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Item Approach to Hypophosphatemic Rickets(Oxford University Press, 2022) Aackah, Sarah A.; Imel, Erik A.; Medicine, School of MedicineHypophosphatemic 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.Item Burosumab Therapy in Children with X-Linked Hypophosphatemia(Massachusetts Medical Society, 2018-05) Carpenter, Thomas O.; Whyte, Michael P.; Imel, Erik A.; Boot, Annemieke M.; Högler, Wolfgang; Linglart, Agnès; Padidela, Raja; van't Hoff, William; Mao, Meng; Chen, Chao-Yin; Skrinar, Alison; Kakkis, Emil; San Martin, Javier; Portale, Anthony A.; Medicine, School of MedicineBACKGROUND X-linked hypophosphatemia is characterized by increased secretion of fibroblast growth factor 23 (FGF-23), which leads to hypophosphatemia and consequently rickets, osteomalacia, and skeletal deformities. We investigated burosumab, a monoclonal antibody that targets FGF-23, in patients with X-linked hypophosphatemia. METHODS In an open-label, phase 2 trial, we randomly assigned 52 children with X-linked hypophosphatemia, in a 1:1 ratio, to receive subcutaneous burosumab either every 2 weeks or every 4 weeks; the dose was adjusted to achieve a serum phosphorus level at the low end of the normal range. The primary end point was the change from baseline to weeks 40 and 64 in the Thacher rickets severity total score (ranging from 0 to 10, with higher scores indicating greater disease severity). In addition, the Radiographic Global Impression of Change was used to evaluate rachitic changes from baseline to week 40 and to week 64. Additional end points were changes in pharmacodynamic markers, linear growth, physical ability, and patient-reported outcomes and the incidence of adverse events. RESULTS The mean Thacher rickets severity total score decreased from 1.9 at baseline to 0.8 at week 40 with every-2-week dosing and from 1.7 at baseline to 1.1 at week 40 with every-4-week dosing (P<0.001 for both comparisons); these improvements persisted at week 64. The mean serum phosphorus level increased after the first dose in both groups, and more than half the patients in both groups had levels within the normal range (3.2 to 6.1 mg per deciliter [1.0 to 2.0 mmol per liter]) by week 6. Stable serum phosphorus levels were maintained through week 64 with every-2-week dosing. Renal tubular phosphate reabsorption increased from baseline in both groups, with an overall mean increase of 0.98 mg per deciliter (0.32 mmol per liter). The mean dose of burosumab at week 40 was 0.98 mg per kilogram of body weight with every-2-week dosing and 1.50 mg per kilogram with every-4-week dosing. Across both groups, the mean serum alkaline phosphatase level decreased from 459 U per liter at baseline to 369 U per liter at week 64. The mean standing-height z score increased in both groups, with greater improvement seen at all time points with every-2-week dosing (an increase from baseline of 0.19 at week 64) than with every-4-week dosing (an increase from baseline of 0.12 at week 64). Physical ability improved and pain decreased. Nearly all the adverse events were mild or moderate in severity. CONCLUSIONS In children with X-linked hypophosphatemia, treatment with burosumab improved renal tubular phosphate reabsorption, serum phosphorus levels, linear growth, and physical function and reduced pain and the severity of rickets.Item Burosumab versus conventional therapy in children with X-linked hypophosphataemia: a randomised, active-controlled, open-label, phase 3 trial(Elsevier, 2019-06-15) Imel, Erik A.; Glorieux, Francis H.; Whyte, Michael P.; Munns, Craig F.; Ward, Leanne M.; Nilsson, Ola; Simmons, Jill H.; Padidela, Raja; Namba, Noriyuki; Cheong, Hae Il; Pitukcheewanont, Pisit; Sochett, Etienne; Högler, Wolfgang; Muroya, Koji; Tanaka, Hiroyuki; Gottesman, Gary S.; Biggin, Andrew; Perwad, Farzana; Mao, Meng; Chen, Chao-Yin; Skrinar, Alison; Martin, Javier San; Portale, Anthony A.; Medicine, School of MedicineBackground X-linked hypophosphatemia in children is characterized by elevated serum FGF23, hypophosphatemia, rickets, lower extremity bowing, and growth impairment. We compared the efficacy and safety of continuing conventional therapy, consisting of oral phosphate and active vitamin D, versus switching to burosumab, a fully human monoclonal antibody against FGF23, in pediatric X-linked hypophosphatemia. Methods In this randomised, active-controlled, open-label, phase 3 trial at 16 clinical sites, we enrolled children with X-linked hypophosphataemia aged 1–12 years. Key eligibility criteria were a total Thacher rickets severity score of at least 2·0, fasting serum phosphorus lower than 0·97 mmol/L (3·0 mg/dL), confirmed PHEX (phosphate-regulating endopeptidase homolog, X-linked) mutation or variant of unknown significance in the patient or a family member with appropriate X-linked dominant inheritance, and receipt of conventional therapy for at least 6 consecutive months for children younger than 3 years or at least 12 consecutive months for children older than 3 years. Eligible patients were randomly assigned (1:1) to receive either subcutaneous burosumab starting at 0·8 mg/kg every 2 weeks (burosumab group) or conventional therapy prescribed by investigators (conventional therapy group). Both interventions lasted 64 weeks. The primary endpoint was change in rickets severity at week 40, assessed by the Radiographic Global Impression of Change global score. All patients who received at least one dose of treatment were included in the primary and safety analyses. The trial is registered with ClinicalTrials.gov, number NCT02915705. Findings Recruitment took place between Aug 3, 2016, and May 8, 2017. Of 122 patients assessed, 61 were enrolled. Of these, 32 (18 girls, 14 boys) were randomly assigned to continue receiving conventional therapy and 29 (16 girls, 13 boys) to receive burosumab. For the primary endpoint at week 40, patients in the burosumab group had significantly greater improvement in Radiographic Global Impression of Change global score than did patients in the conventional therapy group (least squares mean +1·9 [SE 0·1] with burosumab vs +0·8 [0·1] with conventional therapy; difference 1·1, 95% CI 0·8–1·5; p<0·0001). Treatment-emergent adverse events considered possibly, probably, or definitely related to treatment by the investigator occurred more frequently with burosumab (17 [59%] of 29 patients in the burosumab group vs seven [22%] of 32 patients in the conventional therapy group). Three serious adverse events occurred in each group, all considered unrelated to treatment and resolved. Interpretation Significantly greater clinical improvements were shown in rickets severity, growth, and biochemistries among children with X-linked hypophosphataemia treated with burosumab compared with those continuing conventional therapy. Funding Ultragenyx Pharmaceutical Inc. and Kyowa Kirin InternationalItem Burosumab vs Phosphate/Active Vitamin D in Pediatric X-Linked Hypophosphatemia: A Subgroup Analysis by Dose Level(The Endocrine Society, 2023) Imel, Erik A.; Glorieux, Francis H.; Whyte, Michael P.; Portale, Anthony A.; Munns, Craig F.; Nilsson, Ola; Simmons, Jill H.; Padidela, Raja; Namba, Noriyuki; Cheong, Hae Il; Pitukcheewanont, Pisit; Sochett, Etienne; Högler, Wolfgang; Muroya, Koji; Tanaka, Hiroyuki; Gottesman, Gary S.; Biggin, Andrew; Perwad, Farzana; Chen, Angel; Scott Roberts, Mary; Ward, Leanne M.; Medicine, School of MedicineContext: In an open-label, randomized, controlled, phase 3 trial in 61 children aged 1 to 12 years with X-linked hypophosphatemia (XLH), burosumab improved rickets vs continuing conventional therapy with active vitamin D and phosphate. Objective: We conducted an analysis to determine whether skeletal responses differed when switching to burosumab vs continuing higher or lower doses of conventional therapy. Methods: Conventional therapy dose groups were defined as higher-dose phosphate [greater than 40 mg/kg] (HPi), lower-dose phosphate [40 mg/kg or less] (LPi), higher-dose alfacalcidol [greater than 60 ng/kg] or calcitriol [greater than 30 ng/kg] (HD), and lower-dose alfacalcidol [60 ng/kg or less] or calcitriol [30 ng/kg or less] (LD). Results: At week 64, the Radiographic Global Impression of Change (RGI-C) for rickets was higher (better) in children randomly assigned to burosumab vs conventional therapy for all prebaseline dose groups: HPi (+1.72 vs +0.67), LPi (+2.14 vs +1.08), HD (+1.90 vs +0.94), LD (+2.11 vs +1.06). At week 64, the RGI-C for rickets was also higher in children randomly assigned to burosumab (+2.06) vs conventional therapy for all on-study dose groups: HPi (+1.03), LPi (+1.05), HD (+1.45), LD (+0.72). Serum alkaline phosphatase (ALP) also decreased in the burosumab-treated patients more than in the conventional therapy group, regardless of on-study phosphate and active vitamin D doses. Conclusion: Prior phosphate or active vitamin D doses did not influence treatment response after switching to burosumab among children with XLH and active radiographic rickets. Switching from conventional therapy to burosumab improved rickets and serum ALP more than continuing either higher or lower doses of phosphate or active vitamin D.Item Effect of Burosumab Compared With Conventional Therapy on Younger vs Older Children With X-linked Hypophosphatemia(Endocrine Society, 2022) Ward, Leanne M.; Glorieux, Francis H.; Whyte, Michael P.; Munns, Craig F.; Portale, Anthony A.; Högler, Wolfgang; Simmons, Jill H.; Gottesman, Gary S.; Padidela, Raja; Namba, Noriyuki; Cheong, Hae Il; Nilsson, Ola; Mao, Meng; Chen, Angel; Skrinar, Alison; Scott Roberts, Mary; Imel, Erik A.; Medicine, School of MedicineContext: Younger age at treatment onset with conventional therapy (phosphate salts and active vitamin D; Pi/D) is associated with improved growth and skeletal outcomes in children with X-linked hypophosphatemia (XLH). The effect of age on burosumab efficacy and safety in XLH is unknown. Objective: This work aimed to explore the efficacy and safety of burosumab vs Pi/D in younger (< 5 years) and older (5-12 years) children with XLH. Methods: This post hoc analysis of a 64-week, open-label, randomized controlled study took place at 16 academic centers. Sixty-one children aged 1 to 12 years with XLH (younger, n = 26; older, n = 35) participated. Children received burosumab starting at 0.8 mg/kg every 2 weeks (younger, n = 14; older, n = 15) or continued Pi/D individually titrated per recommended guidelines (younger, n = 12; older, n = 20). The main outcome measure included the least squares means difference (LSMD) in Radiographic Global Impression of Change (RGI-C) rickets total score from baseline to week 64. Results: The LSMD in outcomes through 64 weeks on burosumab vs conventional therapy by age group were as follows: RGI-C rickets total score (younger, +0.90; older, +1.07), total Rickets Severity Score (younger, -0.86; older, -1.44), RGI-C lower limb deformity score (younger, +1.02; older, +0.91), recumbent length or standing height Z-score (younger, +0.20; older, +0.09), and serum alkaline phosphatase (ALP) (younger, -31.15% of upper normal limit [ULN]; older, -52.11% of ULN). On burosumab, dental abscesses were not reported in younger children but were in 53% of older children. Conclusion: Burosumab appears to improve outcomes both in younger and older children with XLH, including rickets, lower limb deformities, growth, and ALP, compared with Pi/D.Item Effect of four monthly doses of a human monoclonal anti-FGF23 antibody (KRN23) on quality of life in X-linked hypophosphatemia(Elsevier, 2016-12) Ruppe, Mary D.; Zhang, Xiaoping; Imel, Erik A.; Weber, Thomas J.; Klausner, Mark A.; Ito, Takahiro; Vergeire, Maria; Humphrey, Jeffrey S.; Glorieux, Francis H.; Portale, Anthony A.; Insogna, Karl; Peacock, Munro; Carpenter, Thomas O.; Department of Medicine, IU School of MedicineX-linked hypophosphatemia (XLH) is characterized by lower extremity deformities that lead to bone and/or joint pain that result from decreased renal tubular reabsorption leading to hypophosphatemia caused by elevated levels of fibroblast growth factor 23 (FGF23). Objective Validate the use of SF-36v2 Health Survey (SF-36v2) and the Western Ontario and McMaster Osteoarthritis Index (WOMAC) to measure previously unstudied health-related quality of life (HRQoL) in XLH patients and determine the change in HRQoL before and after treatment with KRN23, a human monoclonal anti-FGF23 antibody. Methods Twenty-eight adult outpatients with XLH received up to four doses of KRN23 administered subcutaneously every 28 days. General HRQoL was measured with the SF-36v2 and condition-related HRQoL with the WOMAC at baseline and study endpoint as a secondary outcome of a Phase 1/2, open-label, multicenter, dose-escalation trial. Results Testing for scale discriminant validity and convergent-divergent validity supported the use of these scales in the assessment of HRQoL in XLH. Both instruments indicated impairment of physical function at baseline with all mean scores showing a trend to improved health at study endpoint compared to baseline. When corrected for multiple comparisons, the score for Role Limitations due to physical health on the SF-36v2 which measures the patient's perception of their own chronic functional impairments due to poor physical health remained significantly improved (P < 0.05), increasing to the mean score of US adults. For the WOMAC, Physical Functioning and Stiffness scores were significantly improved (P < 0.05). Conclusion KRN23 administration was associated with significantly improved patient perception of their Physical Functioning and Stiffness due to their disease. This study demonstrates that the SF-36v2 and WOMAC are valid tools for assessing HRQoL in XLH.Item Health Care Transition From Pediatric- to Adult-Focused Care in X-linked Hypophosphatemia: Expert Consensus(Endocrine Society, 2022) Dahir, Kathryn; Dhaliwal, Ruban; Simmons, Jill; Imel, Erik A.; Gottesman, Gary S.; Mahan, John D.; Prakasam, Gnanagurudasan; Hoch, Allison I.; Ramesan, Prameela; Díaz-González de Ferris, Maria; Medicine, School of MedicineContext: X-linked hypophosphatemia (XLH) is an inherited skeletal disorder that can lead to lifelong deleterious musculoskeletal and functional consequences. Although often perceived as a childhood condition, children and adults both experience the negative effects of XLH. Adolescents and young adults (AYAs) benefit from effective health care transition (HCT) preparation to support the transfer from pediatric- to adult-focused care. Whereas transition timelines, milestones, and educational tools exist for some chronic conditions, they do not meet the unique needs of patients with XLH. Evidence acquisition: To produce the first expert recommendations on HCT preparation for AYAs with XLH developed by clinical care investigators and transition experts, a formal literature search was conducted and discussed in an advisory board meeting in July 2020. A modified Delphi method was used to refine expert opinion and facilitate a consensus position. Evidence synthesis: We identified the need for psychosocial and access-related resources for disease education, genetic counseling, family planning, and AYA emancipation from caregiver-directed care. Additionally, we recognized that it is necessary to facilitate communication with patients through channels familiar and accessible to AYAs and teach patients to advocate for their health care/access to specialists. Conclusion: Clear HCT preparation guidelines and treatment-related goals are defined. Individualized timelines and practical strategies for HCT preparation are proposed to optimize health outcomes resulting from continuous clinical care throughout the patient lifecycle. We provide an expert consensus statement describing a tailored HCT preparation program specifically for AYAs with XLH to aid in the effective transfer from pediatric- to adult-focused health care.Item Iron and fibroblast growth factor 23 in X-linked hypophosphatemia(Elsevier B.V., 2014-03) Imel, Erik A.; Gray, Amie; Padgett, Leah; Econs, Michael J.; Department of Medicine, IU School of MedicineBackground Excess fibroblast growth factor 23 (FGF23) causes hypophosphatemia in autosomal dominant hypophosphatemic rickets (ADHR) and X-linked hypophosphatemia (XLH). Iron status influences C-terminal FGF23 (incorporating fragments plus intact FGF23) in ADHR and healthy subjects, and intact FGF23 in ADHR. We hypothesized that in XLH serum iron would inversely correlate to C-terminal FGF23, but not to intact FGF23, mirroring the relationships in normal controls. Methods Subjects included 25 untreated outpatients with XLH at a tertiary medical center and 158 healthy adult controls. Serum iron and plasma intact FGF23 and C-terminal FGF23 were measured in stored samples. Results Intact FGF23 was greater than the control mean in 100% of XLH patients, and >2SD above the control mean in 88%, compared to 71% and 21% respectively for C-terminal FGF23. In XLH, iron correlated negatively to log-C-terminal FGF23 (r= −0.523, p<0.01), with a steeper slope than in controls (p<0.001). Iron was not related to log-intact FGF23 in either group. The log-ratio of intact FGF23 to C-terminal FGF23 was higher in XLH (0.00 ± 0.44) than controls (−0.28 ± 0.21, p<0.01), and correlated positively to serum iron (controls r= 0.276, p<0.001; XLH r= 0.428, p<0.05), with a steeper slope in XLH (p<0.01). Conclusion Like controls, serum iron in XLH is inversely related to C-terminal FGF23 but not intact FGF23. XLH patients are more likely to have elevated intact FGF23 than C-terminal FGF23. The relationships of iron to FGF23 in XLH suggest altered regulation of FGF23 cleaving may contribute to maintaining hypophosphatemia around an abnormal set-point.Item Long-term Burosumab Administration Is Safe and Effective in Adults With X-linked Hypophosphatemia(Oxford University Press, 2022) Weber, Thomas J.; Imel, Erik A.; Carpenter, Thomas O.; Peacock, Munro; Portale, Anthony A.; Hetzer, Joel; Merritt, J. Lawrence, II; Insogna, Karl; Medicine, School of MedicineContext: Burosumab was developed as a treatment option for patients with the rare, lifelong, chronically debilitating, genetic bone disease X-linked hypophosphatemia (XLH). Objective: Collect additional information on the safety, immunogenicity, and clinical response to long-term administration of burosumab. Methods: UX023-CL203 (NCT02312687) was a Phase 2b, open-label, single-arm, long-term extension study of adult subjects with XLH who participated in KRN23-INT-001 or KRN23-INT-002 studies. The long-term UX023-CL203 study (January 5, 2015 through November 30, 2018) provided data up to 184 weeks. Participants in UX023-CL203 received burosumab based on the last dose in the prior KRN23-INT-001 or KRN23-INT-002 studies (0.3, 0.6, or 1.0 mg/kg given by subcutaneous injection every 4 weeks). At Week 12, burosumab could be titrated upward/downward to achieve fasting serum phosphate levels within the normal range. Primary objectives included long-term safety, the proportion of subjects achieving fasting serum phosphate in the normal range, changes in bone turnover markers, patient-reported outcomes for pain and stiffness, and measures of mobility. Results: Fasting serum phosphate levels at the midpoint of the dosing interval (2 weeks postdose, the time of peak effect) were within the normal range in 85% to 100% of subjects. Measures of phosphate metabolism and bone biomarkers generally improved with burosumab therapy, approaching or reaching their respective normal ranges by study end. Improvements in patient-reported outcomes and mobility were sustained throughout the observation period. No new safety findings emerged with longer-term burosumab treatment. Conclusion: These data support the conclusion that burosumab therapy may be a safe and effective long-term treatment option for adult patients with XLH.Item The metabolic bone disease associated with the Hyp mutation is independent of osteoblastic HIF1α expression(Elsevier, 2017-06) Hum, Julia M.; Clinkenbeard, Erica L.; Ip, Colin; Cass, Taryn A.; Allen, Matt; White, Kenneth E.; Department of Medical and Molecular Genetics, School of MedicineFibroblast growth factor-23 (FGF23) controls key responses to systemic phosphate increases through its phosphaturic actions on the kidney. In addition to stimulation by phosphate, FGF23 positively responds to iron deficiency anemia and hypoxia in rodent models and in humans. The disorder X-linked hypophosphatemia (XLH) is characterized by elevated FGF23 in concert with an intrinsic bone mineralization defect. Indeed, the Hyp mouse XLH model has disturbed osteoblast to osteocyte differentiation with altered expression of a wide variety of genes, including FGF23. The transcription factor Hypoxia inducible factor-1α (HIF1α) has been implicated in regulating FGF23 production and plays a key role in proper bone cell differentiation. Thus the goals of this study were to determine whether HIF1α activation could influence FGF23, and to test osteoblastic HIF1α production on the Hyp endocrine and skeletal phenotypes in vivo. Treatment of primary cultures of osteoblasts/osteocytes and UMR-106 cells with the HIF activator AG490 resulted in rapid HIF1α stabilization and increased Fgf23 mRNA (50-100 fold; p < 0.01-0.001) in a time- and dose-dependent manner. Next, the Phex gene deletion in the Hyp mouse was bred onto mice with a HIF1α/Osteocalcin (OCN)-Cre background. Although HIF1α effects on bone could be detected, FGF23-related phenotypes due to the Hyp mutation were independent of HIF1α in vivo. In summary, FGF23 can be driven by ectopic HIF1α activation under normal iron conditions in vitro, but factors independent of HIF1α activity after mature osteoblast formation are responsible for the disease phenotypes in Hyp mice in vivo.