Burosumab for the Treatment of Tumor-Induced Osteomalacia

Treatment Advances in Tumor-Induced Osteomalacia

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome caused by hypersecretion of fibroblast growth factor 23 (FGF23) by typically benign phosphaturic mesenchymal tumors (PMTs). FGF23 excess causes chronic hypophosphatemia through renal phosphate losses and decreased production of 1,25-dihydroxy-vitamin-D. TIO presents with symptoms of chronic hypophosphatemia including fatigue, bone pain, weakness, and fractures. Definitive treatment is surgical resection of the PMT with wide margins. Other therapeutic options are necessary when the tumor is unable to be localized, not amenable to complete resection, or when the patient is not a good surgical candidate. Alternative ablative approaches such as radiotherapy, radiofrequency ablation, and cryoablation, have been used with variable success and limited follow up. Medical management is warranted both prior to definitive therapy and in non-operable cases to improve symptoms and allow for bone remineralization. Oral phosphate and calcitriol were the mainstay of medical therapy, however, the development of burosumab, a monoclonal blocking antibody to FGF23, has introduced an approved therapy that improves hypophosphatemia and symptoms in patients with TIO. In select cases, cinacalcet can be an effective adjuvant to phosphate and calcitriol. Continued monitoring for tumor growth is necessary while on medical therapy. Infigratinib, a selective FGFR tyrosine-kinase inhibitor targeting a causative tumoral fusion protein, can reverse the biochemical findings of TIO and possibly reduce tumor mass; however, its use is constrained by serious side effects. Overall, innovations in medical and interventional treatments have broadened therapeutic options for patients with PMTs, particularly in cases where a curative surgical resection is not possible.

Effective bone healing after corrective osteotomy in a patient with FGF23-related hypophosphatemic disease using short-term burosumab treatment

Hypophosphatemic rickets is a rare metabolic bone disease caused by renal phosphate wasting, leading to impaired bone mineralization. We present a case of a boy with fibroblast growth factor 23 (FGF23)-related hypophosphatemic rickets who did not achieve callus consolidation after six months of conventional therapy with phosphate and active vitamin D following corrective osteotomy. After one month of therapy with an FGF23 antibody (burosumab), the patient demonstrated significant improvement and no longer required a walking aid. Following six months of burosumab therapy, the bone had nearly fully healed. This report is the first to address the short-term use of burosumab therapy to promote bone healing after orthopedic surgery. Our findings further emphasize the clinical advantages and short-term applications of burosumab in FGF23-related hypophosphatemic diseases, especially for patients undergoing orthopedic surgery.

Emerging concepts on the FGF23 regulation and activity

Fibroblast growth factor 23 (FGF23) discovery has provided new insights into the regulation of Pi and Ca homeostasis. It is secreted by osteoblasts and osteocytes, and acts mainly in the kidney, parathyroid, heart, and bone. The aim of this review is to highlight the current knowledge on the factors modulating the synthesis of FGF23, the canonical and non-canonical signaling pathways of the hormone, the role of FGF23 in different pathophysiological conditions, and the anti-FGF23 therapy. This is a narrative review based on the search of PubMed database in the range of years 2000-2023 using the keywords local and systemic regulators of FGF23 synthesis, FGF23 receptors, canonical and non-canonical pathways, pathophysiological conditions and FGF23, and anti-FGF23 therapy, focusing the data on the molecular mechanisms. The regulation of FGF23 synthesis is complex and multifactorial. It is regulated by local factors and systemic regulators mainly involved in bone mineralization. The excessive FGF23 production is associated with different congenital diseases and with diseases occurring with a secondary high FGF23 production such as in chronic disease kidney and tumor-induced osteomalacia (TIO). The anti-FGF23 therapy appears to be useful to treat chromosome X-linked hypophosphatemia and TIO, but there are doubts about the handle of excessive FGF23 production in CKD. FGF23 biochemistry and pathophysiology are generating a plethora of knowledge to reduce FGF23 bioactivity at many levels that might be useful for future therapeutics of diseases associated with high-serum FGF23 levels.

Healthcare Resource Use Associated With Tumor-Induced Osteomalacia: A Literature Review

CONTEXT

Tumor-induced osteomalacia (TIO) is an ultra-rare, paraneoplastic syndrome caused by tumors that secrete fibroblast growth factor 23 (FGF23). Initial signs and musculoskeletal symptoms can be nonspecific and unrecognized, leading to long delays in diagnosis and treatment, and resulting in severe and progressive disability in patients with TIO.

OBJECTIVE

This review aimed to identify published evidence on healthcare resource use in TIO to better understand the burden of the disease.

EVIDENCE ACQUISITION

A targeted literature review was conducted to identify publications reporting on disease characteristics and healthcare resource use associated with TIO.

EVIDENCE SYNTHESIS

In total, 414 publications were included in the review, of which 376 were case reports. From the case reports, data on 621 patients were extracted. These patients had a mean (SD) age of 46.3 (15.8) years; 57.6% were male. Mean time from first symptoms to diagnosis of TIO was 4.6 (4.7) years and, in cases where imaging tests were reported, patients underwent a mean of 4.1 (2.7) procedures. Tumor resection was attempted in 81.0% of patients and successful in 67.0%. Fracture was reported in 49.3% of patients. Results from association analyses demonstrated that longer time to diagnosis was associated with poorer tumor resection outcomes and a higher probability of tumor recurrence. Unfavorable tumor resection outcomes were associated with greater use of pharmacologic treatment and a greater likelihood of orthopedic surgery.

CONCLUSION

TIO is associated with a substantial healthcare resource burden. Improvements in the diagnostic process could lead to better management of TIO, thereby benefiting patients and reducing that burden.

No longer to be ignored: Hypophosphatemia following intravenous iron administration

Intravenous iron supplementation is increasingly used to safely and effectively correct iron deficiency anemia, but some formulations are linked to a renal phosphate wasting syndrome which is mediated by fibroblast growth factor 23. Unawareness among prescribers and the nonspecific clinical symptoms of hypophosphatemia result in underreporting of this complication. Even though it is often an asymptomatic and self-limiting condition, accumulating evidence from case reports and dedicated randomized controlled trials show that IV iron induced hypophosphatemia may be associated with clinical symptoms. If hypophosphatemia is not recognized and treated, a metabolic bone disease phenotype may develop, pathophysiologically reminiscent of hypophosphatemic rickets as seen in X-linked hypophosphatemic rickets or oncogenic osteomalacia. This syndrome is particularly, but not uniquely, associated with formulations containing ferric carboxymaltose, probably due to specific chemical characteristics of its carbohydrate moiety. Risk factors include repeated infusion, severity of iron deficiency, as well as normal kidney function. Coexisting vitamin D deficiency or hyperparathyroidism increase the risk of metabolic bone disease. Complications can be easily prevented by an early diagnosis and switching to another IV iron formulation. In this review, we describe the epidemiology and pathophysiology of this condition, to raise awareness among prescribing clinicians.

Model-Informed Approach to Recommend Burosumab Dosing Regimens for Pediatric and Adult Patients With the Ultrarare Disease Tumor-Induced Osteomalacia

Burosumab is approved for the treatment of hypophosphatemia in persistent tumor-induced osteomalacia. This work exemplifies a model-informed drug development approach that evaluated burosumab pharmacokinetics and pharmacokinetic/pharmacodynamics in the ultrarare tumor-induced osteomalacia population to support adult and pediatric dosing. Data from tumor-induced osteomalacia participants were combined with data from X-linked hypophosphatemia to understand pharmacokinetic and pharmacokinetic/pharmacodynamic characteristics and covariates specific to tumor-induced osteomalacia. Pharmacokinetic and pharmacokinetic/pharmacodynamic simulations were performed using final models to support dosing recommendations for adults and extrapolation to pediatric patients. Burosumab pharmacokinetics were described using a one-compartment model with first-order absorption and body weight as a significant covariate. Pharmacokinetic/pharmacodynamic relationships were described using a sigmoidal Emax model with significant covariates of baseline fibroblast growth factor 23 on baseline fasting serum phosphate and potency of burosumab response and tumor-induced osteomalacia disease state resulting in a steep slope of response; however, the covariates are not clinically meaningful. Simulations demonstrated that, in pediatric patients, starting doses of burosumab 0.3 and 0.4 mg/kg every 2 weeks at steady state would achieve normal serum phosphate levels in ≥ 30% of patients with relatively low risk of hyperphosphatemia (< 3%). In adults, burosumab 0.3 and 0.5 mg/kg every 4 weeks achieves similar percentages of responders and a relative low risk of hyperphosphatemia (< 7%). Serum phosphate titration-based burosumab dosing increased the probability of achieving normal serum phosphate levels. The models supported a model-informed drug development approach for global approvals of titration-based burosumab dosing, guided by monitoring fasting serum phosphate levels.

Tumour in the dark: a challenging case of osteomalacia

Tumour-induced osteomalacia (TIO), also known as oncogenic osteomalacia, is a rare paraneoplastic syndrome mediated by the overproduction of phosphaturic hormone fibroblast growth factor 23. TIO is most commonly caused by mesenchymal tumours (PMTs), which are typically small, slow-growing and often undetectable on physical examination and conventional imaging techniques. Patients with TIO typically undergo a protracted period of diagnostic workup and medical treatment due to presentation with nonspecific symptoms and difficulty in localising the culprit tumour. During this period, ongoing surveillance is imperative as medical treatment can limit symptom progression, and tumour identification can provide definitive treatment. We report a case of TIO secondary to a PMT, which, despite biochemical diagnosis, medical treatment and serial imaging, took approximately ten years for tumour localisation.

In-depth clinical characterization of intravenous iron infusion-induced hypophosphatemic osteomalacia and its resolution

Iron deficiency anemia is treated by iron supplementation. Increasing evidence has shown that the carbohydrate components in iron infusions can cause hypophosphatemia and subsequent osteomalacia due to excess intact fibroblast growth factor 23 (iFGF23). We here undertook an in-depth characterization of 13 patients with iron infusion-induced osteomalacia (IIIO). Patients were characterized (monocentric institutional practice) by means of laboratory, bone density, HR-pQCT, and virtual osteoid volume estimation. We additionally report a patient who was treated with burosumab because iron infusions had to be continued despite osteomalacia. All 13 patients received ferric carboxymaltose (FCM) infusions and presented with low phosphate levels. Stopping the FCM infusions and supportive treatment by substitution of phosphate, calcium, native, and/or active Vitamin D was the chosen therapeutic approach. Pain, mobility, and biochemical data, such as serum phosphate levels, BMD, bone microstructure, and virtual osteoid volume, were the main outcome measures. Our results indicate biochemical normalization (eg, phosphate levels pre: 0.50 mmol/L ± 0.23 mmol/L, post: 0.93 mmol/L ± 0.32 mmol/L, p<.001) after stopping the FCM infusion and establishing supportive treatment. Additionally, pain levels on the visual analog scale (VAS) decreased (VASpre 7.31 ± 1.22, VASpost 2.73 ± 1.27, p<.0001) and areal BMD (expressed by T-score) improved significantly (T-scorepre: -1.85 ± 1.84, T-scorepost: -0.91 ± 2.13, p<.05). One patient requiring ongoing FCM infusions experienced significant additional benefits from burosumab treatment. In conclusion, our results highlight the importance of monitoring phosphate in patients treated with FCM infusions. Stopping FCM infusions is effective in addressing the excess of iFGF23 and thereby phosphate wasting. Supportive therapy enables quick recovery of the musculoskeletal system and improves pain levels in these patients.

18F-AlF-NOTA-octreotide PET/CT and 3D printing technology for precision diagnosis and treatment of phosphaturic mesenchymal tumors in patients with tumor-induced osteomalacia: two case reports

Objective

This study aims to report the application of 18F-AlF-NOTA-Octreotide PET/CT and 3D printing technology in the diagnosis and treatment of phosphaturic mesenchymal tumors (PMT) in patients with tumor-induced osteomalacia (TIO).

Case presentation

A 68-year-old male patient (Case 1) was admitted to the Weifang People's Hospital in August 2022 with complaints of "persistent pain in the bilateral flank and lumbosacral region". 18F-AlF-NOTA-Octreotide PET/CT showed high octreotide expression in the left femoral region. A 48-year-old male patient (Case 2) was admitted to the Weifang People's Hospital in November 2022, complaining of "pain in the lumbar region and ribs". 18F-AlF-NOTA-Octreotide PET/CT showed high octreotide expression in the pancreatic uncinate process and the left acetabulum. They were diagnosed with hypophosphatemic osteomalacia, with a strong consideration of an underlying neuroendocrine tumor. Preoperative design of 3D virtual surgery, CAD/CAM, and 3D printing technology were used to customize the digital surgical guide plates, and the surgery was carried out. They were both finally confirmed as phosphateuric mesenchymal tumors (PMT) based on postoperative pathology and immunohistochemistry results. Both patients experienced substantial relief from their clinical manifestations after surgery.

Conclusion

18F-AlF-NOTA-Octreotide PET/CT may be a precise diagnostic method for TIO, while 3D printing technology may serve as an effective and dependable adjunct for the treatment of PMT in patients with TIO.

Comprehensive analysis identifies endocrine fibroblast growth factors as promising prognostic markers for colorectal carcinoma

Endocrine fibroblast growth factors (eFGFs) play essential roles in cellular signaling processes, including development and differentiation, and are implicated in various cancers. However, their precise involvement in colon neoplasia and colon adenocarcinoma (COAD) remains incompletely understood. Here, we conducted a comprehensive investigation utilizing multiple databases to explore the multifaceted characteristics of eFGFs. Through integrated analyses of diverse databases, including TIMER2.0, UALCAN, OncoDB, cBioPortal, LinkedOmics, STRING, htfTarget, mirTarBase, circBank, and DGIdb, we explored eFGFs' gene expression, DNA methylation, prognostic significance, genetic alterations, gene regulatory networks, functional analysis, and drug interactions in COAD patients. Our findings revealed elevated expression levels of eFGFs in COAD, with aberrant gene expression potentially linked to promoter methylation. Importantly, hypermethylation of FGF21 and FGF23 and downregulation of FGF23 correlated with poor survival outcomes in COAD patients. Functional analyses highlighted the involvement of eFGF genes in Ras signaling, PI3K-Akt signaling, and cancer pathways. Furthermore, we validated our findings through a cross-sectional study by quantitative real-time polymerase chain reaction (qRT-PCR), confirming significant overexpression of FGF21 in colon polyps compared to normal mucosa. Additionally, we observed elevated RNA expression of FGF21 and FGF23 in adenomatous polyps compared to hyperplastic polyps. This study sheds new light on the critical roles of eFGFs in COAD tumorigenesis and underscores their potential as promising prognostic markers for COAD, as well as discriminative markers for distinguishing high-risk from low-risk polyps. These findings provide valuable insights into the complex molecular mechanisms underlying colorectal neoplasia and offer potential avenues for targeted therapeutic strategies.

Inherited phosphate and pyrophosphate disorders: New insights and novel therapies changing the oral health landscape

BACKGROUND

Mineral metabolism is critical for proper development of hard tissues of the skeleton and dentition. The dentoalveolar complex includes the following 4 mineralized tissues: enamel, dentin, cementum, and alveolar bone. Developmental processes of these tissues are affected by inherited disorders that disrupt phosphate and pyrophosphate homeostasis, although manifestations are distinct from those in the skeleton.

TYPES OF STUDIES REVIEWED

The authors discuss original data from experiments and comparative analyses and review articles describing effects of inherited phosphate and pyrophosphate disorders on dental tissues. A particular emphasis is placed on how new therapeutic approaches for these conditions may affect oral health and dental treatments of affected patients.

RESULTS

Disorders of phosphate and pyrophosphate metabolism can lead to reduced mineralization (hypomineralization) or inappropriate (ectopic) calcification of soft tissues. Disruptions in phosphate levels in X-linked hypophosphatemia and hyperphosphatemic familial tumoral calcinosis and disruptions in pyrophosphate levels in hypophosphatasia and generalized arterial calcification of infancy contribute to dental mineralization defects. Traditionally, there have been few options to ameliorate dental health problems arising from these conditions. New antibody and enzyme replacement therapies bring possibilities to improve oral health in affected patients.

PRACTICAL IMPLICATIONS

Research over the past 2 decades has exponentially expanded the understanding of mineral metabolism, and has led to novel treatments for mineralization disorders. Newly implemented and emerging therapeutic strategies affect the dentoalveolar complex and interact with aspects of oral health care that must be considered for dental treatment, clinical trial design, and coordination of multidisciplinary care teams.

Burosumab, a Transformational Treatment in a Pediatric Patient With Cutaneous-Skeletal Hypophosphatemia Syndrome

Cutaneous-skeletal hypophosphatemia syndrome (CSHS) is a rare disorder characterized by the presence of melanocytic nevi, dysplastic cortical bony lesions, and fibroblast growth factor 23 (FGF23)-mediated hypophosphatemic rickets. Herein, we describe the diagnosis of an 8-year-old girl presenting with short stature, reduced lower limb mobility, and abnormal gait due to muscle weakness and constant pain in the legs. Biochemical parameters demonstrated hypophosphatemia, hyperphosphaturia, slight increase in parathyroid hormone (PTH), high levels of alkaline phosphatase, and elevated FGF23. Burosumab improved phosphate-wasting, serum phosphorus, alkaline phosphatase, and PTH, followed by a significant mineralization in vertebral bodies evidenced by radiographic assessment. Our report shows a long-term follow-up of CSHS with a notable improvement promoted by an anti-FGF23 therapy.

Two Cases of Improved Bone Mineral Density Following Treatment of Hypophosphatemic Osteomalacia Due to FGF23 Excess

Excess fibroblast growth factor-23 (FGF23) causes renal phosphorous wasting and impaired activation of vitamin D leading to osteomalacia. Tumor-induced osteomalacia (TIO) is a rare cause of FGF23-mediated hypophosphatemia. We present 2 patients with FGF23-mediated hypophosphatemia who had low bone mineral density (BMD) at diagnosis and remarkable improvements in BMD with treatment. Patient 1 is a 43-year-old man who had years of progressive pain, difficulty ambulating, and multiple fractures. Patient 2 is a 48-year-old nonverbal man with autism and intellectual disability who had months of progressively declining mobility, presumed pain, and multiple fractures. Workup in both cases revealed hypophosphatemia, evidence of renal phosphorous wasting, and elevated FGF23. Patient 1 was diagnosed with TIO when imaging identified a subcutaneous left flank mass and excision resulted in rapid symptom improvement; he experienced a 96% increase in lumbar spine (LS) BMD after surgery. Patient 2 has had multiple scans over several years, but no FGF23-secreting tumor has been identified. He has been maintained on medical treatment with phosphorous and calcitriol with improvement in functioning and 48% increase in LS BMD. Both patients had improvements in BMD with treatment, with more pronounced improvement in the patient with TIO managed surgically.

Latent metabolic bone disease, skeletal dysplasia and other conditions related to low bone formation among 38 patients with subtrochanteric femoral fractures: a retrospective observational study

Subtrochanteric femoral fracture is rare and intractable due to the possible association with low bone formation. Retrospective analysis of 38 patients with subtrochanteric femoral fractures revealed that four patients suffered from disorders related to low bone formation and there were specific treatments for two of them.

PURPOSE

The main aim of this study was to detect latent metabolic bone diseases and skeletal dysplasia associated with low bone formation among patients with morphologic atypical femoral fracture (AFF). A second aim was to evaluate the frequency of recognized risk factors, such as antiresorptive agents, glucocorticoids, and age.

METHODS

Clinical information was retrospectively analyzed among 38 Japanese patients who were admitted to the Department of Orthopedic Surgery and Spinal Surgery and the Division of Emergency and Critical Care Medicine at the University of Tokyo Hospital with diagnoses of subtrochanteric fractures between February 2012 and March 2022.

RESULTS

Among 38 patients (including 30 females), 21 patients were aged 75 and over. Ten patients had past oral glucocorticoid use, and 18 had past antiresorptive agent use. Two patients were diagnosed with hypophosphatemic osteomalacia after the development of fractures. One patient was suspected to be a carrier of a loss-of-function variant of alkaline phosphatase, biomineralization associated (ALPL), and one other patient had previously been genetically diagnosed with pycnodysostosis. Among four patients with a diagnosis or suspicion of these metabolic bone diseases and skeletal dysplasia, four had past clinical fractures, two had past subtrochanteric femoral fractures, and two had subtrochanteric femoral fractures on both sides.

CONCLUSION

If clinicians encounter patients with morphologic AFF, latent diseases related to low bone formation should be carefully differentiated because appropriate treatment may prevent delayed union and recurrent fractures. Additionally, it may be desirable to exclude these bone diseases in advance before initiating long-term use of antiresorptive agents in osteoporotic patients by screening with serum alkaline phosphatase levels to reduce the risk of morphologic AFF.

Healing of tumor-induced osteomalacia as assessed by high-resolution peripheral quantitative computed tomography is not similar across the skeleton in the first years following complete tumor excision

Tumor-induced osteomalacia is caused by excessive fibroblast growth factor 23 production mainly from phosphaturic mesenchymal tumors. Surgical excision or tumor ablation are the preferred treatment. Information on bone microarchitecture parameters assessed by high-resolution peripheral quantitative computed tomography is limited. We report a woman with hypophosphatemic osteomalacia with generalized pain, weakness and recurrent fractures, and a large thoracic vertebral mass extending to the posterior mediastinum. Detailed radiologic and histopathologic evaluation revealed a phosphaturic mesenchymal tumor. Two surgeries were necessary for complete removal of the mass. Clinical symptoms improved after attaining normophosphatemia. Four-year post-surgical HR-pQCT parameters, compared to baseline, showed in the left distal radius, stable trabecular and cortical volumetric bone mineral density although below reference range. There was stability of trabecular number and thickness. Both stiffness and failure load decreased. A shift in cortical parameters was noted in year 2. In the left distal tibia, trabecular volumetric bone mineral density decreased whereas cortical volumetric bone mineral density markedly increased, as did cortical area. There was stability in the trabecular number and thickness. Both stiffness and failure load improved. Findings from HR-pQCT measurements in this patient disclosed that the healing of osteomalacia is not similar across the peripheral skeletal sites in the first years following tumor removal. Results contrasted low but stable volumetric bone mineral density in the distal radius with increase in the distal tibia at the expense of cortical bone. Our report helps further delineate the pattern of bone healing after treatment of this rare bone disorder.

Tumor-Induced Osteomalacia due to Sarcomatoid Non-Small Cell Lung Carcinoma Confounded by Drug-Induced Fanconi Syndrome

Tumor-induced osteomalacia (TIO) is an exceedingly rare paraneoplastic condition characterized by hypophosphatemia, osteomalacia, fragility fractures, and fatigue. A 39-year-old man was assessed for hemoptysis, pathological rib fractures, and fatigue, and was found to have a chest mass with lung metastasis. Biopsy of the mass suggested high-grade epithelioid and spindle cell neoplasm. He was initially treated for soft tissue sarcoma with an ifosfamide-based regimen and developed Fanconi syndrome that resolved on cessation of ifosfamide. Serum phosphate remained low. A low tubular maximum reabsorption of phosphate to glomerular filtration rate ratio (TmP/GFR) indicated disproportionate phosphaturia, while a severely elevated fibroblast growth factor-23 (FGF23) level enabled a diagnosis of TIO. He was started on phosphate and calcitriol supplementation. Subsequent next-generation sequencing demonstrated a RET-fusion mutation, leading to reclassification of his malignancy to a sarcomatoid non-small cell lung carcinoma. He was switched to selpercatinib, a targeted RET-kinase inhibitor approved for locally advanced or metastatic RET-fusion-positive solid tumors. This induced tumor remission with subsequent normalization of his FGF23 levels and hypophosphatemia. Despite the presence of a confounding etiology like drug-induced Fanconi syndrome, persistence of hypophosphatemia should prompt a workup of TIO, especially in the presence of a tumor.

Tumor-induced osteomalacia

Tumor-induced osteomalacia is one of paraneoplastic syndromes characterized by hypophosphatemia caused by excessive actions of fibroblast growth factor 23 (FGF23). Since the cloning of FGF23 about 20 years ago, more widespread awareness of this disease has been achieved. However, there still remain several difficulties in the management of patients with this disease. In this review, these clinical problems are discussed together with the physiological and pathophysiological functions of FGF23. Personal proposals in the management of patients with suspected patients with tumor-induced osteomalacia are also presented.

Tumor-induced osteomalacia: A systematic literature review

Introduction

Tumor-induced osteomalacia (TIO), is a rare acquired paraneoplastic syndrome characterized by defective bone mineralization, caused by the overproduction of fibroblast growth factor 23 (FGF23) by a tumor.

Material and methods

We conducted a systematic review to identify all case reports of TIO, focusing on those associated with mesenchymal tumors. We followed the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) consensus, and we included patients with a diagnosis of TIO and histological confirmation of phosphaturic mesenchymal tumors or resolution of the condition after treatment of the tumor. Bibliographical searches were carried out until December 2023 in the Cochrane Library, Medline and Embase, as well as congress abstracts online.

Results

We identified 769 articles with 1979 cases reported. Most patients were adults, with a higher incidence on men. Disease duration before diagnosis is a mean of 4.8 years. Most tumors were histologically classified as PMT. Lower limbs were the predominant location. Hypophosphatemia was present in 99.8 % of patients. The FGF23 was elevated at diagnosis in 95.5 %. Resection of the tumor was the treatment of choice in most of patients. After resection, there was a clinical improvement in 97.6 % of cases, and serum phosphorus and FGF23 levels returned to normal ranges in 91.5 % and 81.4 % of the patients, respectively.

Conclusion

TIO is usually misdiagnosed with rheumatological or musculoskeletal disorders. The diagnosis should be suspected in patients with hypophosphatemic osteomalacia, and the measurement of serum FGF23 can be useful for diagnosis and management.

Tumour-induced osteomalacia: the long road to diagnosis and recovery

Tumour-induced osteomalacia is caused by tumorous production of fibroblast growth factor 23 (FGF23) leading to urinary phosphate wasting, hypophosphataemia and decreased vitamin D activation. The resulting osteomalacia presents with muscle weakness and bone pain but progresses to multiple pathological fractures. Patients often remain undiagnosed for years with severe physical, psychological and economic ramifications. A young woman presented with multiple spontaneous fractures including bilateral femoral fractures. Laboratory tests revealed severe hypophosphataemia, elevated bone turnover markers and low to normal calcium and 25-hydroxy-vitamin D levels. Treatment with phosphate, alfalcalcidol, calcium and magnesium was initiated. 68Gallium-DOTATOC positron emission tomography imaging revealed a mass in the right foot and venous sampling of FGF23 from all extremities confirmed this tumour as the culprit. Biopsy and histology were consistent with a phosphaturic mesenchymal tumour, which was surgically resected. Phosphate levels quickly normalised postoperatively but a long convalescence with hungry bone syndrome, fracture healing and physical therapy followed.

Serum Phosphorus as a Driver of Skeletal Morbidity in Fibrous Dysplasia

CONTEXT

Fibrous dysplasia (FD) results in fractures, pain, and deformities. Abnormal osteoprogenitor cells overproduce FGF23, leading to hyperphosphaturia in most patients and frank hypophosphatemia in a subset. Studies suggest hypophosphatemia is associated with increased FD-related morbidity. However, the relationship between phosphorus and skeletal complications has not been investigated, and the optimal therapeutic target has not been determined.

OBJECTIVE

Characterize the impact of serum phosphorus on FD-related morbidity and identify levels associated with increased skeletal complications.

METHODS

Natural history study with 240 subjects at a clinical research center who had ≥1 fasting phosphorus level, determined as age- and sex-adjusted Z-scores. Subjects were categorized based on frank hypophosphatemia (Z-score ≤ -2; n = 48); low-normophosphatemia (> -2 to ≤ -1; n = 66); and high-normophosphatemia (> -1 to ≤ 2; n = 125). Main outcomes were fractures, orthopedic surgeries, and scoliosis.

RESULTS

Subjects with frank and low-normophosphatemia had increased fracture and surgery rates vs high-normophosphatemia. The prevalence of moderate to severe scoliosis was similarly higher in the frank and low-normophosphatemia groups. In a subanalysis of patients matched for Skeletal Burden Score ≥35, fracture and surgery rates remained higher in the frank hypophosphatemia group, suggesting association between phosphorus and skeletal complications is not explained by differences in FD burden alone.

CONCLUSION

Both frank hypophosphatemia and low-normophosphatemia are associated with increased FD-related complications. This supports FGF23-mediated hypophosphatemia as a driver of skeletal morbidity, which may impact a larger proportion of the FD/McCune-Albright syndrome population than previously recognized. These findings enable clinicians to identify at-risk patients and will inform development of prospective studies to determine optimal therapeutic targets.

Extended curettage for tumour-induced osteomalacia in the bone

BACKGROUND

extended curettage is generally used to treat infiltrative bone tumours. However, the extent of the curettage performed in previous studies remains unclear. This study aimed to investigate the efficacy of extended curettage for bone tumour-induced osteomalacia.

METHODS

we included 12 patients with tumour-induced osteomalacia who underwent extended curettage at our hospital between 2000 and 2022. Extended curettage was applied in cases where tumour resection could cause functional impairment or necessitate complex reconstruction. We investigated patients' clinical and oncological outcomes.

RESULTS

patients had a mean age of 55 (24-81) years, and the median follow-up duration after surgery was 3.9 (1.0-14.0) years. The causative tumours were located in the pelvis and lumbar spine. Imaging revealed the tumours to be of the sclerotic, intertrabecular, lytic and mixed types. Intraoperative 3D fluoroscopy was used in 10 patients. Extended curettage with high-speed burring and adjuvant therapy with cauterization using an electric scalpel and ethanol resulted in a remission rate of 83%; no recurrence or metastasis was observed in cases of early postoperative biochemical remission. In cases where the causative tumour was at the lumbar spine and ischium close to the acetabulum, no postoperative biochemical remission was observed, and conservative treatment was continued. Except for one patient with a tumour in the lumbar spine, all patients could walk without a cane.

CONCLUSIONS

extended curettage for bone tumour-induced osteomalacia is oncologically and functionally favourable, especially in cases where resection of the causative tumour could cause functional impairment or necessitate complex reconstruction.

Treatment and Diagnose of Spinal Phosphaturic Mesenchymal Tumor: A Case Report and a Systematic Literature Review

BACKGROUND

Spinal phosphaturic mesenchymal tumor (PMT) is a rare disorder but can be cured once the diagnosis is clear and a complete removal by surgery is performed. To the best of our knowledge, only 22 cases in the spine have been described, and we report a case with the largest number of spinal segments (T12-L5) affected among spine PMT cases.

METHODS

A comprehensive literature search was performed until May 23, 2023, following the Preferred Reporting Items for Systematic Reviews guidelines. Studies were chosen through relevant PubMed, Web of Science, and EMBASE searches to prioritize obtaining the largest studies. The Medical Subject Headings and Boolean operators employed for this search were ("PMT" or "TIO" or "Tumor-induced osteomalacia" or "phosphaturic mesenchymal tumor") and ("spine" or "spinal"). Two researchers (L.S.Z. and D.B.C) independently reviewed and evaluated the included articles. Any differing opinions were discussed until a consensus was reached. A total of 18 studies were included. A case report is also presented.

RESULTS

We report a case of spinal PMT. The full text of the relevant articles was construed. A total of 18 studies were reviewed and consolidated. These articles are roughly divided into the following 5 subcategories: 1) clinical features and baseline distribution, 2) laboratory and imaging findings, 3) pathological manifestations, and 4) surgical methods and treatment options.

CONCLUSIONS

Spinal PMT is very rare with a high rate of misdiagnosis and debilitating complications, so it is of significance to increase awareness of the disease among spine surgeons consulted by patients with spinal PMT. 68Ga-DOTATOC-PET/CT shows very high sensitivity to the spinal PMT but there is no way to exactly determine the location of the tumor. PMT has unique immunohistochemical characteristics and malignant PMT is rare. Once diagnosed, complete surgical excision is the recommended treatment. Burosumab is one of the available options, especially in cases that are recurrent and difficult to surgically resect.

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.

The pathophysiology of hypophosphatemia

After identification of fibroblast growth factor (FGF) 23 as the pivotal regulator of chronic serum inorganic phosphate (Pi) levels, the etiology of disorders causing hypophosphatemic rickets/osteomalacia has been clarified, and measurement of intact FGF23 serves as a potent tool for differential diagnosis of chronic hypophosphatemia. Additionally, measurement of bone-specific alkaline phosphatase (BAP) is recommended to differentiate acute and subacute hypophosphatemia from chronic hypophosphatemia. This article divides the etiology of chronic hypophosphatemia into 4 groups: A. FGF23 related, B. primary tubular dysfunction, C. disturbance of vitamin D metabolism, and D. parathyroid hormone 1 receptor (PTH1R) mediated. Each group is further divided into its inherited form and acquired form. Topics for each group are described, including "ectopic FGF23 syndrome," "alcohol consumption-induced FGF23-related hypophosphatemia," "anti-mitochondrial antibody associated hypophosphatemia," and "vitamin D-dependent rickets type 3." Finally, a flowchart for differential diagnosis of chronic hypophosphatemia is introduced.

Tumor-induced osteomalacia: An overview

Tumor-induced osteomalacia (TIO) is rare paraneoplastic syndrome of hypophosphatemic osteomalacia, caused by phosphaturic factors secreted by small mesenchymal origin tumors with distinct pathological features, called 'phosphaturic mesenchymal tumors'. FGF23 is the most well-characterized of the phosphaturic factors. Tumors are often small and located anywhere in the body from head to toe, which makes the localisation challenging. Functional imaging by somatostatin receptor-based PET imaging is the first line investigation, which should be followed with CT or MRI based anatomical imaging. Once localised, complete surgical excision is the treatment of choice, which brings dramatic resolution of symptoms. Medical management in the form of phosphate and active vitamin D supplements is given as a bridge to surgical management or in inoperable/non-localised patients. This review provides an overview of the epidemiology, pathophysiology, pathology, clinical features, diagnosis, and treatment of TIO, including the recent advances and directions for future research in this field.

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.

Clinical Characteristics of Malignant Phosphaturic Mesenchymal Tumor Causing Tumor-Induced Osteomalacia

CONTEXT

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome usually caused by oversecretion of fibroblast growth factor 23 (FGF23) from a phosphaturic mesenchymal tumor (PMT). PMTs are usually benign neoplasms but some of them show malignant characteristics.

OBJECTIVE

The aim of this study was to compare the clinical characteristics of benign and malignant PMTs inducing TIO.

METHODS

On March 31, 2023, we performed a systematic review of individual patient data analysis in Medline, Google Scholar, Google book, and Cochrane Library using the terms "tumor induced osteomalacia," "oncogenic osteomalacia," "hypophosphatemia," with no language restrictions and according to Preferred Reporting Items for Systematic reviews and Meta-Analyses criteria.

RESULTS

Overall, we collected data from 837 patients with TIO in which the diagnosis of benign and malignant PMT was specified. Of them, 89 were affected by malignant PMT and 748 by benign PMT. Patients with malignant PMTs were younger and presented bone pain, functional impairment, and bone deformities more frequently. Malignant PMTs showed higher values of intact FGF23 and a higher mortality rate.

CONCLUSION

The study results identify the clinical characteristics of patients with malignant TIO, permitting the early identification of patients with PMT at increased risk of malignancy. This may significantly improve the diagnostic approach to disease. Further experimental studies are mandatory to clarify the role of FGF23 in the pathogenesis of malignancy in PMTs.

Fibrous dysplasia in children and its management

PURPOSE OF REVIEW

The purpose of this review is to provide a comprehensive overview into the diagnosis and management of fibrous dysplasia (FD) in children.

RECENT FINDINGS

FD is a mosaic disorder arising from somatic Gα s variants, leading to impaired osteogenic cell differentiation. Fibro-osseous lesions expand during childhood and reach final disease burden in early adulthood. The mainstay of treatment focuses on surgical correction of skeletal deformities, physiatric care, and medical management of associated hyperfunctioning endocrinopathies. Bisphosphonates may be helpful to treat bone pain, but do not alter lesion quality or progression. Emerging evidence suggests that the RANKL inhibitor denosumab may be effective in improving lesion activity and mineralization, however further studies are needed to determine the potential utility of this and other novel therapies, particularly in children with FD.

SUMMARY

Management of children with FD has unique challenges related to skeletal growth and age-related lesion progression. Inclusion of children in clinical research is critical to develop effective treatment strategies to treat FD lesions and prevent their development.

Epidemiology of Tumor-Induced Osteomalacia in Germany Based on Real World Data

Tumor-induced osteomalacia (TIO) is an ultra-rare disease caused mostly by benign tumors that secrete fibroblast growth factor-23. Because of nonspecific symptoms, the diagnostic delay is long, and therapy can be challenging. Moreover, epidemiological data on TIO are scarce owing to its rarity. Therefore, this study aimed to quantify TIO's incidence rates and prevalence in Germany. Retrospective longitudinal and cross-sectional analyses were conducted using anonymized German claims data from the statutory health insurance (SHI) database. This database, which comprises the data of approximately 5 million insurants, is a representative sample of the German population and supports national projections. As there is no unique International Statistical Classification of Diseases and Related Health Problems (ICD) code for TIO, operational categories based on different surrogates were defined to determine the prevalence and incidence rates of TIO among probable patients. This study showed that TIO has a prevalence of (documented code, advanced imaging, medication, or tumor removal) 0.187 per 100,000 persons and an incidence rate of ≤ 0.094 per 100,000 person years. This analysis provides the first epidemiological insight into German patients with TIO. Despite the general limitations associated with the analysis of SHI claims data of ultra-rare diseases, we believe that this analysis provides a sound basis for further analysis, particularly with regard to the care situation of patients with TIO.

Tumor-induced Osteomalacia: A Case Report and Etiological Analysis with Literature Review

BACKGROUND

Tumor-induced osteomalacia (TIO) belongs to a rare disease of the paraneoplastic syndrome. Phosphate uric mesenchymal tumor (PMT) is the most common cause of TIO, while the possibility of other tumors cannot be excluded.

CASE PRESENTATION

We present a case of a 36-year-old female patient with systemic skeletal abnormalities. The woman complained of low back pain with mild motor dysfunction for 2 years. Laboratory examination showed abnormalities in markers of bone metabolism, parathyroid hormone (PTH), vitamin D and serum phosphorus. Pooled imaging examination indicated extension abnormalities in the skeletal system and a single lesion in the right femoral head. The lesion of the right femoral was imaging with somatostatin receptor-positive, which was highly suggestive of a single neuroendocrine tumor. CT guided right femoral tumorectomy and bone grafting were performed when medical treatment failed. Postoperative pathological diagnosis was phosphate urinary mesenchymal tumor secreting fibroblast growth factor 23 (FGF23), which accorded with pre-operative expectations. The postoperative symptoms were effectively relieved, and indicators returned to normal.

CONCLUSION

The tumors causing TIO exhibited significant heterogeneity in terms of tissue origin, pathological characteristics and biological behavior, but the unique common characteristic is the secretion of FGF23. With significant progress in diagnosis and treatment, the clinical follow-up of most TIO patients shows a good prognosis, but the prognosis of those with malignant tumors is relatively poor.

Diagnosis and management of tumor-induced osteomalacia: a single center experience

PURPOSE

The aim of this study is to review the clinical and laboratory characteristics, diagnostic and treatment modalities of tumor-induced osteomalacia (TIO) cases managed in a single center.

MATERIAL METHODS

Demographic and clinical features, biochemical findings, diagnostic procedures, treatment modalities, and outcomes of nine patients who had the diagnosis of TIO were reviewed retrospectively.

RESULTS

Mean age of the study group (F/M: 4/5) was 45.8 ± 10.8 years, and mean time from the onset of symptoms to diagnosis was 4.7 ± 2.8 years. The clinical manifestations were muscle weakness and difficulty in walking (8/9), hip pain (3/9), multiple fractures (2/9), stress fracture (2/9). Mean plasma phosphorus concentration was 1.28 ± 0.4 mg/dl at presentation. We performed radionuclide imaging modalities (18F-FDG PET/CT, Ga68-DOTATATE PET/CT, octreotide scintigraphy) in seven of nine patients, and tumor was detected in all. Lower extremity (n = 6; %67), head region (n = 2; %22) and thorax (n = 1; %11) were the tumor locations of our cases. Eight patients underwent surgery and remission was achieved postoperatively in all of the operated patients and plasma phosphorus level normalized in 4 ± 2 days. Pathological examination revealed mesenchymal tumors with different subtypes. Recurrence occurred in three patients at 13 ± 10.5 months after the first surgery. Two patients were reoperated and radiotherapy was also performed in one of them.

CONCLUSION

Hypophosphatemia necessitates careful evaluation for the etiology. TIO is one of the important causes of adult-onset hypophosphatemic osteomalacia. Diagnosis of TIO is essential because the laboratory and clinical findings resolve after appropriate treatment.

Burosumab Improves Patient-Reported Outcomes in Adults With Tumor-Induced Osteomalacia: Mixed-Methods Analysis

Tumor-induced osteomalacia (TIO) is an ultra-rare disease caused by tumors that secrete fibroblast growth factor 23, leading to chronic hypophosphatemia, poor skeletal health, and impaired physical function. In a phase 2 trial (UX023T-CL201; NCT02304367; n = 14), 48 weeks of burosumab treatment restored phosphate homeostasis, with improvements in skeletal health, functional mobility, and patient-reported pain, fatigue, and health-related quality of life (HRQL) (SF-36 v2). Here, we report an exploratory mixed-methods analysis of change from baseline after 144 weeks of burosumab treatment alongside qualitative data from exit interviews with 8 of 14 trial participants to evaluate meaningful treatment effects from a patient perspective. The interview subset (n = 8) reported pain and fatigue and compromised HRQL at baseline. In the interviews, participants reported that compromised HRQL and pain were the most important aspects of the disease to treat; both were considered more bothersome than fatigue and compromised physical function and activities of daily living. Improvements in pain and fatigue after treatment were reported, some of which achieved statistically and/or clinically meaningful thresholds. Furthermore, improvements in SF-36 v2 scores were most pronounced in the Physical Component Score and its Physical Function and Bodily Pain domains. Overall, the interview subset provided descriptions of symptomatic improvement and its clinical meaningfulness, including physical function, participation in activities of daily living, and mental well-being. Thus, this exploratory mixed-methods analysis provides deeper understanding of patients' perception of clinical meaningfulness beyond that articulated in validated patient-reported outcome instruments. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Tumor-Induced Osteomalacia in Patients With Malignancy: A Meta-analysis and Systematic Review of Case Reports

CONTEXT

Tumor-induced osteomalacia (TIO) due to fibroblast growth factor 23 (FGF23) overexpression is becoming recognized in patients with malignancy. The condition may be underdiagnosed, with a scarce medical literature.

OBJECTIVE

To perform a meta-analysis of case reports to allow a better understanding of malignant TIO and its clinical implications.

METHODS

Full texts were selected according to strict inclusion criteria. All case reports were included where patients had hypophosphatemia, malignant TIO, and FGF23 blood levels. Thirty-two of 275 eligible studies (n = 34 patients) met inclusion criteria. A list of desired data was extracted and graded for methodological quality.

RESULTS

Prostate adenocarcinoma (n = 9) were the most tumors reported. Twenty-five of 34 patients had a metastatic disease and a poor clinical outcome was reported for 15 of 28 patients. The median levels of blood phosphate and C-terminal FGF23 (cFGF23) were 0.40 mmol/L and 788.5 RU/mL, respectively. For most of patients, blood PTH was elevated or within range, and calcitriol levels were inappropriately low or normal. Alkaline phosphatase concentrations were increased for 20 of 22 patients. The cFGF23 values were significantly higher for patients with a poor clinical outcome when compared to other patients (1685 vs 357.5 RU/mL). In case of prostate cancer, cFGF23 levels were significantly lower (429.4 RU/mL) than for other malignancies (1007.5 RU/mL).

CONCLUSION

We report for the first time a detailed description of the clinical and biological characteristics of malignant TIO. In this context, FGF23 blood measurement would be of value for the diagnostic workup, prognostication, and follow-up of patients.

A Case of Tumor-Induced Osteomalacia Detected by Venous Sampling

Tumor-induced osteomalacia (TIO) can cause osteomalacia due to excessive production of fibroblast growth factor 23 (FGF23) by the tumor. Since TIO is a very rare disease, it is often misdiagnosed as intervertebral disc herniation, spondyloarthritis, or osteoporosis. We report a 65-year-old man who developed generalized arthralgia and difficulty walking two years ago and was diagnosed with multiple fractures throughout his body. He was initially diagnosed with osteoporosis and was treated with calcitriol. However, he was referred to our hospital since his symptoms did not improve. We diagnosed tumor-induced osteomalacia based on low serum phosphorus, high bone-type alkaline phosphatase, high FGF23 levels, and the presence of two tumors. The responsible tumor was identified using FGF23 levels in venous sampling. As the location of the tumor made surgical resection difficult, we selected treatment with burosumab, a human monoclonal antibody against FGF23, leading to improvement in the hypophosphatemia and pain, such that he was able to walk with a cane. In cases of osteoporosis with hypophosphatemia, general physicians should keep TIO in mind, and attempt to identify the responsible tumor lesion.

Unusual phosphaturic mesenchymal tumor mimicking osteoid osteoma

Phosphaturic mesenchymal tumor is a rare tumor characterized by paraneoplastic osteomalacia. The diagnosis is often delayed because of nonspecific symptoms and difficulty to localize the tumor. In this study we report a case of PMT of the left femur detected by Ga-68-DOTATATE PET-CT with radiological features mimicking osteoid osteoma. We report a 31-year-old female patient who presented to our hospital for evaluation due to progressive bone pain and muscle weakness. Her laboratory data showed hypophosphatemia and increased fibroblast growth factor 23 (FGF-23) together with reduced bone mineral density on bone densitometry. The diagnosis of PMT was suspected and the tumor was identified on Ga-68-DOTATATE PET-CT as a focal uptake in a lucent lesion of the left femoral head with a central sclerotic dot mimicking a nidus as seen in osteoid osteoma. The lesion was treated with percutaneous radiofrequency ablation. Laboratory tests and bone densitometry rapidly improved post-treatment. The present case emphasizes the difficulty to diagnose PMT due to its nonspecific biochemical and clinical presentation and the relevance of functional imaging for locating these tumors despite different radiological presentation.

Occipital bone and tumor-induced osteomalacia: a rare tumor site for an uncommon paraneoplastic syndrome

INTRODUCTION

Tumor-induced osteomalacia (TIO) is an uncommon paraneoplastic syndrome due to the overproduction of fibroblast growth factor 23 (FGF23). It is predominantly caused by mesenchymal tumors and cured upon their complete removal. Non-surgical treatment is an alternative option but limited to specific clinical conditions.

METHODS

We report a challenging case of TIO caused by a tumor involving the occipital bone. We also performed a literature review of TIO caused by tumors localized at this site, focusing on clinical findings, treatment, and outcomes.

RESULTS

The patient, a 62-year-old male, presented with a long-lasting history of progressive weakness. Biochemical evaluation revealed severe hypophosphatemia due to low renal tubular reabsorption of phosphate with raised intact FGF23 values. A 68 Ga-DOTATATE PET/TC imaging showed a suspicious lesion located in the left occipital bone that MRI and selective venous catheterization confirmed to be the cause of TIO. Stereotactic gamma knife radiosurgery was carried out, but unfortunately, the patient died of acute respiratory failure. To date, only seven additional cases of TIO have been associated to tumors located in the occipital bone. Furthermore, the tumor involved the left side of the occipital bone in all these patients.

CONCLUSION

The occipital region is a difficult area to access so a multidisciplinary approach for their treatment is required. If anatomical differences could be the basis for the predilection of the left side of the occipital bone, it remains to be clarified.

Methodology and Baseline Data of a Comparative Exploratory Double-Blinded Randomized Study of Intravenous Iron on Fibroblast Growth Factor 23 and Phosphate in Chronic Kidney Disease

Modern intravenous iron compounds (e.g., ferric carboxymaltose [FCM] and ferric derisomaltose [FDI]) are utilized in the treatment of iron deficiency anemia in non-dialysis-dependent chronic kidney disease (ND-CKD). Product-specific alterations in the metabolism of fibroblast growth factor 23 (FGF-23) leading to hypophosphatemia have been described for certain intravenous iron compounds, such as FCM, with potential effects on bone and cardiovascular health and quality of life. No prior head-to-head comparison between FCM and FDI exists in ND-CKD. This single-center exploratory double-blind randomized controlled trial primarily aimed to investigate the differential impact of FCM and FDI on FGF-23 and phosphate in patients with iron deficiency +/- anemia and ND-CKD (stages 3a-5 - serum ferritin <200 μg/L or serum ferritin 200-299 μg/L and transferrin saturation <20%). Patients were randomized (1:1) to receive either FCM or FDI over two infusions (1 month apart). Follow-up was 3 months. Measurements of serum intact FGF-23, phosphate, vitamin D metabolites, parathyroid hormone, other bone metabolism, cardiovascular, and quality of life markers were monitored. 168 patients were prescreened. Thirty-five patients were screened; 26 patients were randomized. The mean (standard deviation) age was 67.9 (12.4) years and 17 participants were male. Most participants had stage 4 CKD (median [interquartile range] estimated glomerular filtration rate [eGFR]: 18.0 [11.3] mL/min/1.73 m2). A higher than normal median (interquartile range) level of intact FGF-23 (212.1 [116.4] pg/mL) was noted. Serum phosphate was within normal range, while parathyroid hormone was higher and 1,25 (OH)2 vitamin D lower than the normal range. The "Iron and Phosphaturia - ExplorIRON-CKD" trial will provide important information regarding the differential effect of intravenous iron products in terms of FGF-23, phosphate, and other markers of bone and cardiovascular metabolism, alongside patient-reported outcome measures in patients with ND-CKD.

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.

Global guidance for the recognition, diagnosis, and management of tumor-induced osteomalacia

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome caused by mesenchymal tumors that secrete fibroblast growth factor 23 (FGF23). Patients present with progressive bone pain, muscle weakness, and fragility fractures. TIO is characterized by hypophosphatemia, excess renal phosphate excretion, and low/inappropriately normal 1,25-dihydroxyvitamin D (1,25(OH)₂ D) levels. Rarity and enigmatic clinical presentation of TIO contribute to limited awareness among the medical community. Accordingly, appropriate diagnostic tests may not be requested, leading to delayed diagnosis and poorer patient outcomes. We have developed a global guidance document to improve the knowledge of TIO in the medical community, enabling the recognition of patients with TIO and appropriate referral. We provide recommendations aiding diagnosis, referral, and treatment, helping promote a global standard of patient management. We reviewed the literature and conducted a three-round Delphi survey of TIO experts. Statements were drafted based on published evidence and expert opinions (≥70% consensus required for final recommendations). Serum phosphate should be measured in patients presenting with chronic muscle pain or weakness, fragility fractures, or bone pain. Physical examination should establish features of myopathy and identify masses that could be causative tumors. Priority laboratory evaluations should include urine/serum phosphate and creatinine to assess renal tubular reabsorption of phosphate and TmP/GFR, alkaline phosphatase, parathyroid hormone, 25-hydroxyvitamin D, 1,25(OH)₂ D, and FGF23. Patients with the clinical/biochemical suspicion of TIO should be referred to a specialist for diagnosis confirmation, and functional imaging should be used to localize causative tumor(s). Recommended treatment is tumor resection or, with unresectable/unidentifiable tumors, phosphate salts plus active vitamin D, or burosumab.

A diagnostic challenge of mysterious bone pain caused by tumor-induced osteomalacia with multiple tumors: a case report

Background

Tumor-induced osteomalacia (TIO) is a rare, tumor-induced, metabolic bone disorder, the exact incidence of which is unknown. The most common cause of TIO is hypersecretion of tumor-derived fibroblast growth factor 23 (FGF23). Surgical resection can cure TIO in most cases, while for patients with TIO who are ineligible for surgery, biologic antibodies targeting FGF23 can be used as treatment. However, the diagnosis of TIO is more difficult than its treatment as the initial presentation can be misleading or nonspecific; thus, diagnosing TIO remains a clinical challenge.

Case Description

Herein, we present a case of TIO originating from the nasal cavity neoplasm in which the patient also had a rare, thymic-derived, tumorous lesion. A diagnosis of osteoporosis was subsequently made, and a disorder of phosphorus metabolism was discovered. After determining that the patient was exhibiting signs of TIO, we used gallium-68 dotatate positron emission tomography/computed tomography (⁶⁸Ga-DOTATATE PET/CT) to locate the tumor position.

Conclusions

This case report emphasizes the importance of electrolyte testing, which is potentially helpful for quickly identifying the presence of disorders of phosphorus metabolism in suspected patients. Subsequently, appropriate imaging techniques (e.g., ⁶⁸Ga-DOTATATE PET/CT) should be used to identify potential TIO lesions. Most patients with TIO can be treated successfully following diagnosis.

Keywords

Tumor-induced osteomalacia (TIO); gallium-68 dotatate positron emission tomography/computed tomography (⁶⁸Ga-DOTATATE PET/CT); phosphaturic mesenchymal tumor (PMTs); weakness; case report.

Clinical performance of a new intact FGF23 immunoassay in healthy individuals and patients with chronic hypophosphatemia

While the positive association between automated intact fibroblast growth factor (FGF) 23 measurement kit (Determinar CL FGF23 [CL]) and the former assay (Kainos [KI]), and clinical utility of CL was well established, the clinical performance of Medfrontier FGF23 (MED), which was the manual intact FGF23 measurement kit with same antibody set as CL, has not yet been validated. Therefore, this study aims to compare MED FGF23 levels to KI FGF23 levels. A total of 380 samples were collected from healthy individuals, and 200 samples were collected from 20 patients with chronic hypophosphatemia. The intact FGF23 level of each sample was measured by KI and MED. Among the healthy individuals, the reference range of MED FGF23 levels was 18.6-59.8 pg/mL when calculated as the average ± 2 standard deviations. When compared with KI FGF23 levels, MED FGF23 levels were lower than KI levels both among samples from healthy individuals (KI FGF23, 40.9 [interquartile (IQR), 31.1-50.6]; MED FGF23, 38.0 [IQR, 31.5-45.7]; p value = 0.02) and among samples from patients with chronic hypophosphatemia (KI FGF23, 172.5 [IQR, 115.8-290.7]; MED FGF23, 130.2 [IQR, 93.6-247.0]; p value = 0.003). The linear regression analysis showed that the correlation between KI FGF23 and MED FGF23 was interpreted as a slope of 0.83 with a y-intercept of 0.53, revealing good linearity (R² = 0.99). This study showed that the discrepancy between KI and MED was very similar to the previously reported data between KI and CL.

[Paraneoplastic syndromes in rheumatology]

Paraneoplastic syndromes in rheumatology are a group of canonical rare rheumatic diseases with musculoskeletal involvement that occur in close temporal and causal association with malignancies. Knowledge of these possibly enables a prognostically relevant early diagnosis of the underlying malignant disease. In the era of immune checkpoint inhibitor treatment, there are first indications of an increase in the incidence and severity of paraneoplastic syndromes, so that they are becoming of increasing importance for the practicing rheumatologist. These nine syndromes, paraneoplastic arthritis, palmar fasciitis and polyarthritis, remitting seronegative symmetrical synovitis with pitting edema, pancreatic panniculitis with polyarthritis, paraneoplastic vasculitis, cancer-associated myositis, hypertrophic osteoarthropathy (Marie-Bamberger), eosinophilic fasciitis and tumor-induced osteomalacia, usually occur with characteristic courses and sometimes pathognomonic clinical manifestations, which are presented in this article accompanied by the rational use of a diagnostic algorithm for tumor detection. With frequently disappointing therapeutic response to glucocorticoids, nonsteroidal antirheumatic drugs and immunosuppressants, treatment of the underlying malignant disease represents the crucial step in the treatment of paraneoplastic syndromes.

Long-term use of burosumab for the treatment of tumor-induced osteomalacia

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome caused by tumoral overproduction of FGF-23. Due to local recurrence, we describe the long-term efficacy and safety profile of burosumab, an anti-FGF-23 monoclonal antibody, in a TIO patient after three unsuccessfully surgical attempts.

INTRODUCTION

TIO is a rare paraneoplastic syndrome caused by tumoral overproduction of fibroblast growth factor 23 (FGF23), resulting in hyperphospaturia, hypophosphatemia, and osteomalacia. Surgery is the only definitive treatment, but tumor can locally recur, even after years from primary surgery. Furthermore, some tumors cannot be removed by surgery due to their location.

METHODS

We describe the case of a 54-year-old woman affected by recurrent TIO who, after three unsuccessful surgical attempts of tumor removal, was treated with burosumab, an anti-FGF-23 monoclonal antibody.

RESULTS

The patient was referred to our Bone Unit after experiencing several fractures in different sites, both traumatic and non-traumatic. At the time of first evaluation, at the age of 46, serum-phosphate (SP) was 1.2 mg/dL (reference range (RR) 2.5-4.5), 24-h urinary phosphate was 842 mg (RR 400-1000), and intact-FGF-23 was 117 pg/mL (RR 25-45). Imaging showed a metabolic pre-sacral lesion that firstly underwent to exploratory laparotomy. Then, patient underwent to surgical excision of tumor. After 18 months of well-being, tumor relapsed and even the subsequent surgery was not able to completely remove it. Since 2015, patient was maintained in phosphorus supplements and 1,25(OH)₂vitamin D₃, but SP levels never normalized. In September 2019, she was started on burosumab, initially at the dose of 0.3 mg/kg/month, progressively increased to the current 0.8 mg/kg/month, with great improvement of pain, physical performance, and normalization of SP levels. Burosumab was temporary and cautionary discontinued for COVID-19 pneumonia, with a worsening of SP. After restart of burosumab, biochemistry returned to normal.

CONCLUSIONS

To our knowledge, this is the first European patient affected by TIO treated with burosumab for more than 2 years. Burosumab is a promising therapy in the medical treatment of TIO refractory or not eligible for definitive surgery, with good efficacy and safety profile.

Tumour-induced osteomalacia: 18 months of 2-weekly burosumab treatment

Summary

Tumour-induced osteomalacia (TIO) is due to an overproduction of fibroblast growth factor 23 (FGF23) by mesenchymal tumours, causing hypophosphatemia, osteomalacia and muscle weakness. TIO is usually cured by tumour resection, but neoplasms may be unidentifiable and unresectable or the patient may refuse surgery. In these cases, medical treatment with oral phosphate and calcitriol is mandatory, but it is not fully effective and it is associated with low compliance. Burosumab, a human MAB against FGF23 employed to treat X-linked hypophosphatemia (XLH), has recently been approved for TIO in the USA. Maximum burosumab dose in XLH is 90 mg administered for 2 weeks; there are no data on clinical efficacy and safety of this dose in TIO. We reported the case of a 73 years old male with multiple non-traumatic fractures, low bone mineral density, pain and reduced independence of activities of daily living. Biochemical evaluation showed hypophosphatemia, high alkaline phosphatase (ALP) and C-terminal telopeptide (CTX) and normal albumin-corrected total calcium and parathyroid hormone. Tubular phosphate reabsorption was low (80%), whereas C-terminal tail of FGF23 (cFGF23) was elevated. A 68Ga-DOTATOC PET was performed, identifying a lesion in the first left rib. The patient refused surgery; therefore, burosumab therapy was started. After 18 months of treatment (maximum dose: 60 mg administered for 2 weeks), plasma phosphate normalized and ALP levels improved (138 U/L). Patient clinical symptoms as well as pain severity and fatigue improved. Neither adverse events nor tumour progression was reported during follow-up except for a painless fracture of the second right rib.

Learning points

Our case shows efficacy and safety of burosumab treatment administered every 2 weeks in a tumour-induced osteomalacia (TIO) patient. After 18 months of treatment at a maximum dose of 60 mg every 2 weeks, we found plasma phosphate normalization and ALP reduction as well as improvement in clinical symptoms and fatigue. Neither adverse events nor tumour progression was reported during follow-up, except for a painless fracture of the second right rib.

The efficacy and safety of burosumab in two patients with cutaneous skeletal hypophosphatemia syndrome

Cutaneous skeletal hypophosphatemia syndrome (CSHS) is an ultra-rare mosaic disorder manifesting as skeletal dysplasia and FGF23-mediated hypophosphatemia, with some experiencing extra-osseous/extra-cutaneous manifestations, including both benign and malignant neoplasms. Like other disorders of FGF23-mediated hypophosphatemia including X-linked hypophosphatemia (XLH) and tumor-induced osteomalacia (TIO), patients with CSHS have low serum phosphorus and active 1,25-dihydroxyvitamin D levels. Current treatment options for patients with CSHS include multiple daily doses of oral phosphorus and one or more daily doses of active vitamin D analog to correct the deficits. Recently, the fully human monoclonal antibody against FGF23 burosumab received US approval for the treatment of XLH and TIO, two rare diseases characterized by FGF23-mediated hypophosphatemia leading to rickets and osteomalacia. Given the similarities between the pathobiologies of these disorders and CSHS, we investigated the impact of burosumab on two patients, one pediatric and one adult, with CSHS who participated in separate, but similarly designed trials. In both the pediatric and adult patients, burosumab therapy was well-tolerated and contributed to clinically meaningful improvements in disease outcomes including normalization of phosphorus metabolism and markers of bone health, and improvements in skeletal abnormalities, fractures, and physical function. Reported adverse events were minimal, with only mild injection site reactions attributed to burosumab therapy. Together, these findings suggest that burosumab therapy is a promising therapeutic option for patients with CSHS.

Phosphatonins: From Discovery to Therapeutics

OBJECTIVE

Phosphate is crucial for cell signaling, energy metabolism, nucleotide synthesis, and bone mineralization. The gut-bone-parathyroid-kidney axis is influenced by parathyroid hormone, 1,25-dihydroxyvitamin D, and phosphatonins, especially fibroblast growth factor 23 (FGF23). These hormones facilitate maintenance of phosphate homeostasis. This review summarizes current knowledge regarding the phosphate homeostasis, phosphatonin pathophysiology, and clinical implications of FGF23-related hypophosphatemic disorders, with specific focus on burosumab treatment.

METHOD

A focused literature search of PubMed was conducted.

RESULTS

Phosphatonins including FGF23, secreted frizzled-related protein 4, matrix extracellular phosphoglycoprotein, and fibroblast growth factor 7 play a pathogenic role in several hypophosphatemic disorders. Excess FGF23 inhibits sodium-dependent phosphate cotransporters (NaPi-2a and NaPi-2c), resulting in hyperphosphaturia and hypophosphatemia. Additionally, FGF23 suppresses 1,25-dihydroxyvitamin D synthesis in the proximal renal tubule, and thus, it indirectly inhibits intestinal phosphate absorption. Disorders of FGF23-related hypophosphatemia include X-linked hypophosphatemia (XLH), autosomal dominant hypophosphatemic rickets, autosomal recessive hypophosphatemic rickets, fibrous dysplasia/McCune-Albright syndrome, and tumor-induced osteomalacia (TIO). Complications of conventional therapy with oral phosphate and vitamin D analogs comprise gastrointestinal distress, hypercalcemia, nephrocalcinosis, and secondary/tertiary hyperparathyroidism. In both children and adults with XLH and TIO, the anti-FGF23 antibody burosumab exhibits a favorable safety profile and is associated with healing of rickets in affected children and improvement of osteomalacia in both children and adults.

CONCLUSION

The treatment paradigm for XLH and TIO is changing based on data from recent clinical trials. Research suggest that burosumab is effective and safe for pediatric and adult patients with XLH or TIO.