Burosumab Shows Dramatic Mobility Gains in Children with Severe Fibrous Dysplasia
Key Insights
A phase 2 clinical trial of burosumab in 12 patients with fibrous dysplasia (search) successfully restored phosphate levels to the mid-to-upper normal range in all participants by week 48.
Two severely affected children experienced transformational mobility improvements, with one progressing from full-time wheelchair use to independent walking and another achieving walker-assisted ambulation after never walking independently.
The monoclonal antibody treatment targeting FGF23 (search) was well-tolerated and showed substantial reductions in alkaline phosphatase levels, a marker of disease activity, by 49% at week 48.
A groundbreaking phase 2 clinical trial has demonstrated that burosumab, a monoclonal antibody targeting fibroblast growth factor-23 (search) (FGF23 (search)), can restore mobility in children with severe fibrous dysplasia (search) while safely normalizing phosphate levels. The study, published in Bone Research, represents the first investigation of burosumab in fibrous dysplasia patients and provides compelling evidence for a new therapeutic approach to this rare skeletal disorder.
Trial Design and Patient Population
The open-label study enrolled 12 participants, including seven children and five adults, all with severe disease burden and hypophosphatemia (search). Led by Dr. Alison M. Boyce from the National Institute of Dental and Craniofacial Research (search) at NIH, the trial administered burosumab for 48 weeks while monitoring phosphate metabolism, bone turnover markers, and functional outcomes.
At baseline, participants demonstrated extensive skeletal involvement with a median Skeletal Burden Score of 64.5 out of 75, indicating near-complete skeletal involvement. Eight of the 12 participants required assistive devices including wheelchairs, walkers, or crutches for mobility, reflecting the severe disability associated with this condition.
Biochemical Outcomes Exceed Expectations
The primary endpoint was achieved in all participants, with serum phosphate levels increasing from a median Z-score of -2.88 at baseline to 0.22 at week 48, successfully reaching the target mid-to-upper normal range. This represents a significant improvement over traditional oral phosphate supplementation, which typically fails to normalize phosphate levels.
Treatment also improved phosphate retention in the kidneys, with TmP/GFR increasing from 0.76 to 1.29 mmol/L, and raised levels of active vitamin D from 20.5 to 44 ng/mL. Most notably, alkaline phosphatase levels, a marker associated with abnormal bone turnover and disease activity, declined substantially by 49% (364 U/L) from baseline to week 48.
"The alkaline phosphatase reductions exceeded results from trials in XLH and TIO, which ranged between -20% and -33%," the researchers noted, suggesting that hypophosphatemia (search) may contribute more significantly to disease burden in fibrous dysplasia (search) than previously recognized.
Transformational Mobility Improvements
The most striking results occurred in pediatric participants, where two children experienced dramatic functional gains that exceeded the expected disease course. BUR03, a 13-year-old girl who had been a full-time wheelchair user since age 11, progressed to full-time independent ambulation by week 48, no longer requiring assistive devices for any activities.
BUR06, a 6-year-old girl who had never walked independently, achieved walker-assisted ambulation up to 50 feet by week 48. At baseline, she was unable to perform any weight-bearing activities involving her lower extremities.
"Children with severe fibrous dysplasia (search) often experience progressive mobility loss during critical developmental years," Dr. Boyce explained. "Seeing some patients regain meaningful movement and independence provides strong motivation to continue advancing therapies that directly address the biological drivers of disease."
Patient-Reported Outcomes Show Promise
Pediatric participants demonstrated consistent trends toward improvement across all PROMIS questionnaire domains, including reductions in Pain Intensity (P = 0.06), Pain Interference (P = 0.06), and Fatigue (P = 0.15), along with increased Mobility (P = 0.12). Adult participants showed disparate results with no clear trends toward improvement, suggesting that early intervention may be critical for optimal outcomes.
A qualitative Activities of Daily Living assessment revealed progressive improvements, with an increasing proportion of participants reporting "much" or "very much" improvement in self-identified daily activities between weeks 24 and 48.
Safety Profile Supports Clinical Use
The treatment demonstrated a favorable safety profile, with nine of 12 participants experiencing at least one treatment-related adverse event, all classified as mild. Eight episodes of hyperphosphatemia occurred in six participants, easily managed with protocol-specified dose reductions. Three participants experienced 25 injection site reactions, all mild and self-resolving.
Importantly, detailed PET/CT imaging and lesion biopsies from four adult participants showed no evidence that burosumab accelerated lesion growth or increased abnormal tissue activity, addressing a major safety concern regarding FGF23 (search)-targeting therapies in fibrous dysplasia (search).
Clinical Implications and Future Directions
The study addresses a critical treatment gap for fibrous dysplasia (search) patients, who face unique challenges compared to other FGF23 (search) excess disorders. Unlike X-linked hypophosphatemia (search) (XLH) and tumor-induced osteomalacia (search) (TIO), where complications develop in normally-formed bone, fibrous dysplasia patients have inherently dysplastic and poorly mineralized bone that is particularly vulnerable to hypophosphatemia (search).
"Our findings demonstrate that targeting phosphate levels in the mid to upper normal range can be both safe and clinically meaningful for patients with fibrous dysplasia (search)," Dr. Boyce stated. "The mobility improvements observed in some children suggest that earlier correction of phosphate imbalance may help reduce long-term disability."
The research establishes a higher therapeutic target for phosphate levels than typically used in other FGF23 (search) disorders, where treatment aims for low-normal levels. This approach appears justified given the increased vulnerability of dysplastic bone tissue and the higher skeletal-related morbidity in fibrous dysplasia (search) patients.
Broader Impact on Rare Disease Treatment
The findings may have implications beyond fibrous dysplasia (search), supporting the concept that maintaining optimal phosphate balance could improve outcomes in other disorders driven by excessive FGF23 (search) activity. The results also demonstrate the potential for targeted biologic therapies to become a standard approach for managing rare skeletal diseases associated with phosphate wasting.
For patients and families, burosumab may offer a more effective alternative to traditional phosphate supplementation, potentially improving quality of life while reducing treatment burden. The dramatic mobility gains observed in severely affected children provide hope for preventing lifelong disabilities when treatment is initiated early in the disease course.
This phase 2 trial establishes burosumab as a promising therapeutic option for fibrous dysplasia (search) patients with hypophosphatemia (search), offering both biochemical normalization and the potential for meaningful functional improvements, particularly when initiated during childhood.