Burosumab treatment for fibrous dysplasia

Improvement of Fibrous Dysplasia After Burosumab Therapy in a Pediatric Patient with McCune-Albright Syndrome: A Case Report

CASE

Burosumab is a novel drug developed to treat hereditary fibroblast growth factor 23 (FGF23)-related disorders. We report the case of an 11-year-old girl with McCune-Albright syndrome (MAS) who sustained hypophosphatemia due to excess FGF23 and multiple bone lesions of fibrous dysplasia (FD). Burosumab therapy markedly improved not only the biochemical parameters but also the radiographic appearance of the FD lesions and clinical symptoms.

CONCLUSION

This is the first report to demonstrate that burosumab is effective in improving FD lesions in a patient with MAS.

Diagnosis, treatment, and management of rickets: a position statement from the Bone and Mineral Metabolism Group of the Italian Society of Pediatric Endocrinology and Diabetology

Rickets results from impaired mineralization of growing bone due to alterations in calcium and phosphate homeostasis. Clinical signs of rickets are related to the age of the patient, the duration of the disease, and the underlying disorder. The most common signs of rickets are swelling of the wrists, knees or ankles, bowing of the legs (knock-knees, outward bowing, or both) and inability to walk. However, clinical features alone cannot differentiate between the various forms of rickets. Rickets includes a heterogeneous group of acquired and inherited diseases. Nutritional rickets is due to a deficiency of vitamin D, dietary calcium or phosphate. Mutations in genes responsible for vitamin D metabolism or function, the production or breakdown of fibroblast growth factor 23, renal phosphate regulation, or bone mineralization can lead to the hereditary form of rickets. This position paper reviews the relevant literature and presents the expertise of the Bone and Mineral Metabolism Group of the Italian Society of Pediatric Endocrinology and Diabetology (SIEDP). The aim of this document is to provide practical guidance to specialists and healthcare professionals on the main criteria for diagnosis, treatment, and management of patients with rickets. The various forms of rickets are discussed, and detailed references for the discussion of each form are provided. Algorithms to guide the diagnostic approach and recommendations to manage patients with rare forms of hereditary rickets are proposed.

Acquired Forms of Fibroblast Growth Factor 23-Related Hypophosphatemic Osteomalacia

Fibroblast growth factor 23 (FGF23) is a pivotal humoral factor for the regulation of serum phosphate levels and was first identified in patients with autosomal dominant hypophosphatemic rickets and tumor-induced osteomalacia (TIO), the most common form of acquired FGF23-related hypophosphatemic rickets/osteomalacia (FGF23rHR). After the identification of FGF23, many other inherited and acquired forms of FGF23rHR were reported. In this review article, the detailed features of each acquired FGF23rHR are discussed, including TIO, ectopic FGF23 syndrome with malignancy, fibrous dysplasia/McCune-Albright syndrome, Schimmelpenning-Feuerstein-Mims syndrome/cutaneous skeletal hypophosphatemia syndrome, intravenous iron preparation-induced FGF23rHR, alcohol consumption-induced FGF23rHR, and post-kidney transplantation hypophosphatemia. Then, an approach for the differential diagnosis and therapeutic options for each disorder are concisely introduced. Currently, the majority of endocrinologists might only consider TIO when encountering patients with acquired FGF23rHR; an adequate differential diagnosis can reduce medical costs and invasive procedures such as positron emission tomography/computed tomography and venous sampling to identify FGF23-producing tumors. Furthermore, some acquired FGF23rHRs, such as intravenous iron preparation/alcohol consumption-induced FGF23rHR, require only cessation of drugs or alcohol to achieve full recovery from osteomalacia.

Burosumab: Current status and future prospects

Hypophosphatemic rickets/osteomalacia caused by FGF23 excess is conventionally treated with multiple doses of inorganic phosphate salts and active vitamin D analogs. However, conventional therapy targets the consequences of elevated FGF23 and not the elevated FGF23 itself and is associated with poor adherence and long-term complications such as nephrocalcinosis and secondary/tertiary hyperparathyroidism. Burosumab is a fully human IgG1 monoclonal antibody that binds to and neutralises FGF-23, thereby leading to improvement in phosphate homeostasis and healing of rickets and osteomalacia. Data from phase 2 and 3 trials report overall safety and efficacy and Burosumab is now FDA approved for treatment of XLH and TIO in children and adults. Cost and absence of long-term data are major issues with Burosumab which should be addressed in near future. At present, experts recommend Burosumab use in patients with severe disease or those with mild-moderate disease and a failed response to a trial of conventional therapy.

Metabolic Health and Disease: A Role of Osteokines?

Maintenance of skeletal health is tightly regulated by osteocytes, osteoblasts, and osteoclasts via coordinated secretion of bone-derived factors, termed osteokines. Disruption of this coordinated process due to aging and metabolic disease promotes loss of bone mass and increased risk of fracture. Indeed, growing evidence demonstrates that metabolic diseases, including type 2 diabetes, liver disease and cancer are accompanied by bone loss and altered osteokine levels. With the persistent prevalence of cancer and the growing epidemic of metabolic disorders, investigations into the role of inter-tissue communication during disease progression are on the rise. While osteokines are imperative for bone homeostasis, work from us and others have identified that osteokines possess endocrine functions, exerting effects on distant tissues including skeletal muscle and liver. In this review we first discuss the prevalence of bone loss and osteokine alterations in patients with type 2 diabetes, non-alcoholic fatty liver disease, non-alcoholic steatohepatitis, cirrhosis, and cancer. We then discuss the effects of osteokines in mediating skeletal muscle and liver homeostasis, including RANKL, sclerostin, osteocalcin, FGF23, PGE2, TGF-β, BMPs, IGF-1 and PTHrP. To better understand how inter-tissue communication contributes to disease progression, it is essential that we include the bone secretome and the systemic roles of osteokines.

McCune-Albright Syndrome: A Case Report and Review of Literature

McCune-Albright syndrome (MAS) is a rare sporadic condition defined by the classic triad of fibrous dysplasia of bone, café au lait skin macules, and hyperfunctioning endocrinopathies. The molecular basis of MAS has been ascribed to the post-zygotic somatic gain-of-function mutations in the GNAS gene, which encodes the alpha subunit of G proteins, leading to constitutive activation of several G Protein-Coupled Receptors (GPCRs). The co-occurrence of two of the above-mentioned cardinal clinical manifestations sets the diagnosis at the clinical level. In this case report, we describe a 27-month-old girl who presented with gonadotropin-independent precocious puberty secondary to an estrogen-secreting ovarian cyst, a café au lait skin macule and growth hormone, and prolactin excess, and we provide an updated review of the scientific literature on the clinical features, diagnostic work-up, and therapeutic management of MAS.

Mineral Metabolism in Children: Interrelation between Vitamin D and FGF23

Fibroblast growth factor 23 (FGF23) was identified at the turn of the century as the long-sought circulating phosphatonin in human pathology. Since then, several clinical and experimental studies have investigated the metabolism of FGF23 and revealed its relevant pathogenic role in various diseases. Most of these studies have been performed in adult individuals. However, the mineral metabolism of the child is, to a large extent, different from that of the adult because, in addition to bone remodeling, the child undergoes a specific process of endochondral ossification responsible for adequate mineralization of long bones’ metaphysis and growth in height. Vitamin D metabolism is known to be deeply involved in these processes. FGF23 might have an influence on bones’ growth as well as on the high and age-dependent serum phosphate concentrations found in infancy and childhood. However, the interaction between FGF23 and vitamin D in children is largely unknown. Thus, this review focuses on the following aspects of FGF23 metabolism in the pediatric age: circulating concentrations’ reference values, as well as those of other major variables involved in mineral homeostasis, and the relationship with vitamin D metabolism in the neonatal period, in vitamin D deficiency, in chronic kidney disease (CKD) and in hypophosphatemic disorders.

Approach to Hypophosphatemic Rickets

Hypophosphatemic rickets typically presents in infancy or early childhood with skeletal deformities and growth plate abnormalities. The most common causes are genetic (such as X-linked hypophosphatemia), and these typically will result in lifelong hypophosphatemia and osteomalacia. Knowledge of phosphate metabolism, including the effects of fibroblast growth factor 23 (FGF23) (an osteocyte produced hormone that downregulates renal phosphate reabsorption and 1,25-dihydroxyvitamin-D (1,25(OH)2D) production), is critical to determining the underlying genetic or acquired causes of hypophosphatemia and to facilitate appropriate treatment. Serum phosphorus should be measured in any child or adult with musculoskeletal complaints suggesting rickets or osteomalacia. Clinical evaluation incudes thorough history, physical examination, laboratory investigations, genetic analysis (especially in the absence of a guiding family history), and imaging to establish etiology and to monitor severity and treatment course. The treatment depends on the underlying cause, but often includes active forms of vitamin D combined with phosphate salts, or anti-FGF23 antibody treatment (burosumab) for X-linked hypophosphatemia. The purpose of this article is to explore the approach to evaluating hypophosphatemic rickets and its treatment options.

A case report to assess the safety and efficacy of Burosumab, an investigational antibody to FGF23, in a single pediatric patient with Epidermal Nevus Syndrome and associated hypophosphatemic rickets

Epidermal Nevus Syndrome (ENS), also known as Cutaneous Skeletal Hypophosphatemia Syndrome or Linear Sebaceous Nevus Syndrome, is caused by a mosaic somatic mutation of RAS (Rat Sarcoma genes) which leads to abnormally elevated levels of fibroblast growth factor 23 (FGF23). FGF23 is a major regulator in phosphate homeostasis. There are multiple disorders, along with Epidermal Nevus Syndrome (ENS), that result in unusually high circulating levels of FGF23. This increase ultimately leads to renal phosphate wasting and reduced levels of 1,25-dihydroxy vitamin D. Across these disorders, the clinical symptoms are similar and often include osteomalacia (hypophosphatemic rickets in children), muscle weakness, fatigue, joint deformities, bone pain, and fractures. Burosumab (KRN23), is an IgG1 monoclonal antibody that binds to the FGF23 receptor and inhibits the activity of FGF23. This leads to an increase in serum phosphate levels. Burosumab emerged as a potential therapy in FGF23 overactivity disorders. Burosumab was successful in the treatment of X-linked hypophosphatemia (XLH) and is now FDA-approved for its treatment. Studies have indicated that Burosumab therapy in subjects with XLH consistently increases and sustains serum phosphorus levels and tubular reabsorption of phosphate without a major impact on urine calcium levels or vitamin D metabolism. We studied the effect of Burosumab treatment in a single pediatric patient with Epidermal Nevus Syndrome. Serum phosphorus rose gradually as we titrated the dose of Burosumab upwards. During treatment, a persistent elevation of parathyroid hormone levels was noted along with a persistent elevation of serum calcium. We presumed the patient had tertiary hyperparathyroidism. However, after the removal of three parathyroid glands, the pathology came back with a single enlarged parathyroid adenoma. Subsequently, his calcium and PTH, and phosphorus levels stabilized while taking only Burosumab.

ClinicalTrials.gov NCT04320316.

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Burosumab was superior to conventional therapy in our subject with ENS.

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The subject had persistent hyperparathyroidism during treatment.

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Hyperparathyroidism corrected with removal of parathyroid adenoma

Burosumab Treatment for Autosomal Recessive Hypophosphatemic Rickets Type 1 (ARHR1)

CONTEXT

Autosomal recessive hypophosphatemic rickets (ARHR) are rare, heritable renal phosphate-wasting disorders that arise from overexpression of the bone-derived phosphaturic hormone fibroblast growth factor 23 (FGF23) leading to impaired bone mineralization (rickets and osteomalacia). Inactivating mutations of Dentin matrix protein 1 (DMP1) give rise to ARHR type 1 (ARHR1). Short stature, prominent bowing of the legs, fractures/pseudofractures, and severe enthesopathy are prominent in this patient population. Traditionally, treatment consists of oral phosphate replacement and the addition of calcitriol but this approach is limited by modest efficacy and potential renal and gastrointestinal side effects.

OBJECTIVE

The advent of burosumab (Crysvita), a fully humanized monoclonal antibody to FGF23 for the treatment of X-linked hypophosphatemia and tumor-induced osteomalacia, offers a unique opportunity to evaluate its safety and efficacy in patients with ARHR1.

RESULTS

Monthly administration of burosumab to 2 brothers afflicted with the disorder resulted in normalization of serum phosphate, healing of pseudofracture, diminished fatigue, less bone pain, and reduced incapacity arising from the extensive enthesopathy and soft tissue fibrosis/calcification that characterizes this disorder. No adverse effects were reported following burosumab administration.

CONCLUSION

The present report highlights the beneficial biochemical and clinical outcomes associated with the use of burosumab in patients with ARHR1.

New developments in the biology of fibroblast growth factors

The fibroblast growth factor (FGF) family is composed of 18 secreted signaling proteins consisting of canonical FGFs and endocrine FGFs that activate four receptor tyrosine kinases (FGFRs 1-4) and four intracellular proteins (intracellular FGFs or iFGFs) that primarily function to regulate the activity of voltage-gated sodium channels and other molecules. The canonical FGFs, endocrine FGFs, and iFGFs have been reviewed extensively by us and others. In this review, we briefly summarize past reviews and then focus on new developments in the FGF field since our last review in 2015. Some of the highlights in the past 6 years include the use of optogenetic tools, viral vectors, and inducible transgenes to experimentally modulate FGF signaling, the clinical use of small molecule FGFR inhibitors, an expanded understanding of endocrine FGF signaling, functions for FGF signaling in stem cell pluripotency and differentiation, roles for FGF signaling in tissue homeostasis and regeneration, a continuing elaboration of mechanisms of FGF signaling in development, and an expanding appreciation of roles for FGF signaling in neuropsychiatric diseases. This article is categorized under: Cardiovascular Diseases > Molecular and Cellular Physiology Neurological Diseases > Molecular and Cellular Physiology Congenital Diseases > Stem Cells and Development Cancer > Stem Cells and Development.

Rickets guidance: part II-management

Here, we discuss the management of different forms of rickets, including new therapeutic approaches based on recent guidelines. Management includes close monitoring of growth, the degree of leg bowing, bone pain, serum phosphate, calcium, alkaline phosphatase as a surrogate marker of osteoblast activity and thus degree of rickets, parathyroid hormone, 25-hydroxyvitamin D₃, and calciuria. An adequate calcium intake and normal 25-hydroxyvitamin D₃ levels should be assured in all patients. Children with calcipenic rickets require the supplementation or pharmacological treatment with native or active vitamin D depending on the underlying pathophysiology. Treatment of phosphopenic rickets depends on the underlying pathophysiology. Fibroblast-growth factor 23 (FGF23)-associated hypophosphatemic rickets was historically treated with frequent doses of oral phosphate salts in combination with active vitamin D, whereas tumor-induced osteomalacia (TIO) should primarily undergo tumor resection, if possible. Burosumab, a fully humanized FGF23-antibody, was recently approved for treatment of X-linked hypophosphatemia (XLH) and TIO and shown to be superior for treatment of XLH compared to conventional treatment. Forms of hypophosphatemic rickets independent of FGF23 due to genetic defects of renal tubular phosphate reabsorption are treated with oral phosphate only, since they are associated with excessive 1,25-dihydroxyvitamin D production. Finally, forms of hypophosphatemic rickets caused by Fanconi syndrome, such as nephropathic cystinosis and Dent disease require disease-specific treatment in addition to phosphate supplements and active vitamin D. Adjustment of medication should be done with consideration of treatment-associated side effects, including diarrhea, gastrointestinal discomfort, hypercalciuria, secondary hyperparathyroidism, and development of nephrocalcinosis or nephrolithiasis.

Treatment of fibrous dysplasia: focus on denosumab

INTRODUCTION

Fibrous dysplasia (FD) is a rare bone disease that is associated with various endocrine conditions, such as McCune Albright syndrome. It manifests as abnormal osteolysis, multiple fractures, or deformities that are reported during disease course. The receptor activator of nuclear factor-kappa B (RANK)/RANK ligand (RANKL) pathway is upregulated in FD and can be targeted with denosumab, a blocking monoclonal antibody.

AREAS COVERED

Preclinical and clinical data on the scientific rationale for using denosumab in FD and on the efficacy and safety of this therapy for this condition have been reviewed, in addition to other therapies.

EXPERT OPINION

Denosumab is a potential therapeutic agent against FD. A combined synergic approach involving theranostics might increase its therapeutic potential.

FGF23 and Hypophosphatemic Rickets/Osteomalacia

PURPOSE OF REVIEW

X-linked hypophosphatemia and tumor-induced osteomalacia are diseases characterized by hypophosphatemia with impaired proximal tubular phosphate reabsorption. Complete resection of responsible tumors is the first-line therapy for patients with tumor-induced osteomalacia. In contrast, phosphate and active vitamin D have been used for patients with X-linked hypophosphatemia and inoperable ones with tumor-induced osteomalacia. The purpose of this review is to summarize the pathogenesis of these diseases and discuss about the new treatment.

RECENT FINDINGS

Excessive FGF23 production has been shown to underline several kinds of hypophosphatemic rickets/osteomalacia including X-linked hypophosphatemia and tumor-induced osteomalacia. Burosumab, an anti-FGF23 monoclonal antibody, was approved for clinical use, while the indications of burosumab are different depending on countries. The inhibition of excessive FGF23 activity has been approved as a new therapy for several kinds of hypophosphatemic diseases. Further studies are necessary to clarify the long-term effects and safety of burosumab.