% for NRAS MT, and 0% for HRAS MT, while the frequency of BRAF V600E is 5–10%.1,2 Patients with metastatic CRC (mCRC) and RAS or BRAF V600E MT have a poorer prognosis than those with RAS wild-type (WT) mCRC.3 The TRIBE study established FOLFOXIRI plus bevacizumab as a standard treatment for young patients with right-sided, RAS-mutant CRC. This regimen achieved an overall survival (OS) and progression-free survival (PFS) of 37.0 and 12.8 months in patients with RAS/BRAF WT mCRC, respectively. However, survival outcomes were significantly lower in those with RAS MT mCRC (OS and PFS: 25 and 10.9 months, respectively) and those with BRAF V600E MT (OS and PFS: 13.4 and 7.0 months, respectively).4

Anti-epidermal growth factor receptor (EGFR) antibodies constitute a class of molecular-targeted drugs used in the treatment of mCRC.5 EGFR-targeted antibodies exert anti-tumor effects by binding to EGFR and inhibiting signaling downstream of the receptor.6 However, point MTs in RAS (KRAS/NRAS exons 2, 3, and 4) reduce GTPase activity, causing the RAS protein to remain in its active state bound to GTP. This causes the mutated RAS protein to continuously transmit signals downstream, resulting in resistance to anti-EGFR antibodies, as shown in several clinical trials.7,8,9,10,11,12 International guidelines recommend that RAS and BRAF gene testing should be performed on patients with mCRC before consideration of anti-EGFR treatment and that anti-EGFR antibodies should only be administered to patients with RAS WT mCRC.13,14,15

Repeated tissue biopsies to confirm RAS and BRAF MT status after each line of treatment are not performed in routine clinical practice,13,14,15 and the persistence of the original MT status through multiple lines of therapy remains unclear.16 Recent advances in gene MT testing methods using circulating tumor DNA (ctDNA) in the blood have made it possible to assess MT status easily and repeatedly.17 Among patients with RAS MT mCRC, RAS can revert to WT following treatment in some cases; this phenomenon, known as “Neo__RAS WT,’’ has attracted considerable attention.18,19 Although variations exist depending on the testing method and treatment line, studies have revealed that approximately 10% of patients with RAS MT mCRC convert to RAS WT after treatment,20,21 thereby opening the possibility of anti-EGFR therapy as an effective approach in later lines of treatment.22 However, few reports based on large-scale databases have described the relationship between changes in RAS and BRAF gene status and OS. Confirmation of prognosis based on changes in gene alterations may help optimize treatment. Therefore, in this study, we evaluated the prognostic impact of post-treatment circulating tumor RAS and BRAF mutation dynamics in patients with mCRC.