In an “important first,” the FDA has fast-tracked a cancer treatment based on a biomarker rather than on the location of the tumor.
Pembrolizumab (Keytruda) has already been approved for certain patients with metastatic melanoma, metastatic non-small cell lung cancer, and other cancers. With the new approval, pembrolizumab is now indicated for adult and pediatric patients with unresectable or metastatic solid tumors that have a biomarker known as microsatellite instability-high (MSI-H) or mismatch repair deficient (dMMR). Microsatellite instability-high and dMMR tumors have abnormalities that affect the proper repair of DNA inside the cell. Pembrolizumab blocks a cellular pathway (PD-1/PD-L1—proteins found in immune cells and some cancer cells), which may help the immune system fight the cancer cells.
Tumors with the biomarkers are most commonly found in colorectal, endometrial, and gastrointestinal cancers. About 5% of patients with metastatic colorectal cancer have MSI-H or dMMR tumors. Less commonly, they are found in cancers in the breast, prostate, bladder, thyroid gland, and other locations.
In 5 clinical trials, of 149 patients treated with pembrolizumab for 15 cancer types, 40% had a complete or partial response. For most of those patients, the response lasted ≥ 6 months.
