Exploratory biomarkers for oxaliplatin-induced nivolumab responsiveness in metastatic microsatellite-stable colorectal cancer

Why this matters for people with colon cancer

Most people with advanced colorectal (bowel) cancer do not benefit from today’s powerful immunotherapy drugs, even though these medicines can produce long-lasting remissions in some cancers. This study asks a pressing question: can a short course of standard chemotherapy "open the door" for immunotherapy to work in patients whose tumors are usually resistant, and are there simple blood tests and tumor features that can flag who is most likely to benefit?

Two treatment paths put to the test

Norwegian researchers ran a randomized clinical trial, called METIMMOX, in people with metastatic colorectal cancer that was microsatellite-stable and mismatch repair–proficient—a common form that typically responds poorly to immune checkpoint inhibitors. All patients had inoperable metastases in the abdomen, such as in the liver or peritoneum, and had not received treatment for metastatic disease. One group received a standard oxaliplatin-based chemotherapy regimen alone. The other group received the same chemotherapy, but only in short bursts that alternated with the immunotherapy drug nivolumab. The main outcome the team tracked was how long patients lived without their disease worsening, known as progression-free survival.

Reading the tumor’s mutation "signal"

The investigators focused on tumor mutational burden, a count of how many DNA changes are present per unit of tumor DNA. In these patients, mutation levels were modest: the median was eight mutations per megabase, far below the levels usually seen in cancers already known to respond well to immunotherapy. Still, a pattern emerged. Patients in the alternating chemo–immunotherapy arm whose tumors had at least nine mutations per megabase tended to go longer before their disease progressed than similar patients with lower mutation counts. When the researchers combined this measure with the presence of a specific tumor mutation, BRAF-V600E, the picture sharpened: patients in the experimental arm who had either higher mutational burden or this BRAF change had a median progression-free survival of about 20 months, clearly longer than other groups in the trial.

Inflammation in the bloodstream tips the balance

The team also examined simple blood markers of body-wide inflammation. They measured C-reactive protein (CRP), a protein that rises when inflammation is present. Across all patients, CRP levels tended to fall during the first two cycles of chemotherapy, suggesting that treatment was damping down tumor-driven inflammation. Among those in the experimental arm who had either higher tumor mutations or the BRAF-V600E change, a normal CRP level at the moment nivolumab was started was associated with striking benefit: these patients had a median progression-free survival of 35 months, nearly four times longer than the trial’s overall median. Other, more complex inflammation scores based on multiple blood cell types did not add useful predictive information in this setting.

What the gene studies revealed

Using targeted DNA sequencing, the researchers mapped out the main genes altered in each tumor. The pattern of changes looked typical for colorectal cancer, with well-known genes such as APC, TP53, KRAS and others frequently mutated. Tumors with higher overall mutation counts were more likely to carry changes in some of these genes, including APC and SOX9, but no single additional mutation beyond BRAF-V600E consistently marked the exceptional responders. A few patients carried rare changes in DNA repair genes, but these did not fully explain who did well. This supports the idea that it is the combination of an intermediate mutation load and a low-inflammation environment, rather than any single gene, that primes the cancer to become vulnerable to immunotherapy after oxaliplatin-based chemotherapy.

What this could mean for future care

For patients and clinicians, the study offers cautious optimism that even immunotherapy-resistant colorectal cancers might be coaxed into responding when chemotherapy is used in a tailored way. The findings suggest that three readily measurable factors—tumor mutational burden, the presence of the BRAF-V600E mutation, and a normal CRP level after initial chemotherapy—could help identify a subgroup of patients with metastatic, microsatellite-stable colorectal cancer who stand to gain prolonged disease control from alternating oxaliplatin-based chemotherapy and nivolumab. Because these insights come from a post hoc analysis of a modest-sized trial, the authors stress that larger, prospectively designed studies are needed. If confirmed, these simple tumor and blood markers could guide more precise use of immunotherapy in a patient population that currently has few long-lasting treatment options.

Citation: Ree, A.H., Bousquet, P.A., Visnovska, T. et al. Exploratory biomarkers for oxaliplatin-induced nivolumab responsiveness in metastatic microsatellite-stable colorectal cancer. Br J Cancer 134, 1176–1182 (2026). https://doi.org/10.1038/s41416-026-03357-6

Keywords: metastatic colorectal cancer, immunotherapy, tumor mutational burden, BRAF V600E, systemic inflammation