AstraZeneca's Strensiq Sequel Aced the Kids' Test. Then Came the Adults.

Where the Drug Crushed It

The MULBERRY trial tested efzimfotase alfa against placebo in treatment-naive children aged 2 to 11. The primary endpoint measured bone health using something called an RGI-C score (essentially a radiologist's assessment of whether bones are getting better). Kids on the drug scored a median 1.67 at week 25. Kids on placebo scored zero. The p-value was 0.0003, which in clinical trial terms is about as emphatic as it gets.

Secondary endpoints told the same story. Rickets severity improved significantly. Physical function scores jumped by a median of 9 points. Kids walked about 34.5 meters farther on the six-minute walk test. And the safety profile looked clean, with injection-site reactions running about five times lower than what Strensiq showed in its original trials.

Then there was CHESTNUT, which tested whether kids already on Strensiq could safely switch. The answer was a clear yes. Bone health held steady after the transition, and the safety profile was comparable. This is critical because it means AstraZeneca can eventually migrate its existing pediatric patients to the new drug without losing ground.

For children with HPP, efzimfotase alfa looks like a legitimate upgrade: similar efficacy, better convenience, fewer injection-site reactions. If this were the whole story, you'd call it a slam dunk.

The Adult Problem

It's not the whole story.

The HICKORY trial enrolled adolescents (12 and up) and adults with HPP, and it was supposed to be AstraZeneca's ticket to a much larger market. The primary endpoint was the six-minute walk test (6MWT), which measures how far a patient can walk in six minutes. It's the standard functional yardstick for adult bone diseases.

Efzimfotase alfa failed to beat placebo on that measure at week 25. Full stop.

Now, AstraZeneca is doing what pharma companies always do when a primary endpoint misses: pointing to the secondary data. And to be fair, there are some interesting signals. In a prespecified subgroup of adults whose HPP started in childhood, the drug showed nominally significant improvements in walking distance. Pain scores improved. Fatigue scores improved. Patients who crossed over from placebo to active treatment during the open-label extension showed continued gains through week 48.

But "nominally significant" and "subgroup analysis" are phrases that make regulators reach for their reading glasses. The FDA cares most about the primary endpoint in the full enrolled population, and that number came up empty.

What This Means for AstraZeneca's Rare Disease Empire

The bull case for efzimfotase alfa was never just about protecting the pediatric franchise. It was about expansion. Adults with HPP represent a huge untreated population: many are misdiagnosed with osteoporosis, undertreated, or simply unaware of what's causing their chronic pain and fractures. Only about 6,200 patients carry an HPP diagnosis in the seven major markets. The real number is almost certainly much higher.

AstraZeneca had projected peak annual revenues of $3 to $5 billion for efzimfotase alfa, a figure that baked in substantial adult uptake and global geographic expansion. With the adult primary endpoint missed, those projections need a haircut.

Analysts are framing it as "pediatric thesis intact, adult upside de-rated." The pediatric approval path looks straightforward, and the switch data from CHESTNUT should let AstraZeneca smoothly transition its existing Strensiq base. But the dream of a broad adult label, the kind that would have unlocked the real growth, now hinges on whether regulators will accept secondary endpoints and subgroup signals as enough.

That's a genuine coin flip.

The Bigger Picture

AstraZeneca isn't putting all its rare bone eggs in one basket. Last year, the company closed a $1.05 billion acquisition of Amolyt Pharma, adding eneboparatide (a therapy for hypoparathyroidism, another rare bone and mineral disorder) to its portfolio. That drug's Phase 3 trial met its primary endpoint, giving AstraZeneca a cleaner growth story in a related space.

The rare disease unit also continues to generate strong returns from Ultomiris and Soliris in complement-mediated diseases, so the overall business isn't riding on efzimfotase alfa alone.

Still, for HPP patients, the mixed results carry real consequences. Children will likely get a better, more convenient therapy within a few years. That's genuinely good news. But for the thousands of adults struggling with undiagnosed or undertreated HPP, who inject themselves six times a week or get no disease-modifying treatment at all, the wait continues.

AstraZeneca says it plans to present full data at upcoming medical meetings and submit to regulators based on the "totality of data." The company is betting that the combination of pediatric wins, clean safety, and supportive adult subgroup signals will be enough to earn a meaningful label.

It's a reasonable bet. But in drug development, reasonable bets miss all the time.

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