Although it has shown a lot of promise, the new immune therapy drug used to treat former President Jimmy Carter’s cancer is still a ways off from reaching the status of a “miracle drug.”

Several medical experts told Healthline it’s too early to tell how much credit the drug Keytruda should get for Carter’s successful treatment. The medication is one of a new class of cancer-fighting drugs that are designed to sic the body’s own immune system on cancers. In Carter’s case, the drug appears to have shrunk the tumors in his brain to be undetectable. Now Carter and his doctors will wait to see if the melanoma returns.

Not everyone who might benefit from Keytruda will have access to it. Some insurance providers do not cover the medication, which is estimated to cost $150,000 a year.

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Carter’s Remarkable Recovery

In August, Carter announced that the melanoma doctors had cut out of his liver had materialized in four small spots in his brain.

The former president underwent four radiation treatments that targeted the cancer in his brain. He also started taking Keytruda.

On Sunday, Carter announced at his church that his latest tests showed no cancer in his brain.  That news spurred some to call his recovery a miracle. But several medical experts told Healthline they were not surprised by Carter’s results.

Dr. Len Lichtenfeld, deputy chief medical officer for the American Cancer Society, said the former president was fortunate in several ways. The cancer in Carter’s liver had been surgically removed, and the four cancerous lesions in his brain were relatively small. The surgery and radiation alone might have been enough to do the trick.

“I don’t think we know yet how much of the success is related to Keytruda,” said Lichtenfeld.

Dr. Adil Daud, the co-director of the Melanoma Center at the University of California San Francisco Medical Center, agreed it was premature to solely credit Keytruda in this case. Both doctors said Keytruda has shown remarkable early promise, but the key in Carter’s case will be if the melanoma doesn’t reemerge.

“That will reassure us,” said Daud.

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How Keytruda Works

Keytruda is one of several new drugs that work by basically helping the immune system better do its job.

Scientists have long known that cancer cells send out signals that disable the immune system or trick it into thinking these foreign invaders are no threat. Cancerous cells can emit a protein that deactivates the immune system’s key defenders, T cells.

Dr. Roxana Dronca, an assistant professor of oncology at the Mayo Clinic College of Medicine in Minnesota, likened the situation to a battlefield.

“The T cell soldiers come to the battle and want to fight,” she said, “but when they arrive they are useless.” Keytruda blocks the activity of the proteins, allowing the immune system and its T cells to go after the tumor.

Keytruda, which is manufactured by Merck, was approved by the FDA in September 2014 for use in treatment for late stage melanoma. Two months ago, it was given the OK for use in treatment of advanced non-small cell lung cancer.

In December 2014, the FDA also gave the go-ahead for Bristol-Myers Squibb’s drug Opdivo to be used as part of advanced melanoma treatment. That drug works in similar fashion to Keytruda. Last month, Opdivo also received FDA approval to be used in treatment for a form of advanced kidney cancer. Both drugs have relatively few side effects because they boost the immune system rather than chemically attack cancer cells.

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Moving to the Front Lines

So far, Opdivo, Keytruda, and other drugs have had promising early results.

Dronca said Keytruda causes tumors to shrink in 35 to 45 percent of the melanoma patients who receive the drug. Of those, 10 percent have the tumors disappear. Another 15 to 20 percent of these patients see their tumors stop growing and stabilize.

“It is fairly effective on melanoma,” Dronca said.

So far, Keytruda has been used mostly in patients with stage 4 melanoma. Cancer researchers are hoping the drug can be used for treatment of earlier stages of melanoma.

“We’re looking for Keytruda to become a front-line treatment for patients with melanoma,” said Daud.

Despite the early successes, it’s still a wait-and-see game for these new promising drugs.

Among other things, Dronca said scientists will keep a close eye on cancer cells to see if they mutate to overcome immunology drugs. She said cancer cells have been able to adapt to all other cancer treatments, including chemotherapy.

“All of these treatments have a time stamp on them,” said Dronca.

The key, she added, is to keep developing drugs while the current ones are still effective.

Access and Cost

Who gets these drugs and how do they pay for them?

Lichtenfeld said doctors base their decision on whether to use drugs like Keytruda depending on the patient. Physicians consider factors such as the patient’s genetic make-up and their overall health. Paradoxically, the immune systems of older patients, like Carter, are stronger than younger patients. That’s thanks to a long lifetime of exposure to a vast range of microbes.

Dronca said cancer researchers are trying to determine if there are biomarkers that can predict how patients will react to specific drugs and other treatments. Those markers might even be able to predict which combination of therapies are most likely to succeed, and in what order to administer them.

But most of these therapies are not cheap.

Although the drug is FDA-approved, it’s not a given that insurance companies will agree to cover the medication’s eye-popping price tag.

There are cases where patients are denied potentially life-saving drugs because of price. It happened to a Florida student with a rare form of cancer in the summer of 2015. In the United Kingdom, there were stories this fall about cancer patients being denied drugs by the National Health Service due to high costs.

Overall, though, cancer researchers are optimistic about the new drugs like Keytruda.

“It doesn’t work for everyone,” said Dronca, “but for some people it is kind of a miracle.”