Post hoc analysis links TMB ≥9 or BRAF-V600E to longer PFS with chemo-nivolumab in MSS/pMMR mCRC

This is a post hoc, exploratory biomarker analysis of the METIMMOX randomized controlled trial. The study population consisted of patients with metastatic microsatellite-stable/mismatch repair-proficient colorectal cancer. The analysis compared outcomes based on tumor mutational burden (TMB), BRAF-V600E mutation status, and systemic inflammation (C-reactive protein) among patients who received either alternating oxaliplatin-based chemotherapy and nivolumab or chemotherapy alone.

In the experimental arm, patients with TMB ≥9 or a BRAF-V600E mutation (n=17) had a median progression-free survival of 19.8 months (95% CI, 11.3-28.3), which was statistically longer than other patient groups (p=0.0090). A further subgroup of 11 patients from the experimental arm who had these biomarkers (TMB ≥9 or BRAF-V600E) and a normal, non-inflammatory CRP level at the start of nivolumab treatment had a median PFS of 35.0 months (95% CI, 6.8-63.0; p<0.0001). The median TMB in the cohort was 8 (range, 1-13).

Safety and tolerability data for these specific subgroups were not reported. Key limitations include the post hoc, exploratory nature of the analysis, small subgroup sample sizes (n=17 and n=11 for the main findings), and the need for prospective validation. The findings are associations from a biomarker analysis and do not establish causality.

For clinical practice, the results suggest TMB, somatic BRAF status, and systemic inflammation should be prospectively investigated as potential biomarkers for predicting responsiveness to immune checkpoint inhibitor combinations in this patient population. However, these are not established predictive markers and should not guide treatment decisions outside of a clinical trial context.