Even the FDA called the approval "historic."

"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.

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 other tumors. The therapy is made by harvesting patients' white blood cells and rewiring them to home in on tumors. Novartis's product is the first CAR-T therapy to come before the FDA, leading a pack of novel treatments that promise to change the standard of care for certain aggressive blood cancers.

"This is a brand new way of treating cancer," said Dr. Stephan Grupp of Children's Hospital of Philadelphia quoted by the AP, 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."