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WASHINGTON – The Food and Drug Administration on Thursday gave its speedy approval to a treatment for melanoma that was found to shrink tumors by more than 30 percent in more than a third of patients with advanced cases of the deadly skin cancer.

The FDA designated the immunotherapy drug pembrolizumab – to be marketed as Keytruda – a “breakthrough therapy” after early clinical trials suggested it might offer “substantial improvement over available therapies” for melanoma. That status allowed an accelerated evaluation of Keytruda.

And it allows the drug’s maker, Merck, to offer the drug to patients whose melanoma is no longer responding to other treatments, even as it continues to conduct clinical trials to confirm preliminary findings.

Pembrolizumab is the sixth new treatment for melanoma in three years to get FDA approval. Most of the six work in different ways to beat back skin cancers and can be used in conjunction or sequentially.

“This is the latest in a string of major breakthroughs in melanoma treatment that will galvanize the field of melanoma research and cancer treatment,” said Wendy Selig, president and chief executive of the Melanoma Research Alliance. “Pembrolizumab has demonstrated real potential to save the lives of late-stage melanoma patients who had little hope of survival just a few years ago.”

As an immunotherapy drug, pembrolizumab neutralizes a cascade of cellular signals that hobbles the immune system’s response to melanoma. It is the first medication to target a protein called PD-1, which keeps T-cells from recognizing and attacking cancer cells. Pembrolizumab disables PD-1 and appears to restore the immune system’s ability to fight cancer.

Oncologist Antoni Ribas of the University of California, Los Angeles’ David Geffen School of Medicine, led the early clinical trial in which nearly three-quarters of subjects showed a significant response to pembrolizumab and a third saw significant reduction in their tumors with no regrowth. Ribas called pembrolizumab “a game changer” that will give patients who have not responded to previous therapies “a very real chance to shrink their tumors and the hope of a lasting response to treatment.”

Keytruda has the potential to be used to treat other cancers that the immune system can recognize, including cancers of the lung, bladder, head and neck, Ribas said.

The FDA said Keytruda showed promising early results in 173 clinical trial participants with advanced melanoma whose disease progressed after previous treatment. About a quarter taking one of two doses of Keytruda saw their tumors shrink, according to the FDA. The effect lasted at least 1.4 to 8.5 months and continued beyond that period in most patients.

In a related development, a new study published in JAMA Dermatology says air crews, including pilots and flight attendants, are more than twice as likely than others to get skin cancer.

The likely culprit? As elevation rises, so, too, does exposure to harmful ultraviolet rays.