Chalk up another win for Merck's (NYSE:MRK) blockbuster cancer drug Keytruda, this time in patients with microsatellite instability-high (MSI-H) or mismatch repair deficient (dMMR) unresectable or metastatic colorectal cancer. In layman's terms, those are advanced colon cancers with genetic mutations that reduce the ability of the tumor cells to repair DNA damage.

Keytruda reduced the risk of disease progression or death by 40% compared to chemotherapy. Put another way, patients treated with Keytruda lived for 16.5 months without their tumors starting to grow, compared to 8.2 months for patients treated with chemotherapy.

The gap in response rates between the treatments was narrower -- 43.8% for Keytruda compared to 33.1% for chemotherapy -- but patients responded to Keytruda for longer periods. The median duration of response was 10.6 months for chemotherapy, while the median duration of response for the Keytruda group is long enough that hasn't been determined yet. Looking at the data another way, 83% of patients treated with Keytruda had a response that lasted two years compared to just 35% of patients treated with chemotherapy.

The data are scheduled to be presented on Sunday at the 2020 American Society of Clinical Oncology Annual Meeting, which is being held virtually.

Doctor with her hand on a patient's shoulder

Image source: Getty Images.

Technically, Keytruda is already approved to treat this type of colon cancer because Merck gained FDA approval in 2017 for it to be prescribed as a treatment for any solid tumor with MSI-H or dMMR. But that approval was for previously treated patients, while the patients in this study were earlier in their disease progression, having not been previously treated. Merck didn't mention its plans for submitting a label-expansion request, but presumably, the pharma giant will use this trial data to rack up another approval for Keytruda.