An update on clinical presentation and responses to therapy of patients with hereditary hypophosphatemic rickets with hypercalciuria (HHRH).

Pathogenic variants in solute carrier family 34, member 3 (SLC34A3), the gene encoding the sodium-dependent phosphate cotransporter 2c (NPT2c), cause hereditary hypophosphatemic rickets with hypercalciuria (HHRH). Here, we report a pooled analysis of clinical and laboratory records of 304 individuals from 145 kindreds, including 20 previously unreported HHRH kindreds, in which two novel SLC34A3 pathogenic variants were identified. Compound heterozygous/homozygous carriers show above 90% penetrance for kidney and bone phenotypes. The biochemical phenotype for heterozygous carriers is intermediate with decreased serum phosphate, tubular reabsorption of phosphate (TRP (%)), fibroblast growth factor 23, and intact parathyroid hormone, but increased serum 1,25-dihydroxy vitamin D, and urine calcium excretion causing idiopathic hypercalciuria in 38%, with bone phenotypes still observed in 23% of patients. Oral phosphate supplementation is the current standard of care, which typically normalizes serum phosphate. However, although in more than half of individuals this therapy achieves correction of hypophosphatemia it fails to resolve the other outcomes. The American College of Medical Genetics and Genomics score correlated with functional analysis of frequent SLC34A3 pathogenic variants in vitro and baseline disease severity. The number of mutant alleles and baseline TRP (%) were identified as predictors for kidney and bone phenotypes, baseline TRP (%) furthermore predicted response to therapy. Certain SLC34A3/NPT2c pathogenic variants can be identified with partial responses to therapy, whereas with some overlap, others present only with kidney phenotypes and a third group present only with bone phenotypes. Thus, our report highlights important novel clinical aspects of HHRH and heterozygous carriers, raises awareness to this rare group of disorders and can be a foundation for future studies urgently needed to guide therapy of HHRH.

Burosumab Efficacy and Safety in Patients with X-Linked Hypophosphatemia: Systematic Review and Meta-analysis of Real-World Data.

To assess the efficacy and safety of burosumab in children and adults with X-linked hypophosphatemia based on real-world evidence. MEDLINE (via PubMed) and Cochrane Library were searched until 18 October 2023 for single-arm (before-after) studies. Registries including Clinicaltrials.gov, EU Clinical Trials, WHO International Clinical Trials Registry Platform, and conference abstracts. The outcomes were a change in serum phosphorus concentrations and change in RSS, a change in serum ALP, bone-specific ALP, a change in the ratio of Tubular maximum reabsorption of Phosphate to Glomerular Filtrate rate, a change in serum 1,25(OH)2D and 25(OH)2D concentrations, change in height Z-score, McMaster Universities Osteoarthritis Index (WOMAC) and safety outcomes. An inverse variance random-effects meta-analysis was applied for data synthesis. Fifteen studies (289 participants) were included. Burosumab treatment improved serum phosphate concentrations [mean difference 0.88 mg/dl, 95% confidence interval 0.70 to 1.07, I2 = 92%), Rickets Severity score (mean difference - 1.86, 95% confidence interval - 2.5 to - 1.21, I2 = 71%), serum alkaline phosphate concentrations (mean difference - 1.86, 95% confidence interval - 2.5 to - 1.21, I2 = 71%), serum 1,25(OH)2D concentrations (mean difference 18.91 pg/ml, 95% confidence interval 6.39 to 31.43, I2 = 96%) and renal phosphate reabsorption (mean difference 1.22 mg/dl, 95% confidence interval 0.70 to 1.74, I2 93%). Burosumab treatment improved overall clinical and laboratory findings in patients with X-linked hypophosphatemia.

[Effect and safety of Burosumab in the treatment of 4 children with X-linked hypophosphatemia].

Objective: To evaluate the effectiveness and safety of treatment with Burosumab in pediatric X-linked hypophosphatemia (XLH) patients. Methods: In this retrospective case study, 4 children with pediatric XLH, who were treated with Burosumab in Beijing Children's Hospital, Capital Medical University and Shandong Provincial Hospital affiliated to Shandong First Medical University from July 2022 to December 2023, were selected as the study objects. We collected and analyzed their clinical characteristics, biochemical indicators, imaging results, and treatment. The children were followed up every 3 months until December 2023, and the clinical outcomes and adverse drug reactions after treatment were evaluated. Results: Of the 4 patients, 3 were males and 1 was female; they were aged 6.7, 2.9, 2.1, and 2.3 years, respectively. Three patients had previously received treatment with phosphate supplements and active vitamins, but their wadding gait and lower limb deformities did not improve significantly, neither did their imaging changes of active richets. The initial dose of Burosumab in the 4 patients was 0.8 mg/kg, administered subcutaneously every 2 weeks, with a treatment course of 0.8-1.3 years. The fasting serum phosphorus and tubular maximum reabsorption of phosphate/glomerular filtration rate (TmP/GFR) of the 4 patients before treatment were 0.72, 0.95, 0.81, 0.66 mmol/L and 0.67, 0.85, 0.87, 0.61 mmol/L, respectively. At the last follow-up, the fasting serum phosphorus and TmP/GFR levels were significantly increased (0.96, 1.09, 1.09, 0.90 mmol/L, and 0.85, 0.79, 1.03, 0.98 mmol/L, respectively). Among them, only the TmP/GFR level (1.17 mmol/L) in case 2 achieved normal values at 3 months post-therapy, while the rest did not reach the normal range for children of the same age. After treatment, the alkaline phosphatase levels of all patients gradually decreased (the values were 461, 240, 423, and 237 U/L, respectively), and the ALP levels in cases 2 and 4 returned to normal at the last visit. Case 4 showed a slight increase in parathyroid hormone (PTH) levels after 9 months of treatment, while the PTH levels in the rest 3 cases remained normal. Case 1 underwent a 6-minute walking test, and the walking distance increased from 245 m to 570 m. Abnormal gait, lower limb deformity, and the severity of rickets in the 4 patients had all improved. No adverse drug reactions such as nephrocalcinosis, local skin injection reaction, hyperphosphatemia, or vitamin D deficiency were observed. Conclusion: Burosumab can improve clinical symptoms in children with XLH with a good safety profile.

Efficacy and safety of burosumab compared with conventional therapy in patients with X-linked hypophosphatemia: A systematic review.

Burosumab, a monoclonal antibody directed against the fibroblast growth factor 23 (FGF23), has been approved for the treatment of X-linked hypophosphatemia (XLH). We conducted a systematic review to compare the efficacy and safety of burosumab versus conventional therapy (phosphorus and calcitriol) on XLH treatment. After a comprehensive literature search on MEDLINE/PubMed and Embase, we found nine studies for inclusion in the analysis. Risk of bias was assessed, and a random-effects model was used to determine the effect size. Clinical, biochemical, and radiological parameters of disease severity before and after treatment were analyzed and expressed in standardized mean difference (SMD). Burosumab resulted in normalization of phosphate homeostasis with an increase in renal tubular phosphate reabsorption and significant resolution of skeletal lesions (change in Thacher's total rickets severity score SMD: -1.46, 95% confidence interval [CI]: -1.76 to -1.17, p < 0.001, improvement in deformities, and decline in serum alkaline phosphatase levels [SMD: 130.68, 95% CI: 125.26-136.1, p < 0.001)]. Conventional therapy led to similar improvements in all these parameters but to a lower degree. In adults, burosumab normalized phosphorus levels (SMD: 1.23, 95% CI: 0.98-1.47, p < 0.001) with resultant clinical improvement. Burosumab treatment was well tolerated, with only mild treatment-related adverse effects. The present review indicates a potential role for burosumab in improving rickets, deformities, and growth in children with XLH. Given its superior efficacy and safety profile, burosumab could be an effective therapeutic option in children. We suggest further studies comparing burosumab versus conventional therapy in children and adults with XLH.

Cardiovascular health in pediatric patients with X-linked hypophosphatemia under two years of burosumab therapy.

Introduction: X-linked hypophosphatemia (XLH) is caused by an inactivating mutation in the phosphate-regulating endopeptidase X-linked (PHEX) gene whose defective product fails to control phosphatonin fibroblast growth factor 23 (FGF23) serum levels. Although elevated FGF23 levels have been linked with detrimental cardiac effects, the cardiologic outcomes in XLH patients have been subject to debate. Our study aimed to evaluate the prevalence and severity of cardiovascular morbidity in pediatric XLH patients before, during, and after a 2-year treatment period with burosumab, a recombinant anti-FGF23 antibody. Methods: This prospective observational study was conducted in a tertiary medical center, and included 13 individuals with XLH (age range 0.6-16.2 years) who received burosumab every 2 weeks. Clinical assessment at treatment initiation and after .5, 1, and 2 years of uninterrupted treatment included anthropometric measurements and cardiologic evaluations (blood pressure [BP], electrocardiogram, conventional echocardiography, and myocardial strain imaging). Results: The linear growth of all patients improved significantly (mean height z-score: from -1.70 ± 0.80 to -0.96 ± 1.08, P=0.03). Other favorable effects were decline in overweight/obesity rates (from 46.2% to 23.1%) and decreased rates of elevated BP (systolic BP from 38.5% to 15.4%; diastolic BP from 38.5% to 23.1%). Electrocardiograms revealed no significant abnormality throughout the study period. Cardiac dimensions and myocardial strain parameters were within the normative range for age at baseline and remained unchanged during the study period. Conclusion: Cardiologic evaluations provided reassurance that 2 years of burosumab therapy did not cause cardiac morbidity. The beneficial effect of this treatment was a reduction in cardiovascular risk factors, as evidenced by the lower prevalence of both overweight/obesity and elevated BP.

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

BACKGROUND: Disease-related variants in PHEX cause XLH by an increase of fibroblast growth factor 23 (FGF23) circulating levels, resulting in hypophosphatemia and 1,25(OH)2 vitamin D deficiency. XLH manifests in early life with rickets and persists in adulthood with osseous and extraosseous manifestations. Conventional therapy (oral phosphate and calcitriol) improves some symptoms, but evidence show that it is not completely effective, and it can lead to nephrocalcinosis (NC) and hyperparathyroidism (HPT). Burosumab (anti-FGF23 antibody) has shown to be effective and safety in the clinical trials. METHODS: The current real-world collaborative study evaluated genetic, clinical and laboratory data of XLH Brazilian adult patients treated with burosumab. RESULTS: Nineteen unrelated patients were studied. Patients reported pain, limb deformities and claudication, before burosumab initiation. 78% of them were previously treated with conventional therapy. The severity of the disease was moderate to severe (15 patients with score >5). At the baseline, 3 patients presented NC (16.7%) and 12 HPT (63%). After 16 ± 8.4 months under burosumab, we observed a significant: increase in stature (p = 0.02), in serum phosphate from 1.90 ± 0.43 to 2.67 ± 0.52 mg/dL (p = 0.02); in TmP/GFR from 1.30 ± 0.46 to 2.27 ± 0.64 mg/dL (p = 0.0001), in 1,25 (OH)2 D from 50.5 ± 23.3 to 71.1 ± 19.1 pg/mL (p = 0.03), and a decrease in iPTH from 86.8 ± 37.4 pg/mL to 66.5 ± 31.1 (p = 0.002). Nineteen variants were found (10 novel). HPT tended to develop in patients with truncated PHEX variants (p = 0.06). CONCLUSIONS: This study confirms the efficacy and safety of burosumab on XLH adult patients observed in clinical trials. Additionally, we observed a decrease in iPTH levels in patients with moderate to severe HPT at the baseline.

Hypophosphatemic rickets and short stature.

An 18-month-old male presented with gross motor delay and poor growth (weight z-score -2.21, length z-score -4.26). Radiographs showed metaphyseal irregularities suggesting metaphyseal dysplasia and sagittal craniosynostosis. Biochemical evaluation supported hypophosphatemic rickets [serum phosphorus 2.3 mg/dL (reference range (RR) 4.3-6.8), alkaline phosphatase 754 unit/L (RR 156-369)] due to renal phosphate wasting (TmP/GFR 4.3 mg/dL, normal for age 4.3-6.8), with C-terminal fibroblast growth factor 23 (FGF23) 125 RU/mL (>90 during hypophosphatemia suggests FGF23-mediated hypophosphatemia). Treatment was initiated with calcitriol and phosphate. Genetic analysis showed a pathogenic variant of FGF23: c.527G > A (p.Arg176Gln) indicative of autosomal dominant hypophosphatemic rickets (ADHR). Consistent with reports linking iron deficiency with the ADHR phenotype, low ferritin was detected. Following normalization of ferritin level (41 ng/mL) with oral ferrous sulfate replacement, biochemical improvement was demonstrated (FGF23 69 RU/mL, phosphorus 5.0 mg/dL and alkaline phosphatase 228 unit/L). Calcitriol and phosphate were discontinued. Three years later, the patient demonstrated improved developmental milestones, linear growth (length Z-score -2.01), radiographic normalization of metaphyses, and stabilization of craniosynostosis. While the most common cause of hypophosphatemic rickets is X-linked hypophosphatemia, other etiologies should be considered as treatment differs. In ADHR, normalization of iron leads to biochemical and clinical improvement.

Hereditary vitamin D resistant rickets (HVDRR) case series: phenotype, genotype, conventional treatment, and adjunctive cinacalcet therapy.

INTRODUCTION: Hereditary vitamin D resistant rickets (HVDRR) is a rare autosomal recessive disorder marked by end-organ resistance of 1,25-dihydroxyvitamin D secondary to various mutations in the vitamin D receptor gene. The currently accepted treatment modality involves bypassing the affected receptors in the gut with high-dose intravenous calcium. In a few limited case reports, cinacalcet, a calcimimetic, has been used as an adjunctive therapy. MATERIAL AND METHODS: Retrospective chart reviews were conducted to collect the clinical and biochemical data of 8 patients with HVDRR from 5 Saudi families. Four patients received only high-dose calcium, while the remaining 4 received adjuvant cinacalcet. Serum chemistry and PTH levels were measured before and during cinacalcet treatment. Gene sequencing was performed to identify the disease-causing mutation. RESULTS: All 8 patients exhibited alopecia and secondary hyperparathyroidism. Other clinical and biochemical features of rickets were present to varying degrees. Genetic analysis revealed 3 distinct mutations: a ligand-binding domain mutation in 3 unrelated patients, a ligand-binding domain mutation in 2 sisters, and a missense DNA-binding domain mutation in 3 brothers. While the overall response to therapy was variable, none of the 4 patients who received adjunctive cinacalcet developed hypocalcaemia, and there was some initial promise in improving serum PTH levels. CONCLUSIONS: This series provides new insight into the clinical and biochemical characteristics as well as treatment responses in Saudi children with HVDRR. The findings suggest that cinacalcet is a safe and potentially valuable adjuvant in this understudied population; however, further research is required to verify these results.

Recent advances in fibroblast growth factor 23-related hypophosphatemic disorders.

PURPOSE OF REVIEW: Fibroblast growth factor 23 (FGF23) is a hormone to reduce blood phosphate concentration. Excessive actions of FGF23 induce FGF23-related hypophosphatemic disorders, such as X-linked hypophosphatemic rickets (XLH) and tumor-induced osteomalacia (TIO). We will summarize recent advances in the diagnosis and treatment of FGF23-related hypophosphatemic disorders. RECENT FINDINGS: The measurement of blood FGF23 is useful to make a diagnosis of FGF23-related hypophosphatemic disorders. It was reported that many patients with FGF23-related hypophosphatemic disorders, especially TIO, were misdiagnosed, therefore, it is necessary to enhance the awareness of these diseases. A novel method to inhibit excessive actions of FGF23 by a human monoclonal antibody for FGF23, burosumab, has been approved in several countries. In more long-term observation than clinical trials, burosumab has also been shown to improve biochemical abnormalities and symptoms of rickets/osteomalacia. Following these advances, several registries and consensus recommendations on FGF23-related hypophosphatemic disorders, especially XLH, have been established in each country or region. SUMMARY: Further long-term effects of burosumab and the precise mechanism of FGF23 overproduction in patients with FGF23-related hypophosphatemic disorders need to be clarified in the future studies.

Clinical characteristics and long-term management for patients with vitamin D-dependent rickets type II: a retrospective study at a single center in Saudi Arabia.

Introduction: Hereditary Vitamin D-dependent rickets type II (HVDDR-type II) is a rare autosomal recessive disorder caused by molecular variation in the gene encoding the vitamin D receptor (VDR). This study aims to evaluate phenotype and genotype characteristics and long-term follow-up of the largest group of patients with (HVDDR-type II) in Saudi Arabia. Methodology: We conducted a retrospective chart review to collect the clinical, biochemical, and genetic data for all HVDDR-type II patients currently receiving treatment at King Faisal Specialist Hospital & Research Centre, Riyadh, Saudi Arabia. Results: A total of 42 patients, 57.1% female, and 42.9% male were included in the study. Seven patients were treated with high doses of oral calcium, while 35 patients were treated with IV calcium infusion. The median age at presentation was 15.5 months. Alopecia was found in 97.6%, 21.4% presented with bowing legs, 14.3% with delayed walking, 9.5% with seizure, and 2.4% presented with respiratory failure, while a family history of the disease was positive in 71.4% of total patients. Molecular genetic testing of the VDR gene in our cohort identified six different gene variants c.885 C>A (p.Tyr295Ter), c.88 C>T (p.Arg30Ter), c.1036G>A (p.Val346Met), c.820C>T (p.Arg274Cys), c.803 T>C (p.Ile268Thr), and c.2T>G (p.Met1?). Conclusion: We are describing the largest cohort of patients with HVDDR-type II, their clinical biochemical findings, and the most prevalent genetic variants in our population.

Rickets Types and Treatment with Vitamin D and Analogues.

The definition of "Vitamin D" encompasses a group of fat-soluble steroid compounds of different origins with similar chemical structures and the same biological effects. Vitamin D deficiency and/or a defect in the process of its synthesis or transport predispose individuals to several types of rickets. In addition to cholecalciferol, ergocalciferol, and vitamins D3 and D2, there are also active metabolites for the treatment of this condition which are commercially available. Calcitriol and aphacalcidiol are active metabolites that do not require the renal activation step, which is required with calcifediol, or hepatic activation. The purpose of this review is to summarize current approaches to the treatment of rickets for generalist physicians, focusing on the best vitamin D form to be used in each type, or, in the case of X-linked hypophosphatemic rickets (XLH), on both conventional and innovative monoclonal antibody treatments.

Safety and efficacy of burosumab in improving phosphate metabolism, bone health, and quality of life in adolescents with X-linked hypophosphatemic rickets.

BACKGROUND AND OBJECTIVE: X-linked hypophosphatemic rickets (XLH) is due to loss-of-function mutations in the phosphate-regulating endopeptidase homologue on the X chromosome (PHEX) that lead to increased fibroblast growth factor 23 (FGF23) production. FGF23 excess causes renal phosphate wasting and insufficient 1,25-dihydroxyvitamin D (1,25(OH)2D) synthesis with reduced intestinal phosphate absorption, ultimately resulting in chronic hypophosphatemia. Children with XLH show typical skeletal lesions of rickets, deformities of the lower limbs, stunted growth with disproportionate short stature, bone pain, and physical dysfunctions. Burosumab, a fully human IgG1 monoclonal antibody that binds to FGF23 to inhibit its activity, is more effective to improve the biochemical and clinical signs of XLH than conventional treatment with phosphate supplements and vitamin D active metabolites. Data on adolescents with XLH during the transition period to young adulthood are few. In this prospective case series, we aimed to assess safety and efficacy of burosumab in adolescents with XLH who discontinued long-term conventional therapy. METHODS: Five Caucasian adolescents (4 males, 1 female; mean age 15.4 ± 1.5 years) with XLH were recruited and switched from conventional treatment to burosumab (0.8-1.2 mg/kg, s. c. QW2). Burosumab was continued for 12-48 months and, once discontinued, patients were followed-up for 6-12 months. In all patients, serum calcium, phosphate, alkaline phosphatase (ALP), parathyroid hormone (PTH), and 1,25(OH)2D levels, and renal tubular reabsorption of phosphate (TmP/GFR) values were assessed at entry and during burosumab. Intact FGF23 plasma levels were measured at entry. Patient-reported outcomes (PROs) were assessed at entry and every 3-6 months to evaluate the impact of low extremity pain, stiffness, and difficulties performing daily activities. RESULTS: At entry, all patients showed hypophosphatemia, increased intact FGF23 levels, reduced TmP/GFR, insufficient 1,25(OH)2D levels, and in four out of five increased ALP levels. Two patients had radiological signs of rickets. During burosumab, all patients showed a significant increase in serum phosphate and 1,25(OH)2D levels, and in TmP/GFR values (P < 0.05 - P < 0.0001). Serum ALP levels significantly declined (P < 0.05) to normal values. No changes of serum calcium and PTH levels (PNS) were found during burosumab. PROs significantly improved (P < 0.02 - P < 0.0001) in all patients. Four patients discontinued burosumab when they turned 18 or 19, whereas one continued the treatment since he was still younger than 18 during the study period. Four patients who suspended burosumab showed a rapid decline in serum phosphate and 1,25(OH)2D levels and in TmP/GFR values; serum ALP levels increased, and PROs progressively worsened with a significant reduction in quality of life. These consequences were not observed in the patient who continued burosumab treatment. DISCUSSION: Our data showed that conventional treatment improved only in part the signs and symptoms of XLH. Burosumab was well tolerated and was effective in improving phosphate metabolism, bone health, and PROs. All the benefits of burosumab were lost after its discontinuation. These results suggested that continuing burosumab is required to achieve and maintain the clinical benefits of the treatment during the transition to young adulthood in patients with XLH.

Mutations in the vitamin D receptor gene in four patients with hereditary 1, 25-dihydroxyvitamin D-resistant rickets / Mutações no gene do receptor de vitamin D em quatro pacientes com raquitismo hereditário resistente a 1, 25-dihydroxyvitamin D

Mutations in the vitamin D receptor (VDR) are associated to the hereditary 1,25-dihydroxivitamin D-resistant rickets. The objectives of this work are: search for mutations in the VDR and analyze their functional consequences in four Brazilian children presented with rickets and alopecia. The coding region of the VDR was amplified by PCR e direct sequenced. We identified three mutations: two patients had the same mutation in exon 7 at aminoacid position 259 (p.Q259E); one patient had a mutation in exon 8 at codon 319 (p.G319V) and another one had a mutation in exon 3 leading to a truncated protein at position 73 (p.R73X). Functional studies of the mutant receptors of fibroblast primary culture, from patients' skin biopsy treated with increasing doses of 1,25(OH)2 vitamin D showed that VDR mutants were unable to be properly activated and presented a reduction in 24-hydroxylase expression level.

Mutações no receptor de vitamina D (VDR) são associadas a raquitismo hereditário resistente a 1,25-dihidroxivitamina D. Os objetivos deste trabalho foram procurar mutações no VDR e analisar suas conseqüências funcionais em quatro pacientes com raquitismo e alopécia. A região codificadora do VDR foi amplificada por PCR e seqüenciada diretamente. Identificamos três mutações: dois pacientes apresentavam a mesma mutação no éxon 7 na posição protéica 259 (p.Q259E); um paciente apresentava uma mutação no éxon 8 no códon 319 (p.G319V) e o outro apresentava uma mutação no exon 3 resultando em uma proteína truncada na posição 73 (p.R73X). O estudo funcional dos receptores mutados nos extratos de culturas de fibroblasto primárias obtidas de biópsia de pele dos pacientes, tratados com doses crescentes de 1,25(OH)2 vitamina D demonstraram que os receptores mutantes não apresentam ativação adequada apresentando expressão reduzida de 24-hidroxilase.