Background: X-linked hypophosphatemia (XLH) causes rickets in children and osteomalacia in adults due to lifelong renal phosphate wasting that is mediated by high circulating levels of FGF-23. Burosumab, is a recently approved fully human monoclonal antibody that blocks FGF23, thereby correcting the renal phosphate leak, improving mineral metabolism and reducing osteomalacia by 50-75% in adults . Whether this results in measurable changes in skeletal mass and microarchitecture is unclear. Objective: We examined the impact of burosumab on regional bone mineral density (BMD) and trabecular bone scores (TBS) in study subjects involved in two phase III clinical trials of burosumab.
Methods: In these trails subjects received burosumab 1 mg/kg every 4 weeks. Some patients received placebo for the first 6 months of one trial so we considered their month 6 data as their baseline. Most of the patients had been treated at some point in the past with calcitriol and phosphorus. DXA and TBS were obtained at baseline and then after 6, 12 and 18-24 months of drug treatment. Paired t-tests and ANOVA were performed to assess changes in L-spine BMD, Total Hip BMD and TBS.
Results: 25 subjects with XLH (mean age 38.9 years, 56% female) were enrolled in these studies. Paired data were available in 23 subjects at 6 months, 15 subjects at 12 months and 18 subjects at 18-24 months. Compared to baseline, there were significant increases in L-spine BMD at all time points by paired analysis: 6 months (+6.0%, p=<0.0001), 12 months (+6.95%, p=<0.0001), 18-24 months (+6.13%, p=0.0005). Although there was no significant difference in total hip BMD at 6 months when compared to baseline, there were significant increases at 12 months (+6.72%, p=0.0005) and a further increase at 18-24 months (+10.02%, p=0.0029). When all available subjects were analyzed by one-way ANOVA, there was a significant effect of time of treatment on these regional BMD measurements. There was no change in trabecular bone score over the course of treatment.
Conclusion: Treatment with burosumab is associated with a marked improvement in BMD, particularly in the hip. Since the hip is a frequent site of fracture in XLH, the effect of burosumab at this site is of considerable clinical relevance. The lack of an effect on TBS may relate to the fact that this measurement is much less sensitive to therapeutic interventions than BMD assessed by DXA.