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Former President Jimmy Carter says he has melanoma that has spread to his liver and brain — but he’s fighting it with a drug so promising that its approval was fast-tracked last year.

Carter, 90, has already had his first infusion of the drug, known as Keytruda. It harnesses the immune system to fight melanoma that has spread through the body. Carter also told reporters he would undergo targeted radiation to his brain on Thursday afternoon.

"I'll be prepared for anything that comes," he said.

Keytruda — known generically as pembrolizumab — targets the activity of genes called PD-1 and PD-L1. The interaction between the two genes lets some tumors escape detection and destruction by immune system cells that normally prevent cancer from spreading in the body.

Cancer experts say while Carter will never be cured, the drug has had startling effects in a few people, and at the worst shouldn’t make him too sick — even at age 90.

“It's really a whole new class of therapy."

“It's really a whole new class of therapy, and as president Carter said, it really allows our own immune system to fight a cancer,” said Dr. Wally Curran of Emory University, an expert in brain tumors who is involved in Carter’s treatment.

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“That concept has really been around for decades in cancer but we have not been able to give therapies without terrible side effects," he said.

The Food and Drug Administration gave Keytruda accelerated approval last September for patients whose melanoma has spread.

Keytruda does have side-effects, such as fatigue and rash. But each person is different, and Carter said he only had a “slight reaction” to his infusion of Keytruda on Wednesday night.

“I had a little bit of pain in my shoulder,” he told reporters. “And I went to bed at about six o'clock last night and woke up at 8 a.m. this morning. It was the best night sleep I've had in many years," he added.

"I just thought I had a few weeks left. But I was surprisingly at ease. I've had a wonderful life, I've had thousands of friends, and I've had an exciting and adventurous and gratifying existence."

"We're not looking for cure in patients who have a disease like melanoma that's spread."

Keytruda, which has also shown promise as a treatment for lung cancer, doesn’t require hours of infusion, and doctors said Carter should be able to resume many of his activities.

“Keytruda is basically a 30-minute infusion because it doesn't have nausea as side-effect,” said Dr. Sapna Patel, assistant professor of medicine at MD Anderson Cancer Center in Houston.

“It's not toxic. It doesn't clog up the kidney,” she added.

And even though he’s 90, Carter is strong, Patel pointed out. That's clear from his active life.

“We often think of geriatric patients as being frail and maybe we should be less aggressive,” Patel told NBC News.

“In actuality, we may be doing them a disservice not treating them with the full, broad spectrum of our drugs… it's reasonable to attempt these therapies at full dose in a geriatric population.”

And a cure is not the goal, said Curran. “We're not looking for cure in patients who have a disease like melanoma that's spread,” he said. “What we're seeing in some types of cancer which may not be curable is long-term life with a good quality of life.”

That means keeping the tumors from growing, spreading and causing symptoms, even if they are never eradicated.