The US Food and Drug Administration on Monday approved the first therapy for a rare and devastating condition called metachromatic leukodystrophy, a condition that typically kills affected children before they turn 7.

The one-time treatment, called Lenmeldy, takes stem cells from someone with MLD and uses a harmless virus to insert working copies of a faulty gene. The repaired cells are then infused back to the patient, where they begin to produce an enzyme that’s lacking in children who have the disease.

The personalized treatment and small number of cases contribute to what’s expected to be a high sticker price: Analysts expect that it could list for as much as $3.9 million, which could make it the world’s most expensive drug.

MLD is an inherited disorder, and children born with it lack an enzyme needed break down fatty substances called sulfatides. The build-up of these fatty materials eventually becomes toxic to nerves, leading to the progressive loss of movement and thinking.

Babies with MLD develop normally for a time and but then typically begin to lose the ability to walk and talk around age 2. The disease advances rapidly, causing children to deteriorate into a vegetative state.

“We are over the moon regarding what this means for other families,” said Kendra Riley, 41, of Phoenix, who has two children with MLD.

For their family, the FDA’s action this week was bittersweet.

Riley’s 5-year-old daughter, Olivia, is in hospice after being diagnosed with MLD as a toddler.

Riley says the first clues that something was wrong came when Olivia was around the age of 2. She began to have trouble walking, and her head started to regularly tilt to the side when she would watch TV.

“We thought we just needed some physical therapy,” Riley said.

“Then the irises of her eyes started vibrating. That’s when we knew something else was going on.”

By the time doctors diagnosed Olivia with MLD, it was too late for the gene therapy to help. But knowing that the condition is inherited, they were able to get their younger daughter, Keira, tested and diagnosed.

Keira, now 4, was the 32nd child in the world to get the new therapy, which is most effective before children show symptoms.

The treatment wasn’t yet available in the United States, so the family crowdsourced donations to temporarily relocate to Italy in 2020 to get the therapy. It cost them about $500,000 to live abroad and pay for Keira’s medical care, even though the company provided the gene therapy for free.

“Having this FDA-approved therapy means that if a child does get diagnosed before symptom onset, they have a chance at normal life,” Riley said.

“She is doing amazing. Zero symptoms,” she said of Keira. “You would never know.”

‘Exciting’ drug development

A spokesperson for Orchard Therapeutics, the company that makes Lenmeldy, said the company would announce the treatment’s list price in the coming days. The cost could top Hemgenix, a one-time treatment for hemophilia B, which according to the website Drugs.com is now the most expensive medication in the US at $3.5 million a dose.

Lenmeldy is already approved in Europe, where it’s called Libmeldy; it has a UK list price of 2.8 million pounds, or about $3.5 million US.

The Boston nonprofit Institute for Clinical and Economic Review, or ICER, which evaluates the cost effectiveness of new drugs, published a final report on Lenmeldy last fall and estimated that the cost of the therapy would match its expected benefits to patients if it was priced between $2.3 million and $3.9 million.

Experts agree that even such hefty price tag would be worth its results, as most affected children who get the treatment appear to have long-lasting benefits. Children with the disease who aren’t treated usually die within five years of their diagnosis; the oldest patient to have the therapy is now 12 years old and appears to be developing normally.

“It’s taking a child who would have had a miserable short life and likely giving them a normal life. And that’s worth a lot of money,” said Dr. David Rind, the chief medical officer for ICER. “This is one of the more exciting drugs that we’ve looked at.”

Even with a top tier price, the treatment will not be a blockbuster, Rind noted, because the disease is so rare. Some gene therapies approved for rare diseases have disappeared because the companies that made them couldn’t make enough money to stay in business. Orchard Therapeutics has already struggled to make money on a previous gene therapy it developed, leaving patients hanging.

Rind says one of the reasons ICER reviewed this drug was that it’s for an “ultra-rare disease” and yet the company thought it could offer it at a cost-effective price.

“If they come in below $3.9 million, that actually would be a nice sign that, even for the rarest of the rare diseases, you can find a drug company that that believes it can make therapy at a price that isn’t unreasonable for the benefit,” he said.

New possibilities for patients

This latest approval comes as the FDA is working to clear a backlog of cell and gene therapies that have been waiting for its attention. The agency has hired more staff to review the new treatments and set up a new superoffice, the Office of Therapeutic Products, to oversee their regulation.

Lenmeldy has been available in Europe since 2020, but it took four more years to bring it to the US.

Dr. Nicole Verdun, who was hired to head the new FDA initiative last year, said Lenmeldy’s approval is a sign that the agency is moving in the right direction.

“MLD is a devastating disease that profoundly affects the quality of life of patients and their families,” Verdun said.

“This approval represents important progress in the advancement and availability of effective treatments, including gene therapies, for rare diseases.”

Orchard Therapeutics said the advance had been decades in the making.

“The FDA approval of Lenmeldy opens up tremendous new possibilities for children in the U.S. with early-onset MLD who previously had no treatment options beyond supportive and end-of-life care,” Dr. Bobby Gaspar, co-founder and chief executive officer of Orchard Therapeutics, said in a news release.

Dr. Barbara Burton, who is an attending physician, of genetics, genomics and metabolism at the Ann & Robert H. Lurie Children’s Hospital of Chicago, said it was a watershed moment for doctors, too.

“For too long, my colleagues and I have consoled families at their most vulnerable times — usually following an arduous diagnostic odyssey, coping with a dire prognosis and being told there were no treatments, and then having to watch their young child slip away.

“With this approval, we are now one significant step closer to ensuring future generations of children, families and healthcare professionals no longer need to experience first-hand the terrible manifestations this disease has on untreated patients,” she said.

Riley said Olivia is stable for now, but she lives in fear that Keira could bring germs home.

“The common cold could kill her,” Riley said. “So every time her sister has come home with a cold from school, that could be potentially something that takes her out.”

Olivia can’t walk or talk. She’s fed through a tube. Riley says she requires round-the-clock care. They have physical and occupational therapists come to the house each week “to keep her mind moving. She does understand what’s going on.”

She said that as they have watched Olivia slip away, they’ve come to understand that her life has had profound purpose. But they say they hope that the approval of Lenmeldy means that no other families with MLD will have to lose one child to save another.

“We’ve always called Livvy ‘Keira’s guardian angel on Earth,’ ” Riley said. “She’s here for a reason.”