SAT-283 Tumor-Induced Osteomalacia Disease Monitoring Program: A Longitudinal Cohort Study on An Ultra-Rare Disease

Abstract

Tumor-induced osteomalacia (TIO) is rare disorder usually caused by excess FGF23 produced by mesenchymal tumors resulting in renal phosphate wasting, hypophosphatemia, and decreased 1,25(OH)2D levels, with consequent osteomalacia, fractures and/or pseudofractures, fatigue, musculoskeletal pain and weakness. Burosumab, a fully human monoclonal antibody against FGF23, is FDA-approved for the treatment of TIO when complete surgical removal or localization of the tumor is not possible. The TIO Disease Monitoring Program (DMP) is a prospective, multicenter, observational study describing the disease course and long-term safety and efficacy of burosumab and other treatments in patients with TIO for up to 10 years (NCT04783428), beginning January 31, 2022. Subjects are grouped for analysis by burosumab treatment status as of the data cutoff date (June 2023): Group 1: previously received burosumab as part of a clinical trial, Group 2: started prescription burosumab treatment prior to or during the DMP, Group 3: burosumab-naïve. If subjects in Group 3 initiate burosumab treatment during the DMP, they transition into Group 2. This analysis includes 20 adult subjects enrolled in the study from 4 sites in the US, and 1 in Argentina: Group 1: N=5, Group 2: N=3, Group 3: N=12. The median (range) age of subjects is 59 (41, 69) years in Group 1, 54 (19, 68) years in Group 2, and 48 (28, 70) years in Group 3. One (20%), 2 (67%), and 9 (75%) subjects are female in Groups 1, 2, and 3, respectively. All subjects are white, and 0, 2 (67%), and 8 (67%) subjects are Hispanic or Latino in Groups 1, 2, and 3, respectively. As of June 2023, the mean time from initial TIO diagnosis to enrollment in the study was 5.6 years (range: 0-28 years). The most common clinical manifestations reported by subjects were bone pain (n=19, 95%), muscle weakness (n=15, 75%), difficulty walking (n=14, 70%), fatigue (n=12, 60%), and fractures (n=11, 55%). Of the 8 subjects who were receiving burosumab, the mean dose was 60 mg every 4 weeks (range: 30-90 mg). From the time of enrollment, subjects had received burosumab for a mean of 30.5 weeks (range: 0.1 to 51.1 weeks) as of June 2023. As the TIO DMP progresses and new data are collected, the study will provide a greater understanding of the progression of TIO, and the efficacy and safety of burosumab and conventional therapies. The novel approach of the DMP may ameliorate issues associated with traditional patient registries and increase the knowledge base of this and other rare diseases.