Home › Cancer types › Colorectal cancer › RAS-mutant cancer and immunotherapy

Immunotherapy for KRAS- or NRAS-mutant colorectal cancer: who it helps

By CSE Team · Last updated: October 15, 2026

If your colorectal cancer has a KRAS or NRAS mutation, you may have been told that some drugs won't work for you. It's natural to wonder whether immunotherapy is still on the table. Here's the key point: whether immunotherapy fits depends on a different test result, called MSI or MMR status, not on your RAS result. This guide explains what that means, who benefits, and which treatments fit RAS-mutant cancer when immunotherapy isn't the right match, so you and your loved ones can walk into the next appointment with clear questions.

ℹ️ General information to help you and your loved ones talk with your care team, not medical advice. Read our disclaimer.

Different tests answer different questions

When colorectal cancer has spread, the ASCO guideline (from the American Society of Clinical Oncology) and the NCI describe testing the tumor for several things at once. Each result answers its own question:

These results don't depend on each other. A tumor can be RAS-mutant and MSI-H, or RAS-mutant and MSS. Ask to see each result in your report, and ask which test (tissue or blood) was used.

Why MSI-H or dMMR decides immunotherapy

Mismatch repair proteins fix copying mistakes in DNA. When they're missing, the tumor is called dMMR, and it piles up many mutations, which shows up as MSI-H on testing. Those many changes can make the cancer look more foreign to the immune system. Checkpoint inhibitors, which release the brakes on immune cells, then have more to work with, as the NCI explains.

The ASCO guideline recommends Keytruda (pembrolizumab) as a first treatment for MSI-H or dMMR colorectal cancer that has spread, based on the KEYNOTE-177 trial, which compared it with chemotherapy. ASCO notes that MSI-H or dMMR is found in about 4% of people with advanced colorectal cancer, so it's uncommon, but it's important to know. These FDA-approved options are for MSI-H or dMMR colorectal cancer that can't be removed with surgery or has spread, checked against the DailyMed labels in October 2026:

• Keytruda (pembrolizumab), given by IV, or Keytruda Qlex, given as a shot under the skin, used alone.
• Opdivo (nivolumab) plus Yervoy (ipilimumab), for adults and children 12 and older.
• Opdivo (nivolumab) alone, for cancer that grew after chemo with a fluoropyrimidine, oxaliplatin, and irinotecan.

For the full list, see immunotherapy approved for each cancer and our MSI-high colorectal cancer guide.

If your cancer is RAS-mutant and MSS

This is the most common situation for people with RAS-mutant colorectal cancer. Checkpoint inhibitors given alone generally haven't helped MSS colorectal cancer, and none are FDA-approved for it. That can be hard to hear, especially when you've read hopeful stories about immunotherapy. It isn't a dead end, though. There are proven treatments, and researchers are testing new combinations and RAS-targeted drugs in clinical trials, so it's reasonable to ask whether a trial fits you.

What about TMB? Keytruda also has an approval for TMB-H (high tumor mutational burden) solid tumors that have grown after earlier treatment when there are no satisfactory alternatives. ASCO's guideline experts pointed out that the study behind that approval didn't include people with colorectal cancer. Ask your oncologist what your TMB result means for you.

Treatments that fit RAS-mutant, MSS cancer

Based on the ASCO guideline and the FDA labels, the usual path looks like this. Your team will tailor it to your health, goals, and where the cancer has spread.

• First treatment: two-drug chemo, such as FOLFOX, CAPOX, or FOLFIRI, usually with Avastin (bevacizumab). Some people who are fit enough may be offered three-drug FOLFOXIRI with Avastin, which can work better but causes more side effects.
• EGFR antibodies: Erbitux (cetuximab) and Vectibix (panitumumab) aren't used for RAS-mutant tumors; their labels say so. The exception is KRAS G12C after earlier chemo, where Vectibix (panitumumab) is paired with Lumakras (sotorasib) and Erbitux (cetuximab) with Krazati (adagrasib). See our KRAS G12C colorectal cancer guide.
• Later treatments: Lonsurf (trifluridine and tipiracil), often with Avastin (bevacizumab), Stivarga (regorafenib), or Fruzaqla (fruquintinib).
• Clinical trials: worth asking about at any point, including when you first learn your RAS result.

Side effects to expect

Side effects depend on which path you're on. Checkpoint inhibitors for MSI-H cancer can cause immune-related side effects (when the immune system inflames healthy organs), such as colitis (bowel inflammation), skin rash, or thyroid problems; see our immunotherapy side effects guide. Chemo-based treatment can bring Neuropathy (numbness and tingling), especially with oxaliplatin, Diarrhea (loose, watery stools), especially with irinotecan, Fatigue (extreme tiredness), and Neutropenia (low blood counts and infection risk). Avastin (bevacizumab) can raise blood pressure and, less often, cause bleeding or slow wound healing, so your team checks your blood pressure at visits. With capecitabine, watch for hand-foot syndrome.

❓ Questions to ask your oncologist

• Is my tumor MSI-H or dMMR, or is it MSS?
• Which RAS mutation do I have, and is it KRAS G12C?
• What are my BRAF and HER2 results?
• Was testing done on tissue or blood, and do I need more testing?
• Is there a clinical trial that fits my tumor's results?
• What's the plan if the first treatment stops working?

Questions people often ask

Written by the CSE Team from FDA labels and leading cancer centers. Not medical advice. · Last updated: October 15, 2026 · How we write