Extended Crysvita Use Helps Keep Phosphate Levels Normal and Heal Fractures in Adults with XLH, Trial Data Show

Extended use of Crysvita (burosumab) for up to 48 weeks maintains normal blood levels of phosphate, relieves pain and stiffness, and aids the healing of bone fractures and overall physical condition of adults with X-linked hypophosphatemia (XLH), an extension study of a Phase 3 trial shows.

No new safety concerns were seen with prolonged use of the therapy.

The study, “Continued Beneficial Effects of Burosumab in Adults with X-Linked Hypophosphatemia: Results from a 24-Week Treatment Continuation Period After a 24-Week Double-Blind Placebo-Controlled Period,” was published in the journal Calcified Tissue International.

Crysvita is the only targeted therapy approved for XLH, a hereditary disorder characterized by excessive loss of phosphate in urine leading to low levels of phosphate in the blood.

The medicine, marketed by Ultragenyx Pharmaceutical and Kyowa Kirin and given as an under-the-skin (subcutaneous) injection, is an antibody that binds to FGF23 (fibroblast growth factor 23), a hormone produced in excessive amounts in people with XLH. By blocking FGF23, Crysvita works to increase phosphate reabsorption from the kidney and boost vitamin D production.

Adults with XLH often show a greater risk of fractures due to progressive softening of the bone, or osteomalacia.

In a pivotal, placebo-controlled Phase 3 trial (NCT02526160) in 134 adults with XLH (mean age, 40), 24 weeks of treatment with Crysvita every four weeks rapidly normalized blood phosphate levels and increased the likelihood by 17-fold of healing bone fractures compared to a placebo. It also led to a significant reduction in stiffness and pain, as well as better physical function.

At the end of this 24-week period, participants joined an open-label study where all were treated with Crysvita for another 24 weeks. This was followed by an ongoing extension study, where the therapy will be given be for up to two more years to evaluate long-term efficacy and safety.

Researchers analyzed Crysvita’s efficacy in the first open-label study (weeks 24 to 48), and available safety results from 24 to 74 weeks.

Results revealed that Crysvita continued to maintain normal blood phosphorus levels in 83.8% of patients on this treatment since the start of the placebo-controlled trial, and normalized these levels in 89.4% of patients who switched from placebo.

Among Crysvita-only users, 63.1% of the fractures or pseudofractures (small linear bone cracks) present at the study’s beginning healed completely by week 48, compared to 35.2% in those who had been on placebo.

In both groups, the therapy was associated with clinically significant and lasting reductions in stiffness and pain, as well as better physical function, all reported by patients. Use of Crysvita also led to greater total distances they were able to walk in six minutes.

No new safety concerns were found in the extension. Adverse events were similar with Crysvita and placebo. No treatment-related serious or life-threatening side effects were observed.

Elevated blood phosphorus levels was observed in 7% of people on Crysvita. This rise was not associated with clinical manifestations and resolved after dose reduction.

“These data demonstrate that in participants with XLH, continued treatment with burosumab [Crysvita] is well tolerated and leads to sustained correction of serum phosphorus levels, continued healing of fractures and pseudofractures, and sustained improvement in key musculoskeletal impairments,” the researchers wrote.

“Despite long-standing physical impairments from a lifetime of XLH, participants experienced clinically meaningful benefits that began shortly after burosumab [Crysvita] was initiated and persisted with continued administration, with an overall positive benefit-risk profile through at least 48 weeks of treatment,” they added.

Crysvita is approved in the U.S. for patients ages 6 months and older, and in the E.U. for pediatric patients 1 year and older with signs of bone disease on X-rays.