(Reuters) - The U.S. Food and Drug Administration granted accelerated approval for Merck & Co's immunotherapy, Keytruda, in patients with a form of lung cancer.

Keytruda and another similar treatment from Bristol-Myers Squibb Co called Opdivo block a protein called PD-1, whose natural function is to put checks on the immune system.

Wall Street analysts expect such drugs to capture combined annual sales of over $20 billion by 2020.

Keytruda was approved last year to treat patients with advanced melanoma.