SUN-759 Real-World Outcomes in Late Adolescence Among Individuals With X-linked Hypophosphatemia (XLH): Results From the XLH Disease Monitoring Program

Abstract Disclosure: A.A. Khan: Ascendis, Takeda, Amolyt, Bridge Bio, Alexion Pharmaceuticals, Inc. E.A. Imel: Kyowa Kirin, Ultragenyx. L.M. Ward: Kyowa Kirin, Ultragenyx. T.O. Carpenter: Kyowa Kirin, Ultragenyx. J. Simmons: Kyowa Kirin, Ultragenyx. P.F. Florenzano: Kyowa Kirin, Ultragenyx. J. Li: Kyowa Kirin. S. Tewari: KMK Consulting Inc. S. Sader: Ultragenyx. M. Vincent: Kyowa Kirin. H. Cassinelli: Kyowa Kirin, Ultragenyx. Introduction: Changes in dosing schedules/treating physicians and other life events may affect disease management in persons with X-linked hypophosphatemia (XLH) in late adolescence. To understand the impact of this phase of life on disease progression and management, we describe the longitudinal outcomes of adolescent participants (pts) who reconsented as young adults to the XLH Disease Monitoring Program (DMP; NCT03651505), a multi-country observational study. Methods: The analysis included adolescents treated with burosumab who enrolled in the DMP age <18 years and reconsented as adults. Outcome measures were collected per the DMP visit schedule. Rickets Severity Score (RSS), Rickets Radiographic Global Impression of Change (RGI-C), biochemical and anthropometric z-scores (serum phosphate, serum alkaline phosphatase [ALP], height, weight), Tanner staging, and Patient-Reported Outcomes Measurement Information System (PROMIS) scores were collected pre-reconsent; Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) scores were gathered post-reconsent. Results: Forty-seven pts were included. Mean ± standard deviation (SD) age was 15.5 ± 1.5 years (yrs) at DMP enrollment (baseline [BL]), mean ± SD time to reconsent following enrollment was 3.5 ± 1.1 yrs, and mean ± SD burosumab treatment duration was 3.3 ± 1.1 yrs. At latest visit pre-reconsent, mean ± SD RSS decreased from BL (0.34 ± 0.55 to 0.14 ± 0.35), and Rickets RGI-C change from BL was +1.67 ± 0.93. Mean ± SD weight, height, and height z-scores were 72.3 ± 19.5 kg, 162.3 ± 10.3 cm, and −1.69 ± 1.29 for male pts, and 62.1 ± 14.7 kg, 150.9 ± 7.9 cm, and −1.87 ± 1.23 for female pts, at latest visit pre-reconsent. Tanner stage ≥4 was exhibited in 63.8% of pts at latest visit pre-reconsent. At the latest visit pre-reconsent, mean ± SD serum phosphate was 3.02 ± 0.51 mg/dL (z-score: −1.62 ± 0.74), compared to a BL of 3.08 ± 0.75 mg/dL (z-score: −1.58 ± 0.99). Mean ± SD serum ALP was 151.43 ± 94.48 U/L (z-score: −0.09 ± 1.40), compared to a BL of 236.11 ± 126.60 U/L (z-score: 1.19 ± 1.52). PROMIS outcomes (mobility, pain interference, fatigue) were stable from BL to pre-reconsent. At latest visit post-reconsent, mean ± SD WOMAC scores were pain: 9.7 ± 13.8; stiffness: 13.04 ± 20.1; and physical function: 4.49 ± 7.95. XLH provider change was reported in 9 pts (n=4) post-reconsent. To date, 17/47 (36.2%) pts have switched to the adult monthly dosing schedule post-reconsent. Conclusion: Biochemical parameters and patient-reported outcomes were generally stable for older adolescents treated with burosumab, suggesting that consistent disease management, as anticipated in the DMP, was in place at a time when many persons may otherwise be lost to follow-up. As the DMP evolves, further analyses will be performed to incorporate additional pts and longer follow-up times to enhance the understanding of outcomes during late adolescence. Presentation: Sunday, July 13, 2025