Long-term Burosumab Administration Is Safe and Effective in Adults With X-linked Hypophosphatemia

Recent advances in fibroblast growth factor 23-related hypophosphatemic disorders

PURPOSE OF REVIEW

Fibroblast growth factor 23 (FGF23) is a hormone to reduce blood phosphate concentration. Excessive actions of FGF23 induce FGF23-related hypophosphatemic disorders, such as X-linked hypophosphatemic rickets (XLH) and tumor-induced osteomalacia (TIO). We will summarize recent advances in the diagnosis and treatment of FGF23-related hypophosphatemic disorders.

RECENT FINDINGS

The measurement of blood FGF23 is useful to make a diagnosis of FGF23-related hypophosphatemic disorders. It was reported that many patients with FGF23-related hypophosphatemic disorders, especially TIO, were misdiagnosed, therefore, it is necessary to enhance the awareness of these diseases. A novel method to inhibit excessive actions of FGF23 by a human monoclonal antibody for FGF23, burosumab, has been approved in several countries. In more long-term observation than clinical trials, burosumab has also been shown to improve biochemical abnormalities and symptoms of rickets/osteomalacia. Following these advances, several registries and consensus recommendations on FGF23-related hypophosphatemic disorders, especially XLH, have been established in each country or region.

SUMMARY

Further long-term effects of burosumab and the precise mechanism of FGF23 overproduction in patients with FGF23-related hypophosphatemic disorders need to be clarified in the future studies.

Non-Classical Effects of FGF23: Molecular and Clinical Features

This article reviews the role of fibroblast growth factor 23 (FGF23) protein in phosphate metabolism, highlighting its regulation of vitamin D, parathyroid hormone, and bone metabolism. Although it was traditionally thought that phosphate-calcium homeostasis was controlled exclusively by parathyroid hormone (PTH) and calcitriol, pathophysiological studies revealed the influence of FGF23. This protein, expressed mainly in bone, inhibits the renal reabsorption of phosphate and calcitriol formation, mediated by the α-klotho co-receptor. In addition to its role in phosphate metabolism, FGF23 exhibits pleiotropic effects in non-renal systems such as the cardiovascular, immune, and metabolic systems, including the regulation of gene expression and cardiac fibrosis. Although it has been proposed as a biomarker and therapeutic target, the inhibition of FGF23 poses challenges due to its potential side effects. However, the approval of drugs such as burosumab represents a milestone in the treatment of FGF23-related diseases.

Effect of Burosumab on Muscle Function and Strength, and Rates of ATP Synthesis in Skeletal Muscle in Adults With XLH

CONTEXT

In clinical trials, burosumab ameliorates symptoms of pain, fatigue, and stiffness and improves performance on certain muscle function studies in patients with X-linked hypophosphatemia (XLH).

OBJECTIVE

This work aimed to determine if burosumab increases adenosine triphosphate (ATP) synthesis in skeletal muscle of treatment-naive adults with XLH, and if so, whether that correlates with improved muscle function.

METHODS

Ten untreated, symptomatic adults with XLH had ATP synthesis rates measured in the right calf using the 31P magnetic resonance spectroscopy saturation transfer technique. Baseline muscle function tests and symptoms of pain, fatigue, stiffness, and lower-extremity joint pain were quantified. All participants were treated with burosumab, 1 mg/kg every 4 weeks for 12 weeks. ATP synthesis rates and muscle function tests were repeated 2 weeks ("peak") and 4 weeks ("trough") after the third dose of burosumab.

RESULTS

All symptoms improved with treatment. Performance on the 6-Minute Walk Test (6MWT) and Sit to Stand (STS) tests also improved. Muscle strength and ATP synthesis rates did not change over the 3 months of the study. When individuals whose performances on the 6MWT and STS test were at or better than the median outcome for those tests were compared to those whose outcomes were below the median, no difference was observed in the rate of change in ATP synthesis. Intracellular muscle concentrations of phosphate were normal.

CONCLUSION

The improvement in the 6MWT and STS test without changes in muscle strength or ATP synthesis rates suggests that reductions in pain, fatigue, and stiffness may partly explain the improved performance. Intracellular phosphate in skeletal muscle is insulated from hypophosphatemia in XLH.

Real-world data of Brazilian adults with X-linked hypophosphatemia (XLH) treated with burosumab and comparison with other worldwide cohorts

BACKGROUND

Disease-related variants in PHEX cause XLH by an increase of fibroblast growth factor 23 (FGF23) circulating levels, resulting in hypophosphatemia and 1,25(OH)2 vitamin D deficiency. XLH manifests in early life with rickets and persists in adulthood with osseous and extraosseous manifestations. Conventional therapy (oral phosphate and calcitriol) improves some symptoms, but evidence show that it is not completely effective, and it can lead to nephrocalcinosis (NC) and hyperparathyroidism (HPT). Burosumab (anti-FGF23 antibody) has shown to be effective and safety in the clinical trials.

METHODS

The current real-world collaborative study evaluated genetic, clinical and laboratory data of XLH Brazilian adult patients treated with burosumab.

RESULTS

Nineteen unrelated patients were studied. Patients reported pain, limb deformities and claudication, before burosumab initiation. 78% of them were previously treated with conventional therapy. The severity of the disease was moderate to severe (15 patients with score >5). At the baseline, 3 patients presented NC (16.7%) and 12 HPT (63%). After 16 ± 8.4 months under burosumab, we observed a significant: increase in stature (p = 0.02), in serum phosphate from 1.90 ± 0.43 to 2.67 ± 0.52 mg/dL (p = 0.02); in TmP/GFR from 1.30 ± 0.46 to 2.27 ± 0.64 mg/dL (p = 0.0001), in 1,25 (OH)2 D from 50.5 ± 23.3 to 71.1 ± 19.1 pg/mL (p = 0.03), and a decrease in iPTH from 86.8 ± 37.4 pg/mL to 66.5 ± 31.1 (p = 0.002). Nineteen variants were found (10 novel). HPT tended to develop in patients with truncated PHEX variants (p = 0.06).

CONCLUSIONS

This study confirms the efficacy and safety of burosumab on XLH adult patients observed in clinical trials. Additionally, we observed a decrease in iPTH levels in patients with moderate to severe HPT at the baseline.

Recurrent Soft Tissue Infections Associated With Burosumab Therapy in X-Linked Hypophosphatemic Rickets

X-linked hypophosphatemic rickets (XLH) is a genetic disorder characterized by elevated fibroblast growth factor 23 (FGF23), resulting in renal phosphate wasting and inadequate bone mineralization. Burosumab, a monoclonal antibody that inhibits FGF23 activity, has shown promise in improving renal phosphate reabsorption and clinical outcomes in XLH patients. However, the potential side effects of burosumab, particularly its impact on immune function and susceptibility to infections, remain a subject of concern. In this case report, we describe a 57-year-old male individual with XLH who experienced recurrent soft tissue infections while receiving burosumab therapy. The infections included an olecranon abscess, a cervical retropharyngeal phlegmon with a sternocleidomastoid abscess, and suprapubic cellulitis, all of which were treated with antibiotic therapy. Following discontinuation of burosumab therapy, the patient did not experience further soft tissue infections. These observations suggest a potential association between burosumab therapy and an increased risk of soft tissue infections. Mechanistically, disruption of the FGF23-Klotho signaling axis may lead to impaired humoral immunity mediated by B lymphocytes and compromised innate immune response mediated by macrophages. Further investigation is warranted to better understand the immunological effects of burosumab and its implications for infectious complications in XLH patients.

Benefit of burosumab in adults with X-linked hypophosphataemia (XLH) is maintained with long-term treatment

OBJECTIVES

To report the impact of continued burosumab treatment on clinical laboratory tests of efficacy, patient-reported outcomes (PROs) and ambulatory function in adults with X-linked hypophosphataemia who continued from a 96-week phase 3 study into a 48-week open-label extension.

METHODS

Eligible participants from the phase 3 study continued on the burosumab regimen received at the end of the phase 3 study for a further 48 weeks (n=31). Some (not all) received compassionate burosumab treatment between the two studies (a period of 6-18 months). The primary efficacy outcome was fasting serum phosphate concentration; secondary outcomes were serum 1,25 dihydroxyvitamin D concentration, renal phosphate reabsorption, PROs and ambulatory function.

RESULTS

Improvements in fasting serum phosphate, serum 1,25 dihydroxyvitamin D and renal phosphate reabsorption at 96 weeks were maintained through the 48-week extension. Improvements were also maintained in stiffness and physical function measured using the Western Ontario and McMaster Universities Osteoarthritis Index, pain and fatigue endpoints measuring using the Brief Pain Inventory short-form and Brief Pain Inventory, respectively, and in ambulatory function (6-Minute Walk Test).A post-hoc exploratory analysis exploring outcomes in participants who discontinued burosumab treatment between the studies (n=7) and those who received at least one dose (n=23) indicated that the benefits of burosumab on clinical laboratory tests of efficacy, PROs and ambulatory function may be lost when treatment is interrupted but recover over time when treatment is reinstated.

CONCLUSION

Continued treatment with burosumab appears necessary for sustained clinical benefit.

TRIAL REGISTRATION NUMBERS

Phase 3: NCT02526160; open-label extension: NCT03920072.

Osteomalacia Is Not a Single Disease

Among bone-material qualities, mineralization is pivotal in conferring stiffness and toughness to the bone. Osteomalacia, a disease ensuing from inadequate mineralization of the skeleton, is caused by different processes leading to decreased available mineral (calcium and/or phosphate) or enzymatic alterations. Vitamin D deficiency, which remains the major cause of altered mineralization leading to inadequate intestinal calcium and phosphate absorption, may be also associated with other conditions primarily responsible for abnormal mineralization. Given the reality of widespread vitamin D inadequacy, a full biochemical assessment of mineral metabolism is always necessary to rule out or confirm other conditions. Both too-high or too-low serum alkaline phosphatase (ALP) levels are important for diagnosis. Osteomalacic syndrome is reversible, at least in part, by specific treatment. Osteomalacia and bone mineralization themselves constitute largely unexplored fields of research. The true prevalence of the different forms of osteomalacia and the recovery after proper therapy have yet to be determined in the real world. Although non-invasive techniques to assess bone mineralization are not available in clinical practice, the systematic assessment of bone quality could help in refining the diagnosis and guiding the treatment. This review summarizes what is known of osteomalacia recent therapeutic developments and highlights the future issues of research in this field.