US regulators will undertake a priority review of Merck's application to market a combination of its immunotherapy drug Keytruda and chemotherapy as an initial treatment for metastatic lung cancer.

The drug has already been approved as a monotherapy by the US Food and Drug Administration for first-line treatment of patients with advanced non-small cell lung cancer (NSCLC) whose tumours express a high level of PD-L1.

Approval of the Keytruda (pembrolizumab)/chemotherapy combination would significantly expand the eligible patient population because it would include those with advanced NSCLC regardless of their level of PD-L1 expression.

Merck's announcement of a priority review caught analysts by surprise given that the firm had not revealed that it was close to filing the combo with regulators.

"This comes as an important surprise because if FDA approves the application, Merck would suddenly be catapulted ahead of all other (immunotherapy) competitors who are also pursuing competing combination regimens of their own," Bernstein analyst Tim Anderson said in a research note, according to Reuters.

The FDA is to make a decision on the filing by May 10.

"If approved, this could be the first regimen combining chemotherapy with an immuno-oncology agent for patients with advanced non-small cell lung cancer," noted Dr Roger Dansey, senior vice president and therapeutic area head, oncology late-stage development, Merck Research Laboratories.