A recently approved agent, burosumab (Crysvita), was better than placebo across a range of efficacy outcomes for 14 predefined subgroups of adults with X-linked hypophosphatemia (XLH), new research shows.

The authors analyzed data from the initial 24-week randomized blinded phase of the pivotal phase 3 trial that led to regulatory approval of this drug in the United States in 2018 for XLH, a rare form of rickets characterized by low serum phosphorus levels, skeletal defects, pain, and stiffness.

As in the main analysis, in the subgroups, among patients who received burosumab, serum phosphorus levels were improved, and outcomes were better on the following measures: Western Ontario and McMaster Universities Arthritis Index (WOMAC) stiffness scale, the WOMAC physical function measure, and the Brief Pain Inventory (BPI), which were the main efficacy outcomes. Improvements were seen for many other outcomes as well.

Maria-Luisa Brandi, MD, Careggi University Hospital, Florence, Italy, presented the new subanalysis during the virtual American Society of Bone and Mineral Research (ASBMR) 2020 annual meeting.

The subgroup results were consistent with the overall trial findings, "showing a favorable direction of effect of burosumab relative to placebo" except for results in patients recruited in Asia and non-White patients; those results were considered inconclusive because there were too few participants in those categories, she told Medscape Medical News,.

Lorenz Hofbauer, MD, scientific chair of the ASBMR meeting, said that the take-away message is that the drug "works to reduce pain and disability" in adults with XLH with more severe/less severe symptoms, and "it provides new hope for many patients suffering from this disease," he told Medscape Medical News.

Burosemab also appears superior to what has previously been considered standard therapy for XLH, phosphate/calcitriol, the experts say.

"The disease prevalence is 1 to 9 in a million," Brandi said. "Undiagnosed adults are treated by the doctor that makes the diagnosis, usually a nephrologist or a rheumatologist or a bone doctor; this depends on the prevalent complications in a given patient. The endocrinologist who treats this patient is the one expert in bone disorders."

Hofbauer noted, however, that "[r]are is relative. If you run a bone clinic, you will see four to five patients with XLH; if you are a regional center, 20 to 30 patients. People with rare disease travel more than 1000 miles to see experts."

The US Food and Drug Administration approved burosumab for use in children and adults with XLH 2 years ago. The European Medicines Agency (EMA) approved it for use in children.

The drug is expected to be approved by the EMA for adults with XLH some time this year, said Hofbauer, who is from Dresden Technical University, Dresden, Germany.

Burosumab is a "game changer" with respect to previous treatments, he stressed.

This study is one of the top five clinical abstracts of the ASBMR meeting, which are selected on the basis of "scientific content/novelty, making a difference in clinical practice," Hofbauer explained. He noted that "new drugs that work are always in the top ranks."

Craig Munns, PhD, who was senior author of a recent review about burosumab, agrees.

"Burosumab is transformative, as it is a paradigm shift in the way we manage XLH," he told Medscape Medical News.

"Standard therapy for children is with oral phosphate and calcitriol, and many adults do not receive any therapy," said Munns, from the University of Sydney, Sydney, Australia.

"Phosphate and calcitriol need to be taken multiple times per day, is an incomplete therapy, and has many complications. Burosumab offers a 2-weekly (children) or 4-weekly (adult) dosing regime with superior outcomes compared to no treatment or phosphate/calcitriol," he emphasized.

"Burosumab is an anti-FGF-23 [antifibroblast growth factor-23] antibody for a rare genetic disease, XLH, in which the gene for PHEX is defective," Hofbauer explained.

"PHEX is an enzyme that clears FGF-23; if it does not work, then FGF-23 accumulates in the body and causes phosphate wasting with wide consequences for bone, muscle, and joints. Burosumab is a smart approach, since it blocks these excessive FGF-23 effects."

Children with XLH have rickets, deformities in the lower skeleton, and short stature, Brandi noted, whereas adults have fractures, pseudofractures, enthesopathy (calcification of joint capsule, tendon insertions, and ligaments), pain, stiffness, and impaired physical function.

However, "treatment with oral phosphate and vitamin D is associated with nephrocalcinosis and hyperparathyroidism," she said.

In the phase 3 trial, 134 adults (aged 18 to 65 years) with XLH were randomly assigned in a double-blind manner to receive either burosumab or placebo for 24 weeks, followed by 24 weeks of open-label burosumab. The patients' serum phosphorus levels were <2.5 mg/dL, and they were experiencing measurable bone/joint pain.

Baseline characteristics were similar for the patients who received placebo (66) and those who received burosumab (68). The mean age of the patients was 40 years; 65% were women; and 81% were White.

The current exploratory analysis examined efficacy outcomes in patients grouped according to the following factors and characteristics: sex; age (41 years or >41 years); race (non-White, White); region (Asia, North America/Europe); baseline WOMAC pain score; WOMAC total pain; WOMAC stiffness; WOMAC physical function; BPI worst pain; BPI average pain; opioid use; pain medication use; active fractures and pseudofractures; and 6-minute walking test distance.

The efficacy outcomes were as follows: serum phosphorus level (primary outcome), BPI worst pain, WOMAC stiffness, and WOMAC physical function (key secondary outcomes); and WOMAC pain, WOMAC total score, BPI average pain, BPI pain interference, BPI worst fatigue, BPI global score, patient global impression (PGI), and 6-minute walking distance.

In the overall cohort, at 24 weeks, in comparison with patients who received placebo, patients who received burosumab had favorable responses with respect to serum phosphorus level, WOMAC stiffness (P =. 012),WOMAC physical function (P = .048), and BPI worst pain (P = .092, not significant), as well as significant improvements in WOMAC total score and the 6-minute walk test. There were nonsignificant improvements in WOMAC pain and BPI average pain.

In the subgroup analysis, burosumab was superior to placebo for the primary outcome (serum phosphorus) in all subgroups. It was also superior to placebo for the key secondary outcomes (worst pain, stiffness, and physical function) across all subgroups except for patients from Asia (18 patients) and non-White patients (26).

The study was funded by Kyowa Kirin in partnership with Ultragenyx. Brandi receives consultancy and speaker fees as well as research grants from Kyowa Kirin and other pharmaceutical companies. Munns has received research funding from Kyowa Kirin.

The American Society of Bone and Mineral Research (ASBMR) 2020 Annual Meeting Virtual Event: Presented September 12, 2020.

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Burosumab Is a 'Game Changer,' Effective in All Subgroups of XLH - Medscape

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