After decades without many options for treating their disease, sickle cell patients now have two cutting-edge therapies with the potential to cure the illness. On Dec. 8, the U.S. Food and Drug Administration approved two gene therapies to treat sickle cell: Lyfgenia, from Bluebird Bio, and Casgevy, from Vertex Pharmaceuticals and CRISPR Therapeutics.

Both new treatments work by helping patients' bodies make healthier and more durable forms of hemoglobin, hopefully for the rest of their lives. Both also require nearly a year of procedures, including a bone marrow transplant and high-dose chemotherapy to remove the diseased cells and make room for the modified ones that can produce normal hemoglobin. Studies show that nearly all patients receiving either treatment are able to avoid the disease's characteristic periodic episodes of extreme pain that occur when sickled cells clump together and block small blood vessels,

Casgevy is especially groundbreaking, as it is the first CRISPR-based treatment to be approved in the U.S., and could open the door for other such therapies for different genetic diseases. It works kind of like "removing the stop sign and allowing traffic—in this case, [healthy] fetal hemoglobin—to go forward down the road,” says Dr. Sharl Azar, medical director of the comprehensive sickle cell disease treatment center at Massachusetts General Hospital.

The FDA will require both companies to continue studying patients for 15 years to track any long term side effects.

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