The U.S. Food and Drug Administration (FDA) has cleared the way for an immunologic drug to be offered as the main drug treatment of lung cancer when the disease is first treated.

The FDA’s approval of Keytruda (pembrolizumab) on Monday marks the first time this type of therapy has been approved as the first-line drug treatment for non-small metastatic lung cancer (NSCLC), the nation’s No. 1 cancer killer.

The drug, which primes the immune system to fight cancer, will be offered to about 30 percent of such patients for which it is likely to be effective. Non-small cell lung cancer accounts for up to 90 percent of all people who have the disease.

The regulators approved the drug based on two studies, including one which hailed the drug's effectiveness as a breakthrough when it was discussed earlier this month at a meeting in Copenhagen of the of the European Society of Medical Oncology (ESMO).

"This study may change current practice for the treatment of patients with advanced NSCLC,” said Johan Vansteenkiste, a Belgium medical professor, commenting on the study, which had just been published in the New England Journal of Medicine.

That study of 305 patients found that Keytruda significantly progression-free survival by approximately four months compared to chemotherapy, and that 80 percent of the patients on it were alive at six months compared to 72 percent on chemotherapy.

Immunotherapy drugs, which the FDA began fast-tracking just two years ago, are already used in melanoma, Hodgkin lymphoma, and cancers of the bladder, kidney, head and neck, but its approval for first-line use in lung cancer is by far the biggest nod for this type of therapy yet.

Lung cancer is the greatest cancer killer in the world, accounting for one-of-four caner deaths in the U.S. Each year more people die of it than of colon, breast, and prostate cancers combined.

Keytruda, which takes the brakes off the immune system, is not for all lung cancer patients, but for the estimated 30 percent of patients with tumors that show a high expression of protein marker called PD-L1, a finding that indicates the drug is likely to be effective.

The drug has received a great deal of publicity, most notably because it is the medication credited with saving the life of former President Jimmy Carter, who was given it after metastatic melanoma spread to his brain.

Also in its decision, the FDA widely broadened the number of patients who can be offered Keytruda as a second-line treatment, which is given after previous chemotherapy.

Keytruda had originally only been cleared in this second-line setting for patients with high PD-L1 levels but the FDA also expanded this to allow for treatment where PD-L1 expression is just one percent or more.