Effect of Burosumab Compared With Conventional Therapy on Younger vs Older Children With X-linked Hypophosphatemia.

CONTEXT: 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.

Sustained Efficacy and Safety of Burosumab, a Monoclonal Antibody to FGF23, in Children With X-Linked Hypophosphatemia.

PURPOSE: In X-linked hypophosphatemia (XLH), excess fibroblast growth factor-23 causes hypophosphatemia and low calcitriol, leading to musculoskeletal disease with clinical consequences. XLH treatment options include conventional oral phosphate with active vitamin D, or monotherapy with burosumab, a monoclonal antibody approved to treat children and adults with XLH. We have previously reported outcomes up to 64 weeks, and here we report safety and efficacy follow-up results up to 160 weeks from an open-label, multicenter, randomized, dose-finding trial of burosumab for 5- to 12-year-old children with XLH. METHODS: After 1 week of conventional therapy washout, patients were randomized 1:1 to burosumab every 2 weeks (Q2W) or every 4 weeks (Q4W) for 64 weeks, with dosing titrated based on fasting serum phosphorus levels between baseline and week 16. From week 66 to week 160, all patients received Q2W burosumab. RESULTS: Twenty-six children were randomized initially into each Q2W and Q4W group and all completed treatment to week 160. In 41 children with open distal femoral and proximal tibial growth plates (from both treatment groups), total Rickets Severity Score significantly decreased by 0.9 ±â€…0.1 (least squares mean ±â€…SE; P < 0.0001) from baseline to week 160. Fasting serum phosphorus increases were sustained by burosumab therapy throughout the study, with an overall population mean (SD) of 3.35 (0.39) mg/dL, within the pediatric normal range (3.2-6.1 mg/dL) at week 160 (mean change from baseline P < 0.0001). Most adverse events were mild to moderate in severity. MAIN CONCLUSIONS: In children with XLH, burosumab administration for 160 weeks improved phosphate homeostasis and rickets and was well-tolerated. Long-term safety was consistent with the reported safety profile of burosumab. CLINICALTRIALS.GOV: NCT02163577.

Burden of disease of X-linked hypophosphatemia in Japanese and Korean patients: a cross-sectional survey.

The burden of disease of X-linked hypophosphatemia (XLH) in East Asia is poorly understood. This was a cross-sectional study using an online questionnaire to evaluate health-related quality of life (HRQOL) and disease complications in Japanese and Korean patients with XLH. Adults with XLH and the caregivers of children <18 years of age with XLH in Japan and Korea were surveyed. Respondents disclosed demographic data, family history, diagnostic history, medical history, surgical history, disease-specific clinical symptoms, treatment, medications, and use of ancillary equipment. Patient-reported outcomes (PROs; the Western Ontario and McMaster Universities Osteoarthritis Index, the brief pain inventory, and the 36-item short form health survey version 2) were used to assess pain, disability, and HRQOL in adults. Of those surveyed, all 14 children (100%) and 30/32 adults (93.8%) were receiving treatment for XLH. However, despite oral phosphate and active vitamin D use, short stature, gait abnormalities, dental conditions, and decreased physical function were reported. Stapling of the growth plates was reported in 14.3% of children but no adults. Adult patients reported high rates of bone pain (59.4%) and joint pain (65.6%). Caregivers of children with XLH also reported the occurrence of bone pain (35.7%) and joint pain (35.7%). Many adult patients had a history of impaired renal function (9.5%), nephrocalcinosis (15.6%), hyperparathyroidism (15.6%), and parathyroidectomy (6.3%), all of which are associated with conventional XLH treatments. These data show that patients (both pediatric and adult) continue to have symptoms such as pain, disability, and various complications despite receiving conventional therapies.

Validation of the Radiographic Global Impression of Change (RGI-C) score to assess healing of rickets in pediatric X-linked hypophosphatemia (XLH).

BACKGROUND: Rickets is a primary manifestation of pediatric X-linked hypophosphatemia (XLH) - a rare progressive hereditary phosphate-wasting disease. Severity is quantified from radiographs using the Rickets Severity Scale (RSS). The Radiographic Global Impression of Change (RGI-C) is a complementary assessment in which a change score is assigned based on differences in the appearance of rickets on pairs of radiographs compared side by side. OBJECTIVE: The current study evaluated the reliability, validity, and sensitivity to change of the RGI-C specifically in pediatric XLH. METHODS: The reliability, validity, and sensitivity to change of the RGI-C were evaluated using data from two studies in pediatric XLH (113 children aged 1-12 years) in which burosumab treatment significantly improved rickets severity. Intra-rater and inter-rater reliability were assessed by three pediatric radiologists. RESULTS: Intra-rater reliability for RGI-C global score was >90% for agreement within 1 point, with weighted kappa values >0.5, indicating moderate to almost perfect agreement. Inter-rater reliability was also >90% (0.47-0.52 for all reader pairs; moderate agreement). The RGI-C global score showed significant relationships with changes from baseline to week 64 in serum phosphorus (r = -0.397), alkaline phosphatase (-0.611), total RSS (-0.672), standing height (0.268), and patient-reported global functioning (0.306) and comfort/pain functioning (0.409). Based on standardized response means, RGI-C global scores were sensitive to change in RSS, differentiating between those considered improved and greatly improved. Results for validity and sensitivity to change were similar for the RGI-C wrist, knee, and standing long leg scores. CONCLUSION: The RGI-C is a reliable, valid, and sensitive measure in pediatric XLH, and complementary to the RSS.

Results from a 3-year Non-interventional, Observational Disease Monitoring Program in Adults with GNE Myopathy.

BACKGROUND: GNE myopathy is a rare, autosomal recessive, muscle disease caused by mutations in GNE and is characterized by rimmed vacuoles on muscle biopsy and progressive distal to proximal muscle weakness. OBJECTIVE: Investigate the clinical presentation and progression of GNE myopathy. METHODS: The GNE Myopathy Disease Monitoring Program was an international, prospective, observational study in subjects with GNE myopathy. Muscle strength was assessed with hand-held dynamometry (HHD), with upper extremity (UE) and lower extremity (LE) composite scores reflecting upper and lower extremity muscle groups, respectively. The GNE myopathy-Functional Activity Scale (GNEM-FAS) was used to further assess impairment in mobility, upper extremity function, and self-care. RESULTS: Eighty-seven of 101 enrolled subjects completed the trial until study closure by the sponsor; 60 completed 36 months. Mean (SD) HHD UE composite score decreased from 34.3 kg (32.0) at baseline to 29.4 kg (32.6) kg at month 36 (LS mean change [95%CI]: -3.8 kg [-5.9, -1.7]; P = 0.0005). Mean (SD) HHD LE composite score decreased from 32.0 kg (34.1) at baseline to 25.5 kg (31.2) at month 36 (LS mean change [95%CI]: -4.9 [-7.7, -2.2]; P = 0.0005). GNEM-FAS scores were more severe at baseline in subjects who walked <200 meters versus ≥200 meters in 6 minutes; in both groups, GNEM-FAS total, mobility, UE, and self-care scores decreased from baseline through month 36. CONCLUSIONS: These findings demonstrate progressive decline in muscle strength in GNE myopathy and provide insight into the appropriate tools to detect clinically meaningful changes in future GNE myopathy interventional trials.

Patient-Reported Outcomes from a Randomized, Active-Controlled, Open-Label, Phase 3 Trial of Burosumab Versus Conventional Therapy in Children with X-Linked Hypophosphatemia.

Changing to burosumab, a monoclonal antibody targeting fibroblast growth factor 23, significantly improved phosphorus homeostasis, rickets, lower-extremity deformities, mobility, and growth versus continuing oral phosphate and active vitamin D (conventional therapy) in a randomized, open-label, phase 3 trial involving children aged 1-12 years with X-linked hypophosphatemia. Patients were randomized (1:1) to subcutaneous burosumab or to continue conventional therapy. We present patient-reported outcomes (PROs) from this trial for children aged ≥ 5 years at screening (n = 35), using a Patient-Reported Outcomes Measurement Information System (PROMIS) questionnaire and SF-10 Health Survey for Children. PROMIS pain interference, physical function mobility, and fatigue scores improved from baseline with burosumab at weeks 40 and 64, but changed little with continued conventional therapy. Pain interference scores differed significantly between groups at week 40 (- 5.02, 95% CI - 9.29 to - 0.75; p = 0.0212) but not at week 64. Between-group differences were not significant at either week for physical function mobility or fatigue. Reductions in PROMIS pain interference and fatigue scores from baseline were clinically meaningful with burosumab at weeks 40 and 64 but not with conventional therapy. SF-10 physical health scores (PHS-10) improved significantly with burosumab at week 40 (least-squares mean [standard error] + 5.98 [1.79]; p = 0.0008) and week 64 (+ 5.93 [1.88]; p = 0.0016) but not with conventional therapy (between-treatment differences were nonsignificant). In conclusion, changing to burosumab improved PRO measures, with statistically significant differences in PROMIS pain interference at week 40 versus continuing with conventional therapy and in PHS-10 at weeks 40 and 64 versus baseline.Trial registration: ClinicalTrials.gov NCT02915705.

Growth Curves for Children with X-linked Hypophosphatemia.

CONTEXT: We characterized linear growth in infants and children with X-linked hypophosphatemia (XLH). OBJECTIVE: Provide linear growth curves for children with XLH from birth to early adolescence. DESIGN: Data from 4 prior studies of XLH were pooled to construct growth curves. UX023-CL002 was an observational, retrospective chart review. Pretreatment data were collected from 3 interventional trials: two phase 2 trials (UX023-CL201, UX023-CL205) and a phase 3 trial (UX023-CL301). SETTING: Medical centers with expertise in treating XLH. PATIENTS: Children with XLH, 1-14 years of age. INTERVENTION: None. MAIN OUTCOME MEASURE: Height-for-age linear growth curves, including values for the 5th, 10th, 25th, 50th, 75th, 90th, and 95th percentiles for children with XLH compared to population norms. RESULTS: A total of 228 patients (132 girls, 96 boys) with 2381 height measurements were included. Nearly all subjects (> 99%) reported prior management with supplementation therapy. Compared to the Center for Disease Control and Prevention growth curves, boys at age 3 months, 6 months, 9 months, 1 year, and 2 years had median height percentiles of 46%, 37%, 26%, 18%, and 5%, respectively; for girls the median height percentiles were 52%, 37%, 25%, 18%, and 7%, respectively. Annual growth in children with XLH fell below that of healthy children near 1 year of age and progressively declined during early childhood, with all median height percentiles < 8% between 2 and 12 years old. CONCLUSION: Children with XLH show decreased height gain by 1 year of age and remain below population norms thereafter. These data will help evaluate therapeutic interventions on linear growth for pediatric XLH.

The Lifelong Impact of X-Linked Hypophosphatemia: Results From a Burden of Disease Survey.

CONTEXT: X-linked hypophosphatemia (XLH) is characterized by excess fibroblast growth factor 23 (FGF23), hypophosphatemia, skeletal abnormalities, and growth impairment. We aimed to understand the burden of disease of XLH across the lifespan. METHODS: Responses were collected from adults with XLH and parents/caregivers of a child with XLH in an online survey, including multiple-choice and open-ended questions on demographics, disease manifestations, treatment history, assistive device use, and age-specific patient-reported outcomes (PROs). RESULTS: Data were collected from 232 adults with XLH (mean age, 45.6 years; 76% female) and 90 parents/caregivers of a child with XLH (mean age, 9.1 years; 56% female). Mean age recalled for symptom onset was 3.2 years for adults and 1.3 years for children. When surveyed, nearly all children (99%) and 64% of adults were receiving oral phosphate, active vitamin D, or both. Prior participation in a trial investigating burosumab, a fully human monoclonal antibody against FGF23, was reported in 3% of children and 10% of adults; of these respondents, only one child reported current treatment with burosumab at the time of the survey. Both children and adults reported typical features of XLH, including abnormal gait (84% and 86%, respectively), bowing of the tibia/fibula (72% and 77%), and short stature (80% and 86%). Nearly all adults (97%) and children (80%) reported bone or joint pain/stiffness. Adults reported a history of fractures (n/N = 102/232; 44%), with a mean (SD) age at first fracture of 26 (16) years. Adults reported osteophytes (46%), enthesopathy (27%), and spinal stenosis (19%). Mean scores for PROs evaluating pain, stiffness, and physical function were worse than population norms. Analgesics were taken at least once a week by 67% of adults. CONCLUSIONS: Despite the common use of oral phosphate and active vitamin D established in the 1980s, children with XLH demonstrate a substantial disease burden, including pain and impaired physical functioning that persists, as demonstrated by similar responses reported in adults with XLH.

A phase 3 randomized study evaluating sialic acid extended-release for GNE myopathy.

OBJECTIVE: To investigate the efficacy and safety of aceneuramic acid extended-release (Ace-ER), a treatment intended to replace deficient sialic acid, in patients with GNE myopathy. METHODS: UX001-CL301 was a phase 3, double-blind, placebo-controlled, randomized, international study evaluating the efficacy and safety of Ace-ER in patients with GNE myopathy. Participants who could walk ≥200 meters in a 6-minute walk test at screening were randomized 1:1, and stratified by sex, to receive Ace-ER 6 g/d or placebo for 48 weeks and assessed every 8 weeks. The primary endpoint was change in muscle strength over 48 weeks measured by upper extremity composite (UEC) score. Key secondary endpoints included change in lower extremity composite (LEC) score, knee extensor strength, and GNE myopathy-Functional Activity Scale (GNEM-FAS) mobility domain score. Safety assessments included adverse events (AEs), vital signs, and clinical laboratory results. RESULTS: Eighty-nine patients were randomized (Ace-ER n = 45; placebo n = 44). Change from baseline to week 48 for UEC score between treatments did not differ (least square mean [LSM] Ace-ER -2.25 kg vs placebo -2.99 kg; LSM difference confidence interval [CI] 0.74 [-1.61 to 3.09]; p = 0.5387). At week 48, there was no significant difference between treatments for the change in key secondary endpoints: LEC LSM difference (CI) -1.49 (-5.83 to 2.86); knee extension strength -0.40 (-2.38 to 1.58); and GNEM-FAS mobility domain score -0.72 (-2.01 to 0.57). Gastrointestinal events were the most common AEs. CONCLUSIONS: Ace-ER was not superior to placebo in improving muscle strength and function in patients with GNE myopathy. CLASSIFICATION OF EVIDENCE: This study provides Class I evidence that for patients with GNE myopathy, Ace-ER does not improve muscle strength compared to placebo.

Burosumab versus conventional therapy in children with X-linked hypophosphataemia: a randomised, active-controlled, open-label, phase 3 trial.

BACKGROUND: X-linked hypophosphataemia in children is characterised by elevated serum concentrations of fibroblast growth factor 23 (FGF23), hypophosphataemia, 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 paediatric X-linked hypophosphataemia. 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 and Kyowa Kirin International.

Rickets severity predicts clinical outcomes in children with X-linked hypophosphatemia: Utility of the radiographic Rickets Severity Score.

The Rickets Severity Score (RSS) was used to evaluate X-linked hypophosphatemic rickets (XLH), a genetic disorder mediated by increased circulating FGF23. The reliability of the RSS was assessed using data from a randomized, phase 2 clinical trial that evaluated the effects of burosumab, a fully human anti-FGF23 monoclonal antibody, in 52 children with XLH ages 5 to 12 years. Bilateral knee and wrist radiographs were obtained at baseline, week 40, and week 64. We evaluated the relationships of the RSS to the Radiographic Global Impression of Change (RGI-C), serum alkaline phosphatase (ALP), height Z-score, 6-minute walk test (6MWT) percent predicted, and the Pediatric Orthopedic Society of North America Pediatric Outcomes Data Collection Instrument (POSNA-PODCI). The RSS showed moderate-to-substantial inter-rater reliability (weighted kappa, 0.45-0.65; Pearson correlation coefficient (r), 0.83-0.89) and substantial intra-rater reliability (weighted Kappa, 0.66; r = 0.91). Baseline RSS correlated with serum ALP (r = 0.47). Baseline RSS identified two subgroups (higher [RSS ≥1.5] and lower RSS [RSS <1.5]) that discriminated between subjects with greater and lesser rachitic disease. Higher RSS was associated with more severe clinical features, including impaired growth (Z-score, -2.12 vs -1.44) and walking ability (6MWT percent predicted, 77% vs 86%), more severe self-reported pain (29.9 [more severe] vs 45.3 [less severe]) and less physical function (29.6 [more severe] vs 40.9 [less severe]). During burosumab treatment, greater reductions in RSS corresponded to higher RGI-C global scores (r = -0.65). Improvements in RSS correlated with decreased serum ALP (r = 0.47). These results show the reliability of the RSS in XLH, and demonstrate that higher RSS values are associated with greater biochemical, clinical, and functional impairments in children with XLH.

Efficacy and safety of burosumab in children aged 1-4 years with X-linked hypophosphataemia: a multicentre, open-label, phase 2 trial.

BACKGROUND: Children with X-linked hypophosphataemia have high concentrations of circulating phosphatonin fibroblast growth factor 23 (FGF23), which causes renal phosphate wasting and hypophosphataemia, rickets, skeletal deformities, and growth impairment. Burosumab, a human monoclonal antibody against FGF23, improves phosphate homoeostasis and rickets in children aged 5-12 years with X-linked hypophosphataemia. We aimed to assess the safety and efficacy of burosumab in younger children with X-linked hypophosphataemia. METHODS: In this open-label, phase 2 trial at three hospitals in the USA, children (aged 1-4 years) with X-linked hypophosphataemia received burosumab (0·8 mg/kg) via subcutaneous injection every 2 weeks for 64 weeks. The dose was increased to 1·2 mg/kg if two consecutive pre-dose serum phosphorus concentrations were below 1·03 mmol/L (3·2 mg/dL), serum phosphorus had increased by less than 0·16 mmol/L (<0·5 mg/dL) from baseline, and a dose of burosumab had not been missed. Participants could continue to receive burosumab for up to an additional 96 weeks during the extension period. Key inclusion criteria were age 1-4 years at the time of informed consent; fasting serum phosphorus concentration of less than 0·97 mmol/L (3·0 mg/dL); serum creatinine 8·8-35·4 µmol/L (0·1-0·4 mg/dL); radiographic evidence of rickets (at least five participants were required to have a Thacher Rickets Severity Score of ≥1·5 at the knee); and a confirmed PHEX mutation or a variant of unknown significance in the patient or direct relative also affected with X-linked hypophosphataemia. Conventional therapy was stopped upon enrolment. The coprimary endpoints were safety and change from baseline to week 40 in fasting serum phosphorus concentrations. Changes in rickets severity from baseline to weeks 40 and 64 (assessed radiographically using Thacher Rickets Severity Score and an adaptation of the Radiographic Global Impression of Change), and recumbent length or standing height, were key secondary outcomes. This trial is registered with ClinicalTrials.gov, number NCT02750618, and is ongoing. FINDINGS: Between May 16, 2016, and June 10, 2016, we enrolled 13 children with X-linked hypophosphataemia. All 13 children completed 64 weeks of treatment and were included in the efficacy and safety analysis; none exceeded 70 weeks of treatment at the time of analysis. Serum phosphorus least squares mean increase from baseline to week 40 of treatment was 0·31 mmol/L (SE 0·04; 95% CI 0·24-0·39; 0·96 mg/dL [SE 0·12]; p<0·0001). All patients had at least one adverse event. 14 treatment-related adverse events, mostly injection site reactions, occurred in five children. One serious adverse event considered unrelated to treatment (tooth abscess) occurred in a child with a history of tooth abscess. All other adverse events were mild to moderate, except a severe food allergy considered unrelated to treatment. No instances of nephrocalcinosis or noteworthy changes in the results of a standard safety chemistry panel emerged. Total Thacher Rickets Severity Score decreased by a least squares mean of -1·7 (SE 0·1; p<0·0001) from baseline to week 40 and by -2·0 (SE 0·1; p<0·0001) by week 64. The Radiographic Global Impression of Change score also indicated significant improvement, with a least squares mean score of +2·3 (SE 0·1) at week 40 and +2·2 (0·1) at week 64 (both p<0·0001). Mean length or standing height Z score was maintained from baseline to week 64. INTERPRETATION: Burosumab had a favourable safety profile, increased serum phosphorus, and improved rickets and prevented early declines in growth in children aged 1-4 years with X-linked hypophosphataemia. These findings could substantially alter the treatment of young children with X-linked hypophosphataemia. FUNDING: Ultragenyx Pharmaceutical and Kyowa Kirin International.

Qualitative Research to Explore the Patient Experience of X-Linked Hypophosphatemia and Evaluate the Suitability of the BPI-SF and WOMAC® as Clinical Trial End Points.

BACKGROUND: X-linked hypophosphatemia (XLH) is a rare genetic disorder characterized by renal phosphate wasting and defective bone mineralization. Symptoms include bone pain, joint pain, stiffness, and fatigue. Published evidence regarding the patient experience of XLH is sparse and no XLH-specific outcome measures have been validated. OBJECTIVES: To understand the symptoms, impacts, and patient experience of XLH and to evaluate the face and content validity of the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC®) and the Brief Pain Inventory Short Form (BPI-SF) for use as end points in XLH clinical trials. METHODS: Face-to-face, qualitative, semistructured interviews were conducted with 18 adults with XLH in the United States using concept elicitation and cognitive debriefing techniques. Open-ended questioning elicited spontaneous concepts focusing on XLH-associated symptoms and functional limitations. Cognitive debriefing of the WOMAC® and BPI-SF assessed the relevance and patient understanding of item wording, recall period, and response options. RESULTS: Various distinct symptom concepts were elicited including pain symptoms, dental symptoms, sensory symptoms, tiredness/fatigue symptoms, and musculoskeletal symptoms. Participants reported experiencing significant bone and joint pain, stiffness, mobility limitations, and an impact on their ability to work. Cognitive interviewing found both instruments to be relevant and well understood by most patients. CONCLUSIONS: The interviews generated rich, qualitative insights into the patient experience of XLH. Cognitive debriefing of the BPI-SF and WOMAC® supported their value as XLH clinical trial end points. Future research will assess the psychometric properties of these instruments for use in the XLH population.

Burosumab Therapy in Children with X-Linked Hypophosphatemia.

BACKGROUND: 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. (Funded by Ultragenyx Pharmaceutical and Kyowa Hakko Kirin; ClinicalTrials.gov number, NCT02163577 ; EudraCT number, 2014-000406-35 ).

Phenotypic stratification and genotype-phenotype correlation in a heterogeneous, international cohort of GNE myopathy patients: First report from the GNE myopathy Disease Monitoring Program, registry portion.

GNE myopathy is a rare distal myopathy, caused by mutations in the GNE gene, affecting sialic acid synthesis. Clinical presentation varies from asymptomatic early stage patients to severely debilitating forms. This first report describes clinical presentations and severity of the disease, using data of 150 patients collected via the on-line, patient-reported registry component of the GNE Myopathy Disease Monitoring Program (GNEM-DMP). Disease progression was prospectively analysed, over a 2-year period, using the GNE myopathy functional activity scale (GNEM-FAS). The average annual rates of decline in function were estimated at -9.6% and -3.2% in ambulant and non-ambulant patients respectively. 4.3% of participants became non-ambulant within one year. The mean time from onset to required use of a wheelchair was 11.9 years. Mean delay of genetic diagnosis from symptom onset was 5.2 years. Mutation specific analysis demonstrated genotype-phenotype relationships; i.e. p.Ala662Val may be associated with a more severe phenotype, compared to p.Val727Met. Patients with compound heterozygous mutation in epimerase and kinase domain appeared to have a more severe phenotype compared to patients with both mutations located within one domain. Acknowledging the limitations of the study, these findings suggest that the severity of the GNE mutations affects disease severity. The GNEM-DMP is a useful data collection tool, prospectively measuring the progression of GNE myopathy, which could play an important role in translational and clinical research and further understanding of genotype-phenotype correlations.

Development and preliminary evidence of the psychometric properties of the GNE myopathy functional activity scale.

AIM: GNE myopathy, a rare, severe, progressive myopathy, presents with lower extremity distal muscle weakness. The GNE myopathy functional activity scale (GNEM-FAS) evaluates the impact of GNE myopathy on functioning in adults. This paper presents the psychometric validation of the GNEM-FAS. PATIENTS & METHODS: Validation of the GNEM-FAS was performed using data from a randomized, double-blind, placebo-controlled Phase-II study (n = 46). RESULTS: Domain score distributions were acceptable. Moderate inter-item correlations (typical range, 0.40-0.70), strong item convergent and discriminant validity and high internal consistency reliability (α = 0.88-0.92) supported the instrument structure. Test-retest reliability was strong (ICC range: 0.87-0.95). Scale scores distinguished among subjects with differing disease severity (p < 0.05). CONCLUSION: This study provides preliminary evidence of the GNEM-FAS as a valid, reliable assessment.

Characterization of Strength and Function in Ambulatory Adults With GNE Myopathy.

OBJECTIVE: To characterize the pattern and extent of muscle weakness and impact on physical functioning in adults with GNEM. METHODS: Strength and function were assessed in GNEM subjects (n = 47) using hand-held dynamometry, manual muscle testing, upper and lower extremity functional capacity tests, and the GNEM-Functional Activity Scale (GNEM-FAS). RESULTS: Profound upper and lower muscle weakness was measured using hand-held dynamometry in a characteristic pattern, previously described. Functional tests and clinician-reported outcomes demonstrated the consequence of muscle weakness on physical functioning. CONCLUSIONS: The characteristic pattern of upper and lower muscle weakness associated with GNEM and the resulting functional limitations can be reliably measured using these clinical outcome assessments of muscle strength and function.

Substantial deficiency of free sialic acid in muscles of patients with GNE myopathy and in a mouse model.

GNE myopathy (GNEM), also known as hereditary inclusion body myopathy (HIBM), is a late- onset, progressive myopathy caused by mutations in the GNE gene encoding the enzyme responsible for the first regulated step in the biosynthesis of sialic acid (SA). The disease is characterized by distal muscle weakness in both the lower and upper extremities, with the quadriceps muscle relatively spared until the late stages of disease. To explore the role of SA synthesis in the disease, we conducted a comprehensive and systematic analysis of both free and total SA levels in a large cohort of GNEM patients and a mouse model. A sensitive LC/MS/MS assay was developed to quantify SA in serum and muscle homogenates. Mean serum free SA level was 0.166 µg/mL in patients and 18% lower (p<0.001) than that of age-matched control samples (0.203 µg/mL). In biopsies obtained from patients, mean free SA levels of different muscles ranged from 0.046-0.075 µg/µmol Cr and were markedly lower by 72-85% (p<0.001) than free SA from normal controls. Free SA was shown to constitute a small fraction (3-7%) of the total SA pool in muscle tissue. Differences in mean total SA levels in muscle from patients compared with normal controls were less distinct and more variable between different muscles, suggesting a small subset of sialylation targets could be responsible for the pathogenesis of GNEM. Normal quadriceps had significantly lower levels of free SA (reduced by 39%) and total SA (reduced by 53%) compared to normal gastrocnemius. A lower SA requirement for quadriceps may be linked to the reported quadriceps sparing in GNEM. Analysis of SA levels in GneM743T/M743T mutant mice corroborated the human study results. These results show that serum and muscle free SA is severely reduced in GNEM, which is consistent with the biochemical defect in SA synthesis associated with GNE mutations. These results therefore support the approach of reversing SA depletion as a potential treatment for GNEM patients.

Aceneuramic Acid Extended Release Administration Maintains Upper Limb Muscle Strength in a 48-week Study of Subjects with GNE Myopathy: Results from a Phase 2, Randomized, Controlled Study.

BACKGROUND: GNE Myopathy (GNEM) is a progressive adult-onset myopathy likely caused by deficiency of sialic acid (SA) biosynthesis. OBJECTIVE: Evaluate the safety and efficacy of SA (delivered by aceneuramic acid extended-release [Ace-ER]) as treatment for GNEM. METHODS: A Phase 2, randomized, double-blind, placebo-controlled study evaluating Ace-ER 3 g/day or 6 g/day versus placebo was conducted in GNEM subjects (n = 47). After the first 24 weeks, placebo subjects crossed over to 3 g/day or 6 g/day for 24 additional weeks (dose pre-assigned during initial randomization). Assessments included serum SA, muscle strength by dynamometry, functional assessments, clinician- and patient-reported outcomes, and safety. RESULTS: Dose-dependent increases in serum SA levels were observed. Supplementation with Ace-ER resulted in maintenance of muscle strength in an upper extremity composite (UEC) score at 6 g/day compared with placebo at Week 24 (LS mean difference +2.33 kg, p = 0.040), and larger in a pre-specified subgroup able to walk ≥200 m at Screening (+3.10 kg, p = 0.040). After cross-over, a combined 6 g/day group showed significantly better UEC strength than a combined 3 g/day group (+3.46 kg, p = 0.0031). A similar dose-dependent response was demonstrated within the lower extremity composite score, but was not significant (+1.06 kg, p = 0.61). The GNEM-Functional Activity Scale demonstrated a trend improvement in UE function and mobility in a combined 6 g/day group compared with a combined 3 g/day group. Patients receiving Ace-ER tablets had predominantly mild-to-moderate AEs and no serious adverse events. CONCLUSIONS: This is the first clinical study to provide evidence that supplementation with SA delivered by Ace-ER may stabilize muscle strength in individuals with GNEM and initiating treatment earlier in the disease course may lead to better outcomes.

Neurocognitive clinical outcome assessments for inborn errors of metabolism and other rare conditions.

Well-defined and reliable clinical outcome assessments are essential for determining whether a drug provides clinically meaningful treatment benefit for patients. In 2015, FDA convened a workshop, "Assessing Neurocognitive Outcomes in Inborn Errors of Metabolism." Topics covered included special challenges of clinical studies of inborn errors of metabolism (IEMs) and other rare diseases; complexities of identifying treatment effects in the context of the dynamic processes of child development and disease progression; and the importance of natural history studies. Clinicians, parents/caregivers, and participants from industry, academia, and government discussed factors to consider when developing measures to assess treatment outcomes, as well as tools and methods that may contribute to standardizing measures. Many issues examined are relevant to the broader field of rare diseases in addition to specifics of IEMs.