Evaluation of Emergent Mutations in Circulating Cell-Free DNA and Clinical Outcomes in Patients with Metastatic Colorectal Cancer Treated with Panitumumab in the ASPECCT Study.

PURPOSE: Mutations in EGFR pathway genes are poor prognostic indicators in patients with metastatic colorectal cancer. Plasma analysis of cell-free DNA is a minimally invasive and highly sensitive method to detect somatic mutations in tumors. EXPERIMENTAL DESIGN: Plasma samples collected from panitumumab-treated patients in the ASPECCT study at baseline and safety follow-up (SFU) were analyzed by a next-generation sequencing-based approach for extended RAS mutant allele frequency as a continuous variable and their association with clinical outcomes and the mutational prevalence of 63 cancer-related genes. The correlation between patient outcome and baseline mutational status of EGFR pathway genes was also examined. RESULTS: Overall, 261 patients in the panitumumab arm had evaluable plasma samples. Patients with a higher RAS mutant allele frequency at baseline had worse clinical outcomes than those with a lower frequency (P < 0.001, Cox PH model); however, RAS mutations did not necessarily preclude patients from deriving benefits. The objective response rate (complete or partial response) was 10.8% for patients with baseline RAS mutations and 21.7% for those with BRAF mutations. The 63-gene panel analysis revealed an increase in tumor mutational burden from baseline to SFU (P < 0.001, Wilcoxon signed rank test). Baseline mutations in EGFR pathway genes, when analyzed both categorically and continuously, were associated with shorter survival. CONCLUSIONS: When mutations in EGFR pathway genes were analyzed continuously, higher mutant allele frequency correlated with poorer outcomes. However, extended RAS mutation, by itself, did not preclude clinical responses to panitumumab in a monotherapy setting.

Impact of Emergent Circulating Tumor DNA RAS Mutation in Panitumumab-Treated Chemoresistant Metastatic Colorectal Cancer.

Purpose: The accumulation of emergent RAS mutations during anti-EGFR therapy is of interest as a mechanism for acquired resistance to anti-EGFR treatment. Plasma analysis of circulating tumor (ct) DNA is a minimally invasive and highly sensitive method to determine RAS mutational status.Experimental Design: This biomarker analysis of the global phase III ASPECCT study used next-generation sequencing to detect expanded RAS ctDNA mutations in panitumumab-treated patients. Plasma samples collected at baseline and posttreatment were analyzed categorically for the presence of RAS mutations by the PlasmaSelect-R 64-gene panel at 0.1% sensitivity.Results: Among panitumumab-treated patients with evaluable plasma samples at baseline (n = 238), 188 (79%) were wild-type (WT) RAS, and 50 (21%) were mutant RAS Of the 188 patients with baseline ctDNA WT RAS status, 164 had evaluable posttreatment results with a 32% rate of emergent RAS mutations. The median overall survival for WT and RAS mutant status by ctDNA at baseline was 13.7 (95% confidence interval, 11.5-15.4) and 7.9 months (6.4-9.6), respectively (P < 0.0001). Clinical outcomes were not significantly different between patients with and without emergent ctDNA RAS mutations.Conclusions: Although patients with baseline ctDNA RAS mutations had worse outcomes than patients who were WT RAS before initiating treatment, emergent ctDNA RAS mutations were not associated with less favorable patient outcomes in panitumumab-treated patients. Further research is needed to determine a clinically relevant threshold for baseline and emergent ctDNA RAS mutations. Clin Cancer Res; 24(22); 5602-9. ©2018 AACR.