U.S. regulators have approved the most expensive medicine ever, a therapy meant to cure a disorder that rapidly destroys a baby's muscle control and kills most within a couple years.

The one-time gene therapy developed by Novartis, Zolgensma, will cost $2.125 million. It treats a rare condition called spinal muscular atrophy, or SMA, which strikes about 400 babies born in the U.S. each year.

The therapy, given in a one-hour infusion, was approved for children under age 2 and will be available within two weeks.

Out-of-pocket costs for patients will vary based on insurance coverage.

Novartis said it will let insurers make payments over five years and will give partial rebates if Zolgensma doesn't work. Company executives said that financial assistance will be available.