What Is Immunotherapy for Colon Cancer?

Immunotherapy for colon cancer is a treatment approach that uses drugs to help your own immune system recognize and attack tumor cells. The most established form involves checkpoint inhibitors, which block proteins that cancer cells exploit to hide from immune surveillance. These drugs work remarkably well for a specific subset of colon cancers, but the catch is that only about 15 to 20 percent of patients have tumors with the biological features that make them strong candidates. That distinction, and the ongoing effort to extend immunotherapy’s reach to the remaining majority, is what shapes the landscape of this field.

How Checkpoint Inhibitors Target Colon Cancer

Your immune system has built-in brakes called checkpoints. These exist for good reason: they prevent immune cells from attacking your own healthy tissue. Cancer cells, though, can hijack these checkpoints to avoid being destroyed. One of the most important of these involves a protein on immune cells called PD-1 and a partner protein called PD-L1 that sits on the surface of some tumor cells. When PD-L1 latches onto PD-1, it tells the immune cell to stand down. Checkpoint inhibitors are drugs designed to block that handshake, freeing T cells to do their job and attack the tumor.¹

The checkpoint inhibitors approved for colon cancer include pembrolizumab (a PD-1 blocker) and the combination of nivolumab (also PD-1) with ipilimumab (which blocks a different checkpoint called CTLA-4). Using two checkpoint inhibitors together hits the brakes from two different angles, which can produce a stronger immune response but also carries a higher risk of side effects.

Why Your Tumor’s MSI Status Changes Everything

Not all colon cancers are equally visible to the immune system. The single most important factor in determining whether checkpoint inhibitors will help is something called mismatch repair status. Every time a cell divides, its DNA-copying machinery makes small errors. A set of repair proteins normally catches and fixes those mistakes. When those repair proteins are missing or broken, a condition called mismatch repair deficiency (dMMR), errors pile up across the genome. This creates a hypermutated tumor loaded with abnormal proteins called neoantigens, which act like red flags that the immune system can spot.²

The lab test for this is called microsatellite instability testing. Tumors with high microsatellite instability (MSI-H) are the ones with deficient mismatch repair, and they respond strongly to checkpoint inhibitors. Tumors with stable microsatellites (MSS), which represent roughly 80 to 85 percent of colorectal cancers, have far fewer mutations, produce fewer neoantigens, and largely resist checkpoint blockade on its own.³

Current guidelines recommend that every patient diagnosed with colorectal cancer have their tumor tested for mismatch repair status at diagnosis.⁴ This is partly because dMMR tumors respond so differently to treatment, and partly because the finding can reveal Lynch syndrome, an inherited condition that raises the risk of several cancers.

Checkpoint Inhibitors for Advanced MSI-H Colon Cancer

For patients whose MSI-H colon cancer has spread, checkpoint inhibitors have become a first-line treatment option, replacing traditional chemotherapy in many cases. The pivotal KEYNOTE-177 trial compared pembrolizumab against standard chemotherapy in this population and found that pembrolizumab roughly doubled progression-free survival: a median of about 16.5 months versus 8.2 months with chemotherapy.⁵ Severe side effects were also far less common, occurring in about 22 percent of patients on pembrolizumab compared with 66 percent on chemotherapy.⁶

The combination of nivolumab plus ipilimumab has pushed results even further. In the CheckMate 8HW trial, two-year progression-free survival was 72 percent with the combination versus 14 percent with chemotherapy.⁷ The same trial also compared the combination against nivolumab alone and found a meaningful advantage for the dual-drug approach.⁸ These results have positioned dual checkpoint blockade as a potential new standard of care for this group.

One thing worth noting: even among MSI-H patients who receive checkpoint inhibitors, roughly 45 to 60 percent eventually show primary or acquired resistance, meaning the cancer either does not respond at all or stops responding after an initial period.⁹ Researchers are still working to understand why some MSI-H tumors escape immune attack, and identifying those patients upfront remains a challenge.

Before Surgery: Immunotherapy as a Neoadjuvant

One of the most exciting recent developments is giving immunotherapy before surgery, rather than only after cancer has spread. For patients with locally advanced dMMR colon cancer, a short course of checkpoint inhibitors before the operation can dramatically shrink or even eliminate the tumor. In a landmark study, 111 patients received a combination of nivolumab and ipilimumab before surgery. Ninety-eight percent had a pathological response, meaning the tumor showed clear signs of damage from treatment. Sixty-eight percent achieved a complete pathological response, with no detectable viable tumor cells remaining in the surgical specimen. With a median follow-up of over two years, no patients had recurred.¹⁰

Results from other neoadjuvant trials have been encouraging but more variable. A trial of pembrolizumab alone in stages I through III dMMR colon cancer found complete pathological responses in about 45 percent of patients, with major responses in 63 percent.¹¹ A real-world multicenter study using various immunotherapy regimens reported complete responses in 42 percent of resected tumors.¹² The differences likely reflect differences in drug regimens and patient selection, but the overall trend is clear: for dMMR tumors, pre-surgical immunotherapy can produce striking responses.

This raises a question that researchers and patients are now grappling with: if the tumor is completely destroyed by immunotherapy, is surgery still necessary? A handful of small rectal cancer studies have explored skipping the operation entirely in patients who achieve a complete clinical response, and the colon cancer field is watching closely. For now, surgery remains the standard, but this could change as more long-term data accumulate.

The MSS Problem

The uncomfortable reality is that most colon cancer patients fall into the MSS category, where checkpoint inhibitors alone have shown little benefit. MSS tumors have a low mutational burden, meaning fewer neoantigens for the immune system to target. They also tend to have an immunosuppressive local environment, with cells and signaling pathways that actively keep T cells out. Among the barriers identified: regulatory immune cells that dampen the attack, signaling pathways like Wnt and MAPK that exclude immune cells from the tumor, metabolic changes that starve T cells of fuel, and even disruptions in the gut microbiome.³

Researchers are testing combination strategies designed to break through these defenses. One approach pairs a checkpoint inhibitor with drugs that remodel the tumor environment. A randomized trial tested a three-drug combination of a PD-1 inhibitor, a drug that modifies how genes are read (an HDAC inhibitor), and an anti-VEGF antibody that disrupts the tumor’s blood supply. The triplet appeared to increase the infiltration of killer T cells into the tumor, creating a more immune-active environment.¹³ Another strategy under study is combining immunotherapy with radiation, which can cause tumor cells to release debris that primes the immune system.¹⁴ None of these approaches has yet become standard care for MSS colon cancer, but the field is moving quickly.

Side Effects of Immune Checkpoint Inhibitors

Because checkpoint inhibitors work by releasing the brakes on the immune system, they can cause the immune system to attack healthy tissue. These immune-related side effects can affect virtually any organ, but for colon cancer patients, there is an ironic twist: one of the most common and serious is colitis, inflammation of the colon itself.¹⁵

How often this happens depends on the drug. PD-1 inhibitors alone cause diarrhea in about 10 percent of patients and colitis in roughly 2 percent. The CTLA-4 inhibitor ipilimumab causes diarrhea in about a third of patients and colitis in about 7 percent. When the two are combined, diarrhea rates range from 21 to 37 percent and colitis from 4 to 8 percent, depending on dosing.¹⁶ Colitis typically shows up five to ten weeks after the second or third dose, though cases have been reported anywhere from immediately to two years into treatment.¹⁷

Untreated, immune-related colitis can become life-threatening. Current management relies on broad immunosuppression, typically corticosteroids, which carries its own problem: suppressing the immune system to control the side effect may also reduce the anti-tumor effect of the drug. More targeted treatments are being studied. Other immune-related side effects beyond the gut include thyroid disorders, skin rashes, liver inflammation, and lung inflammation, though these tend to be less frequent with the regimens used in colon cancer.

Quality of Life Compared With Chemotherapy

Beyond tumor shrinkage and survival, how patients feel during treatment matters enormously. In the KEYNOTE-177 trial, patients on pembrolizumab reported clinically meaningful improvements in overall quality of life compared with those on chemotherapy by week 18 of treatment. The time it took for patients to experience worsening in physical functioning, social functioning, and fatigue was substantially longer in the immunotherapy group.¹⁸ This is partly because checkpoint inhibitors spare patients the cumulative nerve damage, nausea, and bone marrow suppression that often accompany standard chemotherapy regimens.

The Immunoscore and Predicting Who Benefits

MSI status is not the only biomarker that matters. Researchers have developed something called the Immunoscore, which measures the density and location of specific immune cells (primarily two types of T cells) within the tumor and at its edges. A large international validation study found that patients with early-stage colon cancer and a high Immunoscore had a five-year recurrence rate of just 8 percent, compared with 32 percent for those with a low Immunoscore. That association held regardless of age, sex, tumor stage, sidedness, and MSI status, and it contributed more to predicting outcomes than any other clinical factor tested.¹⁹

Further studies have confirmed the Immunoscore’s value in stage III disease, where patients with low scores had meaningfully shorter disease-free survival regardless of the chemotherapy regimen they received.²⁰ Even among patients with T4 tumors and no lymph node involvement who did not receive chemotherapy, those with a high Immunoscore fared well, raising the possibility that the Immunoscore could one day help decide which patients can safely skip adjuvant treatment.²¹ The Immunoscore is not yet part of routine staging in most centers, but momentum is building to incorporate it into standard cancer classification systems.

Gut Bacteria and Immunotherapy Response

A growing body of evidence suggests that the composition of your gut microbiome may influence how well checkpoint inhibitors work. A systematic review of studies in colorectal cancer patients found that certain bacterial groups, particularly Faecalibacterium and Prevotellaceae, were associated with better treatment responses. A gut signature enriched in Akkermansia muciniphila and Eubacterium rectale could independently predict better outcomes.²²

This is still early-stage science, and no one can yet prescribe a specific probiotic or dietary change that is proven to improve immunotherapy outcomes. But the findings have practical implications. There is growing concern, for instance, that unnecessary antibiotic use before or during immunotherapy could disrupt beneficial gut bacteria and blunt the treatment’s effectiveness. Some clinical trials are now testing fecal microbiota transplants or targeted bacterial supplements as add-ons to checkpoint inhibitors.

Beyond Checkpoint Inhibitors

Checkpoint inhibitors are the most established form of immunotherapy for colon cancer, but they are not the only approach under investigation. Several newer strategies are in clinical trials, each trying to harness the immune system in a different way.

CAR-T cell therapy involves removing a patient’s T cells, genetically engineering them to recognize a specific protein on the tumor, and infusing them back. This approach has transformed treatment for certain blood cancers but has been harder to apply to solid tumors like colon cancer. Early-phase trials targeting proteins like CEA (carcinoembryonic antigen) have shown the treatment is safe and can produce stable disease in some patients, with occasional tumor shrinkage, but no dramatic radiologic responses so far.²³ Earlier trials targeting a different protein called TAG-72 confirmed that CAR-T cells could traffic to the liver but were largely excluded from large tumor deposits, and persistence of the engineered cells in the bloodstream was short-lived.²⁴ The major challenges include getting the engineered cells deep into solid tumors, keeping them active once they arrive, and dealing with the fact that colon tumors do not all display the same surface proteins.²⁵

Bispecific antibodies represent another emerging class. These are engineered molecules designed to grab onto a tumor cell with one arm and a T cell with the other, physically bringing them together. A preclinical study of a bispecific antibody targeting EGFR on colon cancer cells and CD3 on T cells showed potent tumor regression in mouse models. The design included a “masking” feature that kept the drug inactive in healthy tissue and only unmasked it in the tumor environment, reducing the risk of damage to normal cells expressing EGFR.²⁶ Clinical trials of bispecific antibodies for colorectal cancer are underway.²⁷

Personalized cancer vaccines are a third frontier, and they may be especially relevant for MSS tumors where checkpoint inhibitors alone fail. These vaccines are custom-built from a patient’s own tumor mutations, designed to train the immune system to recognize neoantigens unique to that person’s cancer.²⁸ A small clinical study in six MSS colorectal cancer patients with recurrence after surgery and chemotherapy found that two-thirds developed a measurable immune response to their vaccine, and the four who responded remained progression-free significantly longer than the two who did not.²⁹ These numbers are far too small to draw firm conclusions, but the concept of using vaccines to prevent recurrence after surgery or to sensitize MSS tumors to other immunotherapies is generating serious interest.³⁰

Cost and Access Considerations

Immunotherapy drugs are expensive, and the question of value depends on what they are being compared against. A cost-effectiveness analysis of first-line pembrolizumab versus standard chemotherapy for MSI-H metastatic colorectal cancer found that pembrolizumab cost roughly $11,000 more over a lifetime horizon but added over 1.5 quality-adjusted life years, yielding a very favorable cost-per-benefit ratio by conventional health-economic thresholds.³¹ However, when checkpoint inhibitors are used in later lines of treatment and compared with less expensive salvage therapies, the cost-effectiveness picture is less clear, with substantially higher incremental costs per benefit gained.³²

Access also varies by age. There has been concern that older patients might not tolerate immunotherapy well or might be underrepresented in clinical trials. Current evidence suggests that single-agent immunotherapy is recommended for elderly MSI-H patients on the same basis as for younger patients.³³ Whether older adults benefit equally from more aggressive dual-checkpoint regimens is less well established, and the higher side-effect burden of combination therapy warrants careful discussion between the patient and their oncologist.