A single injected dose will provide lifelong treatment
For children born with hypophosphatasia, a rare disorder that undermines the very architecture of bone, survival has long come at a cost measured in needles — three to six injections each week, indefinitely. Researchers at Sanford Burnham Prebys have now demonstrated in preclinical trials that a single dose of an engineered gene therapy, AAV8-TNAP-D10, may one day replace that burden entirely, delivering a lifelong supply of the missing enzyme from within the body itself. Published in January 2025, the study marks a careful step toward human trials, while also opening a deeper question: as medicine extends the lives of those it once could not save, what new responsibilities does it inherit for the decades that follow?
- Children kept alive by enzyme replacement therapy face an unrelenting schedule of injections that some patients ultimately abandon due to reactions and fatigue.
- A single engineered virus, AAV8-TNAP-D10, has shown in mice that it can restore the missing enzyme naturally — potentially replacing years of weekly treatments with one lifetime dose.
- Researchers identified an optimal dosing window and uncovered an unexpected sex-based difference in how the therapy distributes through the body, a finding that will shape the design of future human trials.
- The team is now actively seeking a commercial partner to advance the therapy into clinical trials, with international collaborators from Nippon Medical School already on board.
- Even as the gene therapy advances, its lead researcher is turning toward a harder horizon: enzyme replacement has saved bones and teeth, but TNAP operates throughout the entire body, and the long-term consequences of its absence remain largely uncharted.
For a decade, children born with hypophosphatasia have survived on a demanding regimen — injections three to six times a week, every week, for life. The enzyme replacement therapy, asfotase alfa, has been genuinely miraculous, rescuing children who would otherwise have died in infancy. But the treatment's weight is real: some patients develop reactions and stop altogether.
Hypophosphatasia is a rare inherited disorder affecting roughly one in 100,000 newborns, ranging from mild fracture risk in adults to life-threatening skeletal failure in infants. It is caused by the absence of a single enzyme, TNAP. Replacing it has, until now, meant those relentless injections.
Researchers at Sanford Burnham Prebys, led by José Luis Millán, have engineered a viral vector — AAV8-TNAP-D10 — incapable of causing disease but able to deliver the gene for TNAP directly into the body. The goal is a single injection that works for life. In January 2025, the team published preclinical findings in the Journal of Bone and Mineral Research, establishing an effective dosing range and safety profile across different disease forms and sexes.
One unexpected finding: adult female mice responded to the therapy more strongly and at lower doses, with the enzyme concentrating in limb muscle rather than the liver as seen in males. Colleagues at a Toronto bone research conference told Millán this sex-based pattern appears in mice but not in primates or humans — something the team will monitor but does not anticipate replicating in trials.
With the preclinical foundation now established, Millán and his collaborators at Nippon Medical School are seeking a commercial partner to move toward human trials. But his attention is already reaching further. Enzyme replacement has given patients long lives it once could not offer — yet it addresses only bones and teeth. TNAP is expressed across the brain, liver, kidney, and immune system, and what complications may emerge over decades of survival remains an open and urgent question.
For a decade, children born with hypophosphatasia have faced a grueling treatment schedule: injections three to six times each week, indefinitely. The enzyme replacement therapy—asfotase alfa—has been nothing short of miraculous. Kids who would have died in infancy now grow into adulthood. But the needle marks accumulate. Some patients develop reactions to the frequent injections and stop treatment altogether. The burden, for all its life-saving power, remains a burden.
Hypophosphatasia is a rare inherited disorder that sabotages bone development and causes premature tooth loss. It ranges from mild—adults at higher risk of fractures—to severe and life-threatening, striking roughly one in every 100,000 newborns. The condition stems from a missing enzyme called tissue-nonspecific alkaline phosphatase, or TNAP. For the past ten years, the only way to replace it has been through those weekly injections.
Now researchers at Sanford Burnham Prebys, led by José Luis Millán, are pursuing a different path. They have engineered a virus—AAV8-TNAP-D10—that cannot cause disease but can deliver the gene needed to produce TNAP naturally inside the body. The vision is stark and simple: a single injection that works for life. "We believe the next evolution in treating HPP will be a gene therapy in which a single injected dose will provide a lifelong treatment for patients," Millán said.
In January 2025, Millán's team published new preclinical evidence in the Journal of Bone and Mineral Research. The study was designed to answer the questions that clinical trials will demand: What dose works? What dose causes harm? Does it matter whether the patient is male or female, young or old? The researchers tested the gene therapy at different dosages in male and female mice, in both early-onset and late-onset forms of the disease. They were hunting for the sweet spot—efficacy without side effects like ectopic calcifications, abnormal mineral deposits in soft organs.
They found it. The data revealed an optimal dosing range and, unexpectedly, a sex-based difference in how the treatment worked. In adult female mice with late-onset HPP, the gene therapy was more effective and required a lower dose. The enzyme activity also distributed differently: in females, it concentrated in the limb muscle where the injection was given; in males, it accumulated in the liver. Millán presented this finding at a bone research conference in Toronto, and physicians told him this sex-based pattern occurs in mice but not in primates or humans. The team now knows to watch for it in future trials, though they don't expect to see it.
The preclinical work is solid enough that Millán and his collaborators—Takashi Shimada and Koichi Miyake of Nippon Medical School in Japan—are now seeking a commercial partner to move the therapy into human trials. But Millán's mind is already turning to a harder problem. The enzyme replacement therapy has given patients years they would never have had. Yet it only fixes the bones and teeth. TNAP is expressed throughout the body—in the brain, liver, kidney, immune system. "We have patients now that will have long lives thanks to enzyme replacement and future therapies, but we're only able to fix the skeletal mineralization," Millán said. He is focused on understanding what long-term complications might emerge in the decades ahead, so medicine can be ready to meet them.
Notable Quotes
Some patients have reactions from frequent injections and discontinue treatment. That has motivated us to find the next step in treating this disease.— José Luis Millán, professor at Sanford Burnham Prebys
We need to anticipate long-term problems before they happen so we can be prepared to help patients with HPP throughout their lives.— José Luis Millán
The Hearth Conversation Another angle on the story
Why does it matter that this is a one-time treatment instead of weekly injections?
Because compliance breaks down. Some patients develop reactions to frequent needles and simply stop. A single dose removes that barrier entirely—you get the therapy and you're done.
The study found that female mice responded better than males. Should patients worry about that?
Not really. Millán's colleagues told him this sex difference exists in mice but doesn't show up in primates or humans. It's a quirk of mouse biology. But now the people running trials will be watching for it, just in case.
What's the bigger concern Millán mentioned at the end?
That we're only treating the bones and teeth. The missing enzyme works everywhere—brain, liver, kidney, immune system. We don't know what problems might surface in ten or twenty years. He wants to anticipate them now, before patients hit those complications.
So this gene therapy isn't the final answer?
It's a major step forward. But for patients who will now live full lives, there are unknowns ahead. That's what's keeping Millán awake.