RAS Mutations in Circulating Tumor DNA and Clinical Outcomes of Rechallenge Treatment With Anti-EGFR Antibodies in Patients With Metastatic Colorectal Cancer.

PURPOSE: Several trials have evaluated the efficacy of rechallenge treatment with anti-epidermal growth factor receptor (EGFR) monoclonal antibody (mAb) in patients with metastatic colorectal cancer (mCRC). A recent trial indicated that RAS status in circulating tumor DNA (ctDNA) may potentially predict patients with RAS wild-type mCRC resistant to anti-EGFR mAb who would benefit from rechallenge treatment, and the findings should be further investigated. MATERIAL AND METHODS: We enrolled patients whose plasma samples were collected in prospective phase II trials, the JACCRO CC-08 (n = 36) and CC-09 (n = 25), which evaluated rechallenge chemotherapy with anti-EGFR mAb for KRAS wild-type mCRC. RAS in ctDNA was analyzed at the time points of baseline, 8 weeks, and progression using OncoBEAM RAS CRC kit. RESULTS: Sixteen patients were enrolled in this study, with a response rate of 0% and a disease control rate (DCR) of 62.5%. RAS mutations were found at baseline in six patients. The DCR was 33% in patients with RAS mutations in ctDNA, whereas it was 80% in patients without RAS mutation at baseline. Patients with RAS mutation at baseline had significantly shorter progression-free survival (PFS) and overall survival (OS) than those without RAS mutation (median PFS, 2.3 v 4.7 months; hazard ratio [HR], 6.2; P = .013; median OS, 3.8 v 16.0 months; HR, 12.4; P = .0028). Six of 10 patients without RAS mutation at baseline acquired RAS mutations at progression. Postprogression survival after rechallenge treatment was numerically shorter in patients with RAS mutation at progression. CONCLUSION: RAS status in ctDNA was significantly associated with clinical outcomes in patients with mCRC receiving rechallenge treatment with anti-EGFR mAb. These findings could support the clinical utility of OncoBEAM RAS CRC kits for anti-EGFR mAb rechallenge in RAS wild-type mCRC.

CTF18 interacts with replication protein A in response to replication stress.

Replication stress response is a protective mechanism against defects in chromosome replication for maintaining genome integrity in eukaryotic cells. An alternative clamp loader complex termed chromosome transmission fidelity protein 18 and replication factor C (CTF18­RFC) has been shown to act as a positive regulator of two types of replication stress response: S­phase checkpoint signaling and translesion DNA synthesis. However, it remains largely unknown how CTF18­RFC responds to replication stress and is recruited to stalled replication forks. The present study demonstrated that endogenous CTF18 forms a physical complex with a single­stranded DNA­binding protein replication protein A (RPA) in mammalian cells. Using an in situ proximity ligation assay (PLA), it was demonstrated that the interaction between CTF18 and RPA occurs in chromatin when replication stress is elicited by treatment with hydroxyurea during S phase. Similar results were obtained after exposure to ultraviolet irradiation, which triggers translesion DNA synthesis. Furthermore, the PLA demonstrated that the kinetics of the interaction between CTF18 and RPA was positively correlated with that of checkpoint kinase 1 phosphorylation, which is an indicator of activation of the ATM and Rad3­related pathway. These findings provide novel insights into the molecular mechanism underlying the participation of CTF18­RFC in the regulation of replication stress response.