X-Linked hypophosphatemic rickets (XLH), caused by mutations in PHEX, is the most common inherited disorder of renal phosphate wasting. The PHEX mutation results in increased expression of fibroblast growth factor 23 (FGF23) in bone cells, and elevated circulating FGF23 concentrations are reported. Dysregulated circulating FGF23 causes hypophosphatemia, renal phosphate wasting, and inappropriately low or normal 1,25-dihydroxyvitamin D concentrations. In order to control the hypophosphatemia and heal the osteomalacia, XLH is treated with high dose phosphate and calcitriol. This treatment does not address the underlying problem of excess FGF23. Based on in vitro and in vivo models, both calcitriol and phosphate are likely to further increase FGF23 concentrations in treated subjects. FIGURE 1: FGF23 concentrations, phosphate dose and calcitriol dose plotted against time on therapy. Time 0 = initial pretreatment sample. ▲= FGF23, ■ = Calcitriol dose, ● = Phosphate dose Doses of phosphate and calcitriol were generally changed together. Thus, there was high correlation between treatment doses with these two agents (R2 = 0.873). The individual FGF23 concentration patterns are shown in Figure 1, along with corresponding doses of phosphate (mg/kg) and calcitriol (ng/kg). Initial pretreatment sample is time = 0 months. In two subjects, treatment was discontinued for 1- 2 months prior to the last sample (due to hypertensive urgency in one and lack of supply in the other). The FGF23 concentration decreased on the next sample (off treatment) in both patients. Five untreated women did not demonstrate consistent elevation of FGF23 over time. OVERVIEW METHODS SUMMARY Erik A. Imel M.D. 1,2, Linda DiMeglio, M.D 2, Siu L. Hui, Ph.D 1, Thomas O. Carpenter, M.D. 4, & Michael J. Econs M.D. 1,3 1 Departments of Internal Medicine; 2 Pediatrics, & 3 Medical and Molecular Genetics, Indiana University School of Medicine, Indianapolis IN., 4 Department of Pediatrics, Yale University, New Haven, Connecticut The effect of treating X-linked hypophosphatemic rickets on FGF23 Average FGF23 Pretreatment Peak FGF23 on Treatment FGF23 at Highest Phosphate dose FGF23 at Highest Calcitriol dose Children Subject Subject Subject Subject Subject Subject Subject Mean329 ± ± ± ± 1171 Adults Subject Subject Subject Mean216 ± ± ± 801 All subjects Mean295 ± ± ± ± 1050 Age in years Months on Treatment Intact FGF23 (pg/ml) Phosphate Dose (mg/kg/day) Calcitriol Dose (ng/kg/day) Serum Phosphorus (mg/dl) Serum Creatinine (mg/dl) Serum Calcium (mg/dl) PTH (pg/ml) Alkaline Phosphatase (U/L) Subject Subject Subject Subject Subject Subject Subject Subject Subject Subject The study was IRB approved and informed consent was obtained from all subjects. Ten subjects were treated clinically for XLH. The subjects ranged in age from 2 to 30 years (7 children and 3 adults). Adult subjects with a previous history of treatment for XLH had been off therapy at least one year prior to beginning this study. We measured FGF23 concentrations on samples obtained before and after starting treatment with phosphate and calcitriol. Pretreatment, 1-3 samples were obtained. Post- treatment, 1-5 samples were obtained over 0.5 to 3.5 years. Samples were stored at -80ºC. We measured intact FGF23 concentrations using an ELISA that recognizes only full-length FGF23 using monoclonal antibodies (Kainos Laboratories, Tokyo, Japan). FGF23 concentrations were also measured on five adult women with XLH who were not receiving concurrent active treatment for XLH. Additional biochemistries were measured using standard clinical labsoratory methods. FGF23 concentrations were compared with calcitriol and phosphate doses. Table 1: Biochemistry values at baseline and at the time of the peak FGF23 concentration during treatment NIH R01AR42228, T32DK and MO1RR00750, and KL2RR Although our sample size was small, treatment of XLH with phosphate and calcitriol was associated with concurrent increase in FGF23 concentrations. This effect is more strongly associated with the calcitriol dose. FGF23 concentrations did not change in 5 subjects monitored without treatment. Previous studies indicated that current therapy improves the bone disease. However, increasing FGF23 concentrations may make the disease more difficult to manage, or contribute to complications of therapy. Although more study is needed, it may be beneficial to adjust therapy to minimize the effect on FGF23 concentrations. SUPPORT Subject Time in Months FGF23 pg/ml Phosphate mg/kg/d or Calcitriol ng/kg/d Subject Time in Months FGF23 pg/ml Phosphate mg/kg/d or Calcitriol ng/kg/d Time in Months Subject Time in Months FGF23 pg/ml Phosphate mg/kg/d or Calcitriol ng/kg/d Subject Time in Months FGF23 pg/ml Phosphate mg/kg/d or Calcitriol ng/kg/d RESULTS Table 2. FGF23 concentrations before and after treatment with phosphate and calcitriol Pretreatment FGF23 concentrations were elevated (compared to a normal controls range of 20% in 7/10 and by >100% in 3/10 subjects during treatment. FGF23 concentrations correlated with both phosphate doses (p < 0.05) and calcitriol doses (p < 0.01). When controlling for combined treatment, calcitriol dose remained a significant predictor of FGF23 concentrations (p = 0.01), while phosphate dose was no longer significant.