Results from a 24-Week Burosumab Treatment Continuation Period After a 24-Week Double-Blind Placebo-Controlled Period. This study aimed to report the impact of continued CRYSVITA® treatment on biochemical profile, patient-reported outcomes, radiological profile and safety in adults with X-linked hypophosphatemia who continued from a 24-week phase 3 study into a 24-week open-label extension. 134 participants had received up to 24 weeks’ further subcutaneous CRYSVITA® treatment in the open-label extension study. Primary endpoint was the fasting serum phosphorus concentration at the end of each dose cycle. Seven secondary efficacy endpoints including 3 patient-reported outcomes were defined. Safety data were obtained throughout the period of CRYSVITA® exposure, including the extension period. Baseline demographic, clinical, and radiographic characteristics were similar between the treatment groups. Seventy participants had a total of 156 fractures or pseudofractures. Osteoarthritis was present in 63.4% of participants, and enthesopathy was present in nearly all participants from both groups. 94.1% in the CRYSVITA® group achieved and sustained a mean serum phosphate concentration above the lower limit of normal compared to only 7.6% in the placebo group (p<0.001) at Week 24. After participants switched from placebo to CRYSVITA® at Week 24, serum phosphorus concentrations promptly increased and remained at or above the LLN until Week 48. CRYSVITA® increased 1,25(OH)2D from baseline through Week 48. CRYSVITA® increased mean phosphate reabsorption from baseline to week 48 compared to the placebo group. CRYSVITA® treatment significantly improved stiffness score, measured by WOMAC compared with placebo. CRYSVITA® significantly reduced pain, measured by BPI Worst Pain, from baseline compared with placebo. CRYSVITA® significantly improved mobility showed by increases in mean distance walked, assessed by the 6 minute walk test, from baseline compared with placebo. CRYSVITA® treatment demonstrated improvement in 80% of baseline fractures and pseudofractures, with 63.1% achieving complete healing and 16.9% showing partial healing at Week 48 in the CRYSVITA® - CRYSVITA® group. CRYSVITA® demonstrated an acceptable safety profile, as indicated by an overall incidence, nature, and severity of treatment-emergent adverse events that was similar to placebo. To conclude, continued treatment with CRYSVITA® in adults with XLH is associated with significant efficacy and an acceptable safety profile through 48 weeks.