WASHINGTON (AP) — Opening a new era in cancer care, U.S. health officials on Wednesday approved a breakthrough treatment that genetically engineers patients' own blood cells into an army of assassins to seek and destroy childhood leukemia.

The Food and Drug Administration called the approval historic, the first gene therapy to hit the U.S. market. Made from scratch for every patient, it's one of a wave of "living drugs" under development to fight additional blood cancers and other tumors, too.

Novartis Pharmaceuticals set the price for its one-time infusion of so-called "CAR-T cells" at $475,0...