President Carter recently announced that his melanoma was gone.

Keytruda: 4 things to know about the drug Jimmy Carter used to treat his cancer

He said he has a new cancer drug, called Keytruda, to thank for it.

>> Photos: How Carter has kept busy since diagnosis

1. What is it?

The drug is known chemically as pembrolizumab.

“It’s technically called a checkpoint inhibitor," Dr. Len Lichtenfeld, Chief Medical Officer of the American Cancer Society, said. "It’s one of several drugs available over the past several years that have been effective in treating melanoma by harnessing the body’s own ability to fight the cancer cells to find the cancer cells and destroy them,” he said.

2. How does it work?

“What Keytruda and similar drugs do is wake up the immune system and allow the immune system to recognize that these cancer cells are not normal and foreign so go after them,” Lichtenfeld said.

>> 5 things to know about Jimmy Carter’s 'cancer-free' declaration

3. Origin of the drug

The drug is made by Merck and was approved in September 2014 when the company sold $55 million worth of it in a few months. This year, Merck has already sold more than $352 million worth of the drug.

Merck said they’ve already sold more than $400 million of Keytruda since the FDA approved it in 2014. A representative said, "While advanced melanoma has historically been considered difficult to treat, recently there have been several advances that have been restoring hope for patients with this devastating disease as well as their physicians."

Research for the drug Carter received started decades ago, Litchenfeld said.

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4. It's effective for more than one type of cancer

Carter told the world he had four spots of melanoma on his brain and Litchenfeld says his doctors gave him the drug to control the disease.

The drug can be used for more than just melanoma, however.

“What’s interesting is they are not only effective in treating melanoma, they have turned out to be effective for other cancers and recently been approved in treatment in lung, stomach, neck and other cancers,” said Lichtenfeld about the drug used.

Click here to learn more about the drug.

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