Merck & Co has filed an application in the US to market its PD-1 checkpoint inhibitor Keytruda for the treatment of patients with advanced non-small cell lung cancer.

Keytruda (pembrolizumab) already carries Breakthrough Therapy designation in this setting, and Merck’s initial filing seeks approval for patients whose disease has progressed despite platinum-containing chemotherapy and an official therapy for certain mutations (if present).

The drug belongs to a class of immunotherapies known as PD-1 inhibitors that block the Programmed Death receptor on tumour cells, thus impeding their ability to escape detection and destruction cytotoxic T-cells.

Its US filing is based on data from KEYNOTE-001 involving patients with greater than or equal to 50% of tumour cells positive for PD-L1 expression. Data presented today (Sunday) at the American Association for Cancer Research meeting in Philadelphia show that these patients taking Keytruda had an overall response rate (ORR) of 45.2%.

The ORR for the entire study - regardless of PD-L1 expression - was 19.4%, and the median duration of response, at 12.5 months, exceeded one year, “which is one of the exciting outcomes with this class of drug," according to researcher Edward Garon, David Geffen School of Medicine, University of California.

Earlier this year, Keytruda’s closest rival - Bristol-Myers Squibb’s Opdivo (nivolumab) - beat it to the post by becoming the first immunotherapy cleared in the US for treatment of advanced squamous NSCLC. Conversely, Keytruda was the first to get approval for advanced melanoma.