The Food and Drug Administration on Friday approved the first gene therapy for a type of spinal muscular atrophy, a lifesaving treatment for infants that will also be the most expensive drug in the world.

Novartis is pricing Zolgensma at $2.125 million, or an annualized cost of $425,000 per year for five years, the company said.

Launching Zolgensma will be a big test for Novartis and CEO Vas Narasimhan, now two years on the job. Shareholders expect the gene therapy to deliver blockbuster sales to justify the $8.7 billion that Novartis spent to acquire it last year.

To achieve commercial success, Novartis must persuade doctors who treat SMA patients that the muscle-preserving benefits from a one-time injection of Zolgensma will be durable. Complex payment and insurance reimbursement arrangements required for expensive gene therapies need to be handled deftly.

Novartis is likely to face backlash from critics who believe charging millions of dollars for any medicine – no matter how effective – renders it unaffordable for a healthcare system already under financial stress.

There’s also competition. Spinraza, approved in late 2016 and sold by Biogen, has already been used to successfully treat thousands of patients with severe and milder forms of SMA. The drug requires regular spinal infusions costing $750,000 in the first year and $375,000 annually thereafter, for life. Sales last year totaled $1.7 billion. Zolgensma may be more convenient than Spinraza, but Roche is developing a daily pill for SMA called risdiplam that could reach the market in 2020.

The FDA approved Zolgensma to treat children under 2 diagnosed with SMA, regardless of genetic mutation. In its pivotal clinical trial and an ongoing clinical trial, a majority of the infants and young children injected once with Zolgensma remained alive, could breathe on their own, and showed improvements in motor milestones like being able to sit up without support.

Zolgensma “is markedly better than any other therapy out there, particularly in the clinical trials of type 1 that we’ve released,” Narasimhan told STAT in a recent interview. “Clearly, parents will know right away that this is a medicine that performs extremely, extremely well in these infants and has this kind of marked effect on their well-being.”

In its announcement, acting FDA Commissioner Ned Sharpless said the approval marks “another milestone in the transformational power of gene and cell therapies to treat a wide range of diseases.”