Former president Jimmy Carter seems to have made an all-but-miraculous recovery. His melanoma — which is typically deadly in advanced stages — had spread to his liver and brain. And now it appears to be gone. The 91-year-old humanitarian announced recently that his latest tests showed no evidence of the cancer.

Carter underwent surgery and radiation, but it’s a novel, cutting-edge drug treatment that gets most of the credit for saving him, doctors say.

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The drug, pembrolizumab (Keytruda), primes the body's immune system to wage war against the cancer. It's one of several promising new immunotherapy drugs that are changing the outlook not only for melanoma patients but those with other cancers, too. The drugs are less toxic than some older cancer drugs, experts say — and in some cases, they're stunningly effective.

About immunotherapy drugs

Immunotherapy isn't a new idea. It’s been around for more than a century, and some immunotherapy drugs are already in use. In the past few years, however, as scientists have learned more about how cancers behave, the drugs have improved, yielding better results.

Keytruda, approved by the FDA in 2014, is one of these new drugs. A type of immunotherapy drug called a monoclonal antibody drug, it’s known as a ''checkpoint inhibitor." That's because it removes one of the ''brakes” that keeps the immune system from fighting cancer cells.

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Immune cells called T cells patrol the body, looking to identify and attack cancer cells and foreign invaders. Proteins on their surface, known as PD-1, or programmed cell death protein 1, keep them in check so they don’t go overboard and attack normal tissue. Melanoma is good at evading the immune system by controlling the PD-1 pathway. But Keytruda blocks the action of PD-1, allowing T cells to hunt down — and destroy — melanoma.

Keytruda is typically given every three weeks as an infusion. Carter will reportedly stay on that schedule indefinitely as a precaution. That's common, says Joseph Skitzki, MD, an associate professor of surgical oncology at Roswell Park Cancer Institute. He is a specialist in immunotherapy drugs for cancer but was not directly involved in Carter's care. "People tolerate it well," he says. Eventually, he says, Carter will probably not need the drug anymore.

Carter may have been at an advantage, he says, due to his age. That's because adults over about age 70 are more likely to have many mutations in their cancers, Skitzki says. "The more mutations, the better it [Keytruda] seems to work."

Is Carter cured? "It's hard to tell," Skitzki says. Research suggests if no recurrences surface after two years, the prognosis is good, he says.

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A reality check

With standard chemotherapy medicines used in the past, Skitzki says, only about 10 percent or fewer people with melanoma were helped. Now, that percent is higher — but it’s nowhere near 100 percent. For instance, just 34 percent of patients responded to Keytruda in clinical trials, Skitzki says.

The cost for immunotherapy drugs, including Keytruda, is often breathtaking. Prices vary, but the annual cost for Keytruda and other similar drugs may be $100,000 to $150,000 a year. Even with health plan coverage, the copayment can be staggering. Pharmaceutical companies may offer patient assistance programs.

Beyond melanoma

The potential of the new immunotherapy drugs is still being investigated, Skitzki says. Keytruda, like other immunotherapy drugs, is not limited to fighting melanoma. As scientists learn more about how some cancers are similar, they can test specific immunotherapy drugs against them.

Keytruda, for instance, is now approved to treat a form of lung cancer known as nonsmall cell lung cancer. The drug may also help people with head and neck cancers associated with smoking, Skitzki says, and those with a hereditary form of colon cancer. It may help treat pancreatic cancer, too, he says.

The trend now, Skitzki says, is to use the immunotherapy drugs, including Keytruda, in combination with standard chemotherapy medicines to see if the results can be improved.

One point seems clear: Immunotherapy drugs are here to stay, with improvements in the pipeline.

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Kathleen Doheny is a Los Angeles journalist specializing in health, behavior and fitness topics.