Merck's (NYSE:MRK) application for permission to treat a large cohort of advanced-stage cancer patients with Keytruda has been accepted for a priority review by the FDA. The company is seeking approval for the drug to be used as a treatment for anyone with solid tumors that harbor more genetic mutations than usual.

Keytruda from Merck and Opdivo from Bristol Myers Squibb (NYSE:BMY) use the same mechanism to make it harder for tumors to hide from the immune system. In January 2019, Bristol Myers Squibb withdrew an FDA application to use Opdivo plus Yervoy as a treatment for newly diagnosed lung cancer patients with a high tumor mutational burden (TMB). It withdrew the application because long-term outcome data didn't suggest a strong survival benefit for the combination compared to standard chemotherapy in the first-line lung cancer setting.

Healthcare provider caring for a hospital patient.

Image source: Getty Images.

Merck, however, is attempting to earn approval for its PD-1 inhibitor as a second-line treatment for solid tumors regardless of their origination site based on response rate data from its single-arm Keynote-158 study. Keytruda shrank tumors for 34.3% of 233 patients with tumors deficient in DNA mismatch repair, a condition that coincides with a high tumor mutational burden 97% of the time.

Straight to the top

In 2019, sales of Keytruda soared 88% to $7.2 billion, and there's still more room for them to grow. Previous approvals for it to be used to treat lung cancer are expected to make Keytruda the world's top-selling drug in a few short years.  

In 2017, the FDA approved Keytruda as a second-line treatment for patients with mismatch repair-deficient tumors based on results from five different studies, including Keynote-158. If it's approved to treat the larger population of patients with high TMB, sales of Merck's blockbuster immunotherapy could get another big boost.