Burosumab improves physical function in X-linked hypophosphatemia

Burosumab improves physical function, reduces pain in X-linked hypophosphatemia Key takeaways: - Patients with X-linked hypophosphatemia reported reduced pain and stiffness with burosumab at 3 years. - No improvements in any questionnaire items were seen with vitamin D and phosphate salts. CHICAGO — Burosumab is tied to greater improvements in pain, stiffness and physical function than active vitamin D and phosphate salts among patients with X-linked hypophosphatemia, according to data presented at ENDO 2026. Burosumab-twza (Crysvita, Kyowa Kirin), a monoclonal anti-fibroblast growth factor 23 antibody, was approved by the FDA as the first therapy to treat X-linked hypophosphatemia, a rare form of rickets, in 2018. In data from a prospective observational study, researchers assessed how burosumab impacted quality of life among people with X-linked hypophosphatemia compared with treatment with vitamin D or phosphate salts in a real-world setting. “This analysis provides insights into burosumab’s impact on activities of daily living across three [Western Ontario and McMaster Universities Osteoarthritis Index] domains in 3 years,” Erik A. Imel, MD, professor of medicine and pediatrics at Indiana University School of Medicine, said during a presentation. “Treatment of burosumab did improve in the domains of pain, stiffness and physical function compared to phosphate and vitamin D.” Researchers analyzed data from the X-linked Hypophosphatemia Disease Monitoring Program from July 2018 to February 2025. Data were collected from 54 participants receiving burosumab (mean age, 40 years; 70.4% women) and 55 patients receiving active vitamin D and phosphate salts (mean age, 39.5 years; 81.8% women). Follow-up data for the burosumab group was collected beginning 1 year after drug initiation. Researchers used the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) questionnaire to assess self-reported pain, stiffness and physical function, with higher scores indicating worse symptoms. Follow-up continued for up to 3 years. At baseline, the burosumab groups had higher scores for pain (mean score, 39 points vs. 28.8 points; P = .031) and stiffness (mean score, 51.2 points vs. 39.1 points; P = .023) than the vitamin D with phosphate salts group. No difference was seen in total score or physical function score. Pain scores decreased by 9.8 points at 3 years with burosumab compared with a 3.3-point increase for the vitamin D and phosphate salt group (P < .001). Patients receiving burosumab had a 15.2-point decrease in stiffness score at 3 years compared with a 3.7-point decline for those receiving vitamin D and phosphate salts (P = .016). Physical function scores dropped by 10 points at 3 years compared with a 3.3-point increase with vitamin D or phosphate salts (P = .001). The burosumab group had a larger increase in serum phosphate than the vitamin D and phosphate salts group at 1 year that persisted at 3 years. When individual questions on the WOMAC questionnaire were assessed, patients receiving burosumab had improvements in 17 of the 24 items, while no improvements were seen with vitamin D and phosphate salts. When responses to individual questions were classified into five categories ranging from “none” to “severe,” Imel noted that most participants receiving burosumab had a reduction in symptom severity over time on multiple items. “Pain walking on a flat surface, you can see a notable shift in the distribution toward the lower scores in the patients on burosumab, and there was not a similar change in distribution with the phosphate and vitamin D scores,” Imel said. “For pain standing upright, again, you have a similar change in shifting toward the zero to two grade scores overall with burosumab, and no significant change with phosphate and vitamin D. It’s going to be a big theme here.” Imel said some of the questionnaire items in which burosumab was not associated with improvements were going up and down stairs, heavy chores, nocturnal symptoms, and putting on or taking off socks or stockings. Imel added that one of the study limitations was that individual item scores were not compared between the burosumab and vitamin D and phosphate groups.