On October 19, 2026, the U.S. Food and Drug Administration (FDA) approved dostarlimab (Jemperli, GSK). This is the first immunotherapy approved to treat adults with stage II or stage III rectal cancer that is microsatellite instability high (MSI-H) or has mismatch repair deficiency (dMMR). It was granted accelerated approval.
According to the American Cancer Society, there will be an estimated 49,990 new cases of rectal cancer diagnosed in the United States in 2026. Around 5% to 10% of patients have MSI-H/dMMR rectal cancer. Currently, the standard treatment for locally advanced rectal cancer is surgery with chemotherapy and radiation therapy. These treatments have long-term side effects that affect quality of life long after treatment ends.
Dostarlimab is an immunotherapy that targets the MSI-H/dMMR biomarker. Treatment with this immunotherapy can avoid the serious long-term side effects of chemotherapy, radiation, and surgery.
In a press release from GSK, Dr. Andrea Cercek of MD Anderson said, “The AZUR-1 results demonstrate the potential for a fundamentally different treatment approach in this biomarker-defined patient population, and today’s approval means MSI-H/dMMR patients can now access a licensed treatment tailored to the biology of their disease.
How does dostarlimab treat rectal cancer?
Dostarlimab is a monoclonal antibody and PD-1 inhibitor. When cancers have a dMMR mutation, T cells have too much of the PD-1 protein. T cells do not grow the way they should and can’t stop the tumor growth. PD-1 inhibitors like dostarlimab block PD-1 on immune T cells, activating the immune system to attack cancer cells.
Dostarlimab is given intravenously. This means it is given as an infusion through an IV. A 500mg infusion is given every 3 weeks for nine cycles.
AZUR-1: The study that led to dostarlimab’s approval
The accelerated approval of dostarlimab was based on results from the phase 2 AZUR-1 clinical trial (NCT05723562). There were 154 patients with previously untreated MSI-H/dMMR locally advanced rectal cancer treated during this trial. Of those, 68% achieved a sustained clinical complete response of 12 months with dostarlimab alone. This means that there were no signs of the tumor.
There were 15 patients who went on to have surgery after treatment with dostarlimab. They had surgery because imaging scans suggested there may be tumor cells remaining. Of those, 12 had a pathological complete response. This means a biopsy confirmed there were no remaining cancer cells.
At a median follow up of two years, all of the patients who responded to the treatment remained cancer free. There are 27 patients who have been followed for at least three years. Of those, 24 remain cancer free and only one patient had a local recurrence. The other two patients are no longer in the trial.
What are the common side effects of dostarlimab?
The most common side effects of dostarlimab for people with locally advanced rectal cancer were:
Fatigue
Rash
Diarrhea
Itching
During the clinical trial, 3% of patients stopped treatment due to side effects.
Questions to ask your care team about dostarlimab
Consider asking your care team the following questions about dostarlimab:
Was my tumor tested for MSI-H/dMMR status?
Would dostarlimab be a beneficial treatment for me? Could I avoid surgery, radiation therapy, and chemotherapy?
What side effects should I watch for? What happens if dostarlimab doesn’t control the cancer?
