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The FDA approved a record 59 drugs last year, but the commercial potential of these drugs is lacklustre.The FDA approved a record 59 drugs last year, but the commercial potential of these drugs is lacklustre.

In an open label, randomized, controlled, phase 3 trial in 61 children 1 to 12 years old with X-linked hypophosphatemia (XLH), burosumab improved rickets versus continuing conventional therapy with active vitamin D and phosphate. Here, we conducted an analysis to determine whether skeletal responses differed when switching to burosumab versus continuing higher or lower doses of conventional therapy. Conventional therapy dose groups were defined as: higher dose phosphate >40 mg/kg [HPi], lower dose phosphate ≤40 mg/kg [LPi], higher dose alfacalcidol >60 ng/kg or calcitriol >30 ng/kg [HD], and lower dose alfacalcidol ≤60 ng/kg or calcitriol ≤30 ng/kg [LD]. At Week 64, the Radiographic Global Impression of Change (RGI-C) for rickets was higher (better) in children randomized to burosumab versus conventional therapy for all pre-baseline dose groups: HPi (+1.72 versus +0.67), LPi (+2.14 versus +1.08), HD (+1.90 versus +0.94), LD (+2.11 versus +1.06). At Week 64, the RGI-C for rickets was also higher in children randomized to burosumab (+2.06) versus conventional therapy for all on-study dose groups: HPi (+1.03), LPi (+1.05), HD (+1.45), LD (+0.72). Serum alkaline phosphatase also decreased in the burosumab treated patients more than in the conventional therapy group, regardless of on-study phosphate and active vitamin D doses. 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 alkaline phosphatase more than continuing either higher or lower doses of phosphate or active vitamin D.

Abstract Context Burosumab has been approved for the treatment of children and adults with X-linked hypophosphatemia (XLH). Real-world data and evidence for its efficacy in adolescents are lacking. Objective To assess the effects of 12 months of burosumab treatment on mineral metabolism in children (aged <12 years) and adolescents (aged 12-18 years) with XLH. Design Prospective national registry. Setting Hospital clinics. Patients A total of 93 patients with XLH (65 children, 28 adolescents). Main Outcome Measures Z scores for serum phosphate, alkaline phosphatase (ALP), and renal tubular reabsorption of phosphate per glomerular filtration rate (TmP/GFR) at 12 months. Results At baseline, patients showed hypophosphatemia (−4.4 SD), reduced TmP/GFR (−6.5 SD), and elevated ALP (2.7 SD, each P < .001 vs healthy children) irrespective of age, suggesting active rickets despite prior therapy with oral phosphate and active vitamin D in 88% of patients. Burosumab treatment resulted in comparable increases in serum phosphate and TmP/GFR in children and adolescents with XLH and a steady decline in serum ALP (each P < .001 vs baseline). At 12 months, serum phosphate, TmP/GFR, and ALP levels were within the age-related normal range in approximately 42%, 27%, and 80% of patients in both groups, respectively, with a lower, weight-based final burosumab dose in adolescents compared with children (0.72 vs 1.06 mg/kg, P < .01). Conclusions In this real-world setting, 12 months of burosumab treatment was equally effective in normalizing serum ALP in adolescents and children, despite persistent mild hypophosphatemia in one-half of patients, suggesting that complete normalization of serum phosphate is not mandatory for substantial improvement of rickets in these patients. Adolescents appear to require lower weight-based burosumab dosage than children.

X-linked hypophosphatemia (XLH), a rare genetic disorder caused by PHEX mutations, leads to fibroblast growth factor 23–mediated phosphate wasting, hypophosphatemia, and impaired bone mineralization. We report a 28-year-old woman with XLH carrying a heterozygous PHEX c.1080-1G>A splice-site mutation and a CLDN16 c.165_166delinsC mutation. Diagnosed at 18 months of age, she received long-term phosphate and active vitamin D metabolites, which resulted in secondary hyperparathyroidism, vertebral fractures, and medullary nephrocalcinosis requiring subtotal parathyroidectomy. Although CLDN16 mutation carriers are usually asymptomatic, coexistence of XLH and prolonged phosphate therapy may exacerbate renal magnesium and calcium handling defects, potentially contributing to nephrocalcinosis. Burosumab was initiated for ongoing complications. Before surgery, she was wheelchair dependent due to severe diffuse bone pain. Postoperatively, pain improved and she regained independent ambulation, which further improved after burosumab initiation. Laboratory findings showed partial but sustained improvements in serum phosphate, alkaline phosphatase, and parathyroid hormone levels. Incomplete biochemical normalization may reflect renal tubular acidosis and medullary calcinosis. This case suggests long-term phosphate therapy can adversely affect parathyroid glands and may exacerbate nephrocalcinosis in the presence of tubular vulnerabilities. It supports burosumab as an effective therapeutic option in patients with coexisting renal tubular disorders.

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

Burosumab, a fully human monoclonal antibody to FGF23, is the only approved treatment for X-linked hypophosphatemia (XLH), a rare genetic disorder characterized by renal phosphate wasting and substantial cumulative musculoskeletal morbidity. During an initial 24-week randomized, controlled trial, 134 adults with XLH received burosumab 1 mg/kg (n = 68) or placebo (n = 66) every 4 weeks. After 24 weeks, all subjects received open-label burosumab until week 48. This report describes the efficacy and safety of burosumab during the open-label treatment period. From weeks 24–48, serum phosphorus concentrations remained normal in 83.8% of participants who received burosumab throughout and were normalized in 89.4% who received burosumab after placebo. By week 48, 63.1% of baseline fractures/pseudofractures healed fully with burosumab, compared with 35.2% with burosumab after placebo. In both groups, burosumab was associated with clinically significant and sustained improvement from baseline to week 48 in scores for patient-reported outcomes of stiffness, pain, physical function, and total distance walked in 6 min. Rates of adverse events were similar for burosumab and placebo. There were no fatal adverse events or treatment-related serious adverse events. Nephrocalcinosis scores did not change from baseline by more than one grade at either week 24 or 48. These data demonstrate that in participants with XLH, continued treatment with burosumab is well tolerated and leads to sustained correction of serum phosphorus levels, continued healing of fractures and pseudofractures, and sustained improvement in key musculoskeletal impairments.

Understanding the effects of burosumab compared to conventional therapy or no treatment on patient-important outcomes in adults with X-linked hypophosphatemia (XLH) is essential to guide evidence-based treatment recommendations. To examine the highest certainty evidence addressing the management of XLH in adults to inform treatment recommendations. We searched Embase, MEDLINE, Web of Science, and Cochrane Central up to May 2023. Eligible studies included randomized controlled trials (RCTs) and observational studies of individuals aged 18+ with clinically or genetically confirmed XLH. Manuscripts comparing burosumab to no treatment or conventional therapy (phosphate and active vitamin D) and conventional therapy to no treatment were included. Two reviewers independently determined eligibility, extracted data, and assessed risk of bias (RoB). GRADE methodology was used to assess evidence certainty. We screened 4114 records, after removing duplicates, and assessed 254 full texts. One RCT and 2 observational studies were eligible. The RCT of burosumab vs no treatment had low RoB. Burosumab probably improves pain from fracture/pseudofracture healing (moderate certainty) but has little or no impact on direct pain measures (moderate certainty). Burosumab may reduce the need for parathyroidectomy (low certainty) but has little or no impact on fatigue (high certainty), stiffness (moderate certainty), and mobility (low certainty) over 24 weeks. Burosumab may increase dental abscess risk (low certainty). Indirect evidence comparing burosumab to conventional therapy provided low certainty regarding burosumab vs conventional therapy. Two observational studies on conventional therapy vs no treatment had high RoB and very low certainty regarding the impact of conventional therapy on patient-important outcomes. No formal comparisons between burosumab and conventional therapy in adults exist. Evidence for conventional therapy vs no treatment is very uncertain. Our review highlights the need for more data on the long-term effects of burosumab and conventional therapy on patient-important outcomes in adult patients with XLH.

Background X-linked hypophosphatemic rickets (XLH) is the most common cause of inherited rickets. Historically, XLH was treated with oral phosphate and calcitriol (conventional treatment). Burosumab, a fibroblast growth factor 23 (FGF-23) monoclonal antibody, was approved by the United States Food and Drug Administration (FDA) in 2018 for XLH treatment. Nevertheless, conventional treatment of XLH continues to be recommended by some specialists due to lack of published experience with burosumab in the clinical setting. We compared laboratory and radiographic changes observed following transition from conventional therapy to burosumab in pediatric XLH patients as part of routine care. Methods This retrospective single-center study identified and retroactively studied twelve patients aged 1–18 years old with XLH previously treated with conventional therapy and transitioned to burosumab. Laboratory studies and radiographs were obtained routinely as standard of care during two treatment periods: (1) conventional therapy and (2) burosumab treatment. Laboratory values and radiologic rickets severity scores were compared between periods. Results All laboratory values demonstrated improvement following 1 month of burosumab treatment, findings which were sustained over the 2-year study period. Rickets severity scores and height z -scores also improved with burosumab. There were no serious adverse events with burosumab, and adverse events overall were very infrequent and mild. One patient developed an asymptomatic mild elevation of serum phosphate while taking burosumab resulting in a temporary pause in therapy. Conclusions Safety and effectiveness of burosumab in treatment of XLH were demonstrated as burosumab yielded statistically significant improvement in laboratory and radiographic markers of rickets and height compared to conventional therapy. Graphical abstract A higher resolution version of the Graphical abstract is available as Supplementary information.

Abstract Context 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.

McCune-Albright syndrome (MAS) is a rare disorder characterized by fibrous dysplasia (FD), café-au-lait spots, and hyperfunctioning endocrinopathies. FD lesions can overproduce fibroblast growth factor-23 (FGF-23), leading to renal phosphate wasting, hypophosphatemia, and impaired bone mineralization. Conventional treatment with oral phosphate and calcitriol is limited by gastrointestinal intolerance. Burosumab, a monoclonal antibody against FGF-23, is approved for X-linked hypophosphatemia and tumor-induced osteomalacia and has shown promise in case reports of pediatric and adult patients with MAS-related FGF-23-mediated hypophosphatemia. We describe a 46-year-old woman with MAS and extensive FD who presented with worsening bone pain and FGF-23-mediated hypophosphatemia. She was started on phosphate and calcitriol with improvement in pain. Due to failure to achieve treatment targets, she was transitioned to burosumab 0.50 mg/kg injections every 4 weeks with normalization of serum phosphorus but persistent elevation of alkaline phosphatase. This case represents the second reported adult and oldest patient treated with burosumab for MAS-related FGF-23-mediated hypophosphatemia after failure of conventional therapy. Although its use in MAS remains off-label, burosumab may provide a more targeted and better-tolerated therapeutic option.