A drug to treat advanced cases of the type of skin cancer known as melanoma, which researchers have characterized as "game changing," has received approval from the Food and Drug Administrations.

Using their fast-track authorization process, the FDA says the drug Keytruda (pembrolizumab) from pharmaceutical firm Merck & Co. can be used as a treatment for patients suffering from advanced melanoma if they have stopped responding to other forms of therapy.

Keytruda is one of a new class of drugs meant to overcome the problem of cancer cells evading by body's immune system by activating a "braking system" in the system that stops it from attacking a tumor.

Keytruda impedes the action of one such "brake," a protein called PD-1, for programmed death receptor 1.

By specifically targeting this protein, Keytruda boosts the ability of immune cells to attack tumor cells.

The FDA based its fast-track approval on the results of clinical trials conducted at UCLA, in which patients diagnosed with advanced cases of melanoma that were not responding to existing treatments were given Keytruda.

"Even the very preliminary results on a handful of patients, 20 or so, indicated a high degree of activity," Dr. Richard Pazdur, who oversees cancer drug research at the FDA, said.

Tumor shrinkage of up to 30 percent was noted in a quarter of the patients in the trials and the tumors did not regrow for a period of months.

"This drug is a game changer, a very significant advance in the treatment of melanoma," says Dr. Antoni Ribas, who conducted the UCLA trial. "For patients who have not responded to prior therapies, this drug now provides a very real chance to shrink their tumors and the hope of a lasting response to treatment."

Keytruda may also prove effective against other forms of cancer that are recognized by the immune system, including lung and bladder cancers and cancers of the head and neck, he says.

Keytruda is the sixth new drug for the treatment of melanoma approved since 2011.

Those drugs have transformed a disease once considered invariably fatal if it spread into one that can be attacked and managed with a number of treatments.

The FDA estimates around 76,000 American will develop melanoma annually and more than 9,000 will die from it each year.

Excessive exposure to the sun is considered by experts to be a major factor in the development of melanoma.

Doctors have known for some time that melanoma is susceptible to attack by the human immune system, which is why immunotherapy drugs have remained at the center of a search for treatments.