Mutational analysis of patients with colorectal cancer in CALGB/SWOG 80405 identifies new roles of microsatellite instability and tumor mutational burden for patient outcome

PURPOSE CALGB/SWOG 80405 was a randomized phase III trial that found no statistically significant difference in overall survival (OS) in patients with first-line metastatic colorectal cancer treated with chemotherapy plus either bevacizumab or cetuximab. Primary tumor DNA from 843 patients has been used to discover genetic markers of OS. PATIENTS AND METHODS Gene mutations were determined by polymerase chain reaction. Microsatellite status was determined by genotyping of microsatellites. Tumor mutational burden (TMB) was determined by nextgeneration sequencing. Cox proportional hazard models were used, with adjusting factors. Interaction of molecular alterations with either the bevacizumab or the cetuximab arms was tested. RESULTS Patients with high TMB in their tumors had longer OS than did patients with low TMB (hazard ratio [HR], 0.73 [95% CI, 0.57 to 0.95]; P = .02). In patients with microsatellite instability-high (MSI-H) tumors, longer OS was observed in the bevacizumab arm than in the cetuximab arm (HR, 0.13 [95% CI, 0.06 to 0.30]; interaction P andlt; .001 for interaction between microsatellite status and the two arms). Patients with BRAF mutant tumors had shorter OS than did patients with wild-type (WT) tumors (HR, 2.01 [95% CI, 1.49 to 2.71]; P,.001). Patients with extended RAS mutant tumors had shorter OS than did patients with WT tumors (HR, 1.52 [95% CI, 1.26 to 1.84]; P andlt; .001). Patients with triple-negative tumors (WT for NRAS/KRAS/BRAF) had a median OS of 35.9 months (95% CI, 33.0 to 38.8 months) versus 22.2 months (95% CI, 19.6 to 24.4 months ) in patients with at least one mutated gene in their tumors (P andlt; .001). CONCLUSION In patients with metastatic colorectal cancer treated in first line, low TMB, and BRAF and RAS mutations are negative prognostic factors. Patients with MSI-H tumors benefited more from bevacizumab than from cetuximab, and studies to confirm this effect of MSI-H are warranted.