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Colon cancer happens when tumors grow in the large intestine. Research estimates that about 15 percent of colorectal cancers turn out to be MSI-high (MSI-H). That means there’s a high amount of instability in the cancer cells.

In recent years, new immunotherapy drugs have become available to treat MSI-H colon cancer. The goal of immunotherapy is to get your immune system to see these cancer cells and launch a strong attack against them.

Keep reading to find answers to some common questions about immunotherapy for MSI-high colon cancer.

All colon cancers are not the same. One distinguishing feature is microsatellite stability status. This is a piece of information you get from a biopsy. Laboratory analysis of the tissue sample shows if the cancer cells are:

  • microsatellite stable (MSS)
  • microsatellite instability-low (MSI-L)
  • microsatellite instability-high (MSI-H)

In MSI-high colon cancer, the cells are extremely unstable. That means there’s a high number of mutations. Under a microscope, these cancer cells look and act in a distinctive way.

Research suggests that MSI-H colorectal cancer tends to have a better outlook and less metastasis, compared with MSS colorectal cancers.

Microsatellite status can be an important factor in choosing a treatment that’s most likely to be effective for colorectal cancer.

It depends on what type of colon cancer you have. Immunotherapy has been shown to be effective in treating MSI-high colon cancers, but it hasn’t worked well in MSS colon cancers.

There are three immunotherapy drugs approved to treat MSI-high colorectal cancer:

  • Nivolumab (Opdivo) is used when the cancer continues to progress after you’ve started chemotherapy.
  • Ipilimumab (Yervoy) is approved for use only in combination with nivolumab.
  • Pembrolizumab (Keytruda) earned approval from the Food and Drug Administration (FDA) in 2020 as first-line therapy for MSI-H colorectal cancer.

All three are immune checkpoint inhibitors that target the cellular pathway of certain proteins found on immune cells and cancer cells. Ipilimumab blocks CTLA-4, while nivolumab and pembrolizumab target PD-1.

These checkpoints normally keep the immune response from going overboard. Blocking them frees up immune cells to mount a powerful offense against cancer.

These medications are usually given through intravenous infusion.

They’re not exactly the same, but they’re related.

In this context, MMR is short for mismatch repair genes. These are genes that help regulate DNA. MMR are supposed to repair the errors that happen during cell division.

Defective MMR stop making repairs, which leads to microsatellite instability.

After your biopsy, your pathology report may have read, “MSI-H/dMMR.” Tumors that are deficient in MMR (dMMR) are considered to have high levels of microsatellite instability.

Your body is always creating new cells to replace older ones as they die off — a process that can result in DNA damage to some cells. Fortunately, MMR acts like a built-in system to detect and repair this damage.

But the system can fail. Instead of dying off as they should, abnormal cells continue to reproduce. Newer cells have an increasing number of errors. They pile up and form tumors, and that’s how microsatellite instability causes cancer.

Immunotherapy is a relatively new treatment for MSI-H colon cancer, but research already shows it has a decent likelihood of success.

CheckMate 142 is a phase 2 study to evaluate the safety and effectiveness of PD-1 inhibitors in 74 people with MSI-high metastatic colorectal cancer, most of whom have already been through chemotherapy.

At a follow-up of 12 months, 23 participants saw their MSI-H colorectal cancer respond to the immunotherapy drug nivolumab. The treatment also provided disease control for at least 12 weeks to 51 participants, while 8 people had a response that lasted at least a year.

The study, which is ongoing, is also evaluating therapy with combination nivolumab plus ipilimumab. Early results show promise, and the treatment appears to be well tolerated.

There’s also a phase 3 clinical trial that compared immunotherapy with pembrolizumab to chemotherapy as first-line therapy for MSI-high metastatic colorectal cancer. The results showed that pembrolizumab doubled participants’ median progression-free survival from 8.2 months to 16.5 months.

That trial also showed that immunotherapy caused fewer serious side effects than chemotherapy. Serious adverse events occurred in 22 percent of participants in the immunotherapy group, compared with 66 percent in the chemotherapy group.

Researchers continue to study the potential of immunotherapy for all stages and types of colorectal cancer.

MSI-high colon cancer is a type of colon cancer that involves highly unstable cells.

One of the newest treatments for this type of cancer is immunotherapy. It helps your body’s immune system see and destroy the cancer cells.

Two immunotherapy drugs are approved to treat MSI-high colon cancer that has progressed during or after chemotherapy. There’s also a new immunotherapy drug approved as a treatment for people with MSI-H colorectal cancer who haven’t tried chemotherapy.

Clinical trials suggest that immunotherapy is associated with longer progression-free survival than chemotherapy, but research is ongoing.

If you have MSI-high colon cancer, speak with your doctor to see if immunotherapy is a good fit for you.