WASHINGTON -- Opening a new era in cancer care, the Food and Drug Administration on Wednesday approved the first treatment that genetically engineers patients' own blood cells into an army of assassins to seek and destroy childhood leukemia.

The CAR-T cell treatment developed by Novartis and the University of Pennsylvania is the first type of gene therapy to hit the U.S. market — and one in a powerful but expensive wave of custom-made "living drugs" being tested against blood cancers and some other tumors, too.

FDA called the approval historic.

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"This is a brand new way of treating cancer," said Dr. Stephan Grupp of Children's Hospital of Philadelphia, who treated the first child with CAR-T cell therapy — a girl who'd been near death but now is cancer-free for five years and counting. "That's enormously exciting."

CAR-T treatment uses gene therapy techniques not to fix disease-causing genes but to turbocharge T cells, immune system soldiers that cancer too often can evade. Researchers filter those cells from a patient's blood, reprogram them to harbor a "chimeric antigen receptor" that zeroes in on cancer, and grow hundreds of millions of copies. Returned to the patient, the revved-up cells can continue multiplying to fight disease for months or years.

"Just the fact that I get a future is completely remarkable — I shouldn't be alive right now," Mitchell Carbon, 19, told CBS News' Jim Axelrod. After just one infusion, he is now cancer-free.

"It's a pretty amazing new treatment," Dr. David Agus, director of the USC Norris Westside Cancer Center and CBS News medical contributor, told "CBS This Morning" in July, when an FDA panel voted to recommend approval. "They take the white [blood] cells out of a child with cancer, they send them to [a lab in] New Jersey, and they put in a gene to reprogram these cells to attack the cancer."

Novartis didn't immediately disclose the therapy's price but it is expected to cost hundreds of thousands of dollars. The health news site STAT reported a course of treatment will cost $475,000. It's made from scratch for every patient.

"We're entering a new frontier in medical innovation with the ability to reprogram a patient's own cells to attack a deadly cancer," said FDA Commissioner Scott Gottlieb.

This first use of CAR-T therapy is aimed at patients desperately ill with a common pediatric cancer — acute lymphoblastic leukemia — that strikes more than 3,000 children and young adults in the U.S. each year. While most survive, about 15 percent relapse despite today's best treatments, and their prognosis is bleak.

In a key study of 63 advanced patients, 83 percent went into remission. It's not clear how long that benefit lasts: Some patients did relapse months later. The others still are being tracked to see how they fare long-term.