Abstract Background Mucopolysaccharidosis VII (MPS VII) is an ultra-rare, autosomal recessive, debilitating, progressive lysosomal storage disease caused by reduced activity of β-glucuronidase (GUS) enzyme. Vestronidase alfa (recombinant human GUS) intravenous enzyme replacement therapy is an approved treatment for patients with MPS VII. Methods This disease monitoring program (DMP) is an ongoing, multicenter observational study collecting standardized real-world data from patients with MPS VII (N ≈ 50 planned) treated with vestronidase alfa or any other management approach. Data are monitored and recorded in compliance with Good Clinical Practice guidelines and planned interim analyses of captured data are performed annually. Here we summarize the safety and efficacy outcomes as of 17 November 2022. Results As of the data cutoff date, 35 patients were enrolled: 28 in the Treated Group and seven in the Untreated Group. Mean (SD) age at MPS VII diagnosis was 4.5 (4.0) years (range, 0.0 to 12.4 years), and mean (SD) age at DMP enrollment was 13.9 (11.1) years (range, 1.5 to 50.2 years). Ten patients (29%) had a history of nonimmune hydrops fetalis. In the 23 patients who initiated treatment prior to DMP enrollment, substantial changes in mean excretion from initial baseline to DMP enrollment were observed for the three urinary glycosaminoglycans (uGAGs): dermatan sulfate (DS), -84%; chondroitin sulfate (CS), -55%; heparan sulfate (HS), -42%. Also in this group, mean reduction from initial baseline to months 6, 12, and 24 were maintained for uGAG DS (-84%, -87%, -89%, respectively), CS (-70%, -71%, -76%, respectively), and HS (+ 3%, -32%, and − 41%, respectively). All adverse events (AEs) were consistent with the known vestronidase alfa safety profile. No patients discontinued vestronidase alfa. One patient died. Conclusions To date, the DMP has collected invaluable MPS VII disease characteristic data. The benefit-risk profile of vestronidase alfa remains unchanged and favorable for its use in the treatment of pediatric and adult patients with MPS VII. Reductions in DS and CS uGAG demonstrate effectiveness of vestronidase alfa to Month 24. Enrollment is ongoing.

Long-chain fatty acid oxidation disorders (LC-FAOD) are a group of rare, autosomal-recessive disorders of fatty acid metabolism in the mitochondria. LC-FAOD may manifest in infants but also commonly presents in older children or adults following periods of increased metabolic demand, including physiologic stress, fasting, and prolonged exercise. Disease presentation can be heterogeneous, unpredictable, and serious, including life-threatening episodes of rhabdomyolysis, hypoglycemia, and cardiomyopathy.

Excess FGF23 causes hypophosphatemia, leading to chronic debilitating musculoskeletal impairments in patients with X-linked hypophosphatemia (XLH). Treatments include burosumab, a fully-human monoclonal antibody to FGF23, or the combination of oral phosphate and active vitamin D (Pi/D). XLH symptoms in adults include bone/joint pain, stiffness, and fatigue. Previous research validated the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) in adults with XLH. This study assesses burosumab impact on WOMAC among adults from an XLH Disease Monitoring Program (XLH-DMP; NCT03651505).

Abstract X-linked hypophosphatemia (XLH) is a rare, heritable disorder wherein excess FGF23 leads to renal phosphate wasting and impaired activation of vitamin D. XLH is characterized by rickets and osteomalacia leading to bone pain, skeletal deformities, and short stature. The XLH Disease Monitoring Program (DMP; NCT03651505) is a prospective, multinational outcomes study of the longitudinal progression of clinical, radiographic, and biochemical features of XLH in children and adults. We assessed the impact of the recombinant human IgG1 monoclonal antibody to FGF23, burosumab, on measures of bone health in children with XLH at Year 1 of the DMP. As of March 2021, 287 children (<18 years) had completed the Year 1 visit. Participants were grouped by when they first received burosumab: Group 1, prior to DMP enrollment (N=68; mean (SD) age 9.5 (3.6) years); Group 2, during the DMP (N=150; 8.7 (5.0) years); Group 3, never, but receiving conventional therapy (N=69; 8.8 (5.3) years). Before receiving burosumab, serum phosphorus levels were below normal; after burosumab initiation, serum phosphorus in Groups 1 and 2 rapidly normalized, with sustained improvements up to 3 years of treatment. Serum phosphorus remained low in Group 3. Before burosumab, serum alkaline phosphatase levels were above normal; with continued burosumab therapy, mean levels gradually declined towards normal and were maintained for up to 4 years. The Radiographic Global Impression of Change (RGI-C) assessed rickets and lower limb deformity from DMP baseline to Year 1, with positive values denoting improvement. Mean (SD) RGI-C rickets global scores were +1.0 (0.9) for Group 1 (n=44), +1.5 (0.7) for Group 2 (n=39), and +0.3 (1.2) for Group 3 (n=3). Lower limb deformity RGI-C scores were +0.3 (0.6) for Group 1 (n=42), +0.4 (0.8) for Group 2 (n=39), and +0.8 (1.1) for Group 3 (n=3). Height Z-score change from DMP baseline at Year 1 was 0.1 (0.3) for Group 1 (n=58), 0.1 (0.4) for Group 2 (n=90), and -0.8 (3.1) for Group 3 (n=18). There were no new safety concerns or reports of hyperphosphatemia. These real-world data affirm that normalization of serum phosphorus is rapid and sustained following burosumab initiation, resulting in improved serum alkaline phosphatase levels which positively impact bone mineralization. Improvements in rickets, growth, and lower limb deformity occur in the first year of burosumab treatment and continue in those who started burosumab prior to entering the DMP. The absence of new safety concerns or reports of hyperphosphatemia up to 4 years after burosumab initiation is also notable. The 10-year DMP study design will provide understanding of long-term effects in these patients with continued burosumab therapy. Presentation: Sunday, June 12, 2022 11:15 a.m. - 11:30 a.m.

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.

Abstract Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome wherein small tumors secrete excess fibroblast growth factor 23 (FGF23) leading to hypophosphatemia, osteomalacia, bone pain, fractures, and muscle weakness. Burosumab is an anti-FGF23 monoclonal antibody approved for treatment of TIO. We report end of study (EOS) data from the open-label phase 2 UX023T-CL201 trial (NCT02304367) of burosumab in adults with TIO. In UX023T-CL201, participants received burosumab (0.3–2.0 mg/kg) by subcutaneous injection every 4 weeks for 48 weeks with an extension up to 300 weeks (∼6 years). Co-primary endpoints were change in serum phosphorus at Week 24 and change in excess osteoid by trans-iliac crest bone biopsy at Week 48. Additional endpoints included changes in bone biomarkers, activity on radionucleotide bone scan representing fractures/pseudofractures, patient-reported outcomes, and safety measures. This analysis included 14/17 UX023T-CL201 participants; excluding those with other diagnoses (n=3). Mean (SD) serum phosphorus improved from 1.6 (0.5) mg/dL at baseline to 2.3 (0.5) mg/dL when averaged across the mid-point of the dose interval through Week 24 (co-primary endpoint). Levels further improved (2.6 [0.4] mg/dL; n=7) to within the normal range (2.5–3.6 mg/dL) at Week 240. Most measures of osteomalacia improved at Week 48 (co-primary endpoint), including osteoid/bone volume, osteoid thickness, and mineralization lag time. However, osteoid surface/bone surface showed no change. Bone biomarkers (P1NP, CTx, BALP) followed similar trends. Upon initiation of burosumab, markers briefly increased by Week 24 as bone healing began, then gradually decreased with further treatment, reaching normal (P1NP, CTx) or near-normal (BALP) ranges by Week 240. At baseline, 249 areas of increased radionucleotide uptake indicating fracture activity were detected across 14 participants. By Week 144, 36% (72/201) and 13% (26/201) regions of abnormal uptake were fully or partially restored, respectively. By EOS, further reduction of active fractures and fewer new areas of uptake were observed. New fractures at Week 144 were observed in a single participant (considered a burosumab partial responder) and between Week 144 and EOS in another participant unrelated to burosumab (car accident). Participants reported sustained mean reduction from baseline pain at Week 240 per Brief Pain Inventory and significantly per SF-36 Bodily Pain score. Significant reductions in baseline fatigue at Week 240 were reported per Brief Fatigue Inventory and SF-36 Vitality scores. SF-36 physical health (Role Physical, Bodily Pain, Component Score) and mental health (Social Function, Vitality) significantly improved from baseline at Week 240. The observed safety profile of long-term burosumab in participants with TIO was consistent with the known safety profile of burosumab and similar to patients with XLH. Long-term burosumab therapy was associated with improved serum phosphorus, osteomalacia, bone turnover, fracture incidence and healing, health-related quality of life, and reductions in pain and fatigue in adults with TIO. Presentation: Sunday, June 12, 2022 11:00 a.m. - 11:15 a.m.