Bristol-Myers Squibb (BMY - Get Report) secured an expanded U.S. approval Wednesday for the use of its checkpoint inhibitor Opdivo to treat a form of advanced lung cancer.

The new Opdivo approval covers patients with squamous non-small cell lung cancer no longer responsive to chemotherapy, according to an announcement made by the FDA. In December, Bristol's drug was approved initially to treat skin cancer. 

The FDA moved exceptionally fast expanding Opdivo's approval. Bristol said the lung cancer application was accepted last week with an approval decision expected in June.

The worldwide commercial market for squamous cell lung cancer patients tops $3 billion, according to an analysis by Barclays.  

Bristol-Myers conducted a study of 272 squamous non-small lung cancer patients, randomizing them to treatment with Opdivo or the chemotherapy drug docetaxel. On average, patients treated with Opdivo lived 3.2 months longer than patients on docetaxel, according to the FDA. A separate study of patients with lung cancer no longer responsive to chemotherapy and then treated with Opdivo demonstrated a response rate of 15%, with  more than half of those responses lasting six months or longer, FDA said. 

The class of immunotherapies known as checkpoint inhibitors work by blocking a protein known as PD-1 which cancer cells use to hide from a patient's immune system. Blocking PD-1 makes tumors visible to killer T cells. Merck's (MRK) Keytruda belongs to the same class of drugs and is also approved to treat skin cancer.

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