BACKGROUND: Hypophosphatemia can complicate intravenous iron therapy, but no head-to-head trials compared the effects of newer intravenous iron formulations on risks and mediators of hypophosphatemia.METHODS: In a randomized, double-blinded, controlled trial of adults with iron deficiency anemia from February 2016 to January 2017, we compared rates of hypophosphatemia in response to a single FDA-approved course of ferric carboxymaltose (n = 1,000) or ferumoxytol (n = 997). To investigate pathophysiological mediators of intravenous iron-induced hypophosphatemia, we nested within the parent trial a physiological substudy (ferric carboxymaltose, n = 98; ferumoxytol, n = 87) in which we measured fibroblast growth factor 23 (FGF23), calcitriol, and parathyroid hormone (PTH) at baseline and 1, 2, and 5 weeks later.RESULTS: The incidence of hypophosphatemia was significantly higher in the ferric carboxymaltose versus the ferumoxytol group (<2.0 mg/dl, 50.8% vs. 0.9%; <1.3 mg/dl, 10.0% vs. 0.0%; P < 0.001), and hypophosphatemia persisted through the end of the 5-week study period in 29.1% of ferric carboxymaltose-treated patients versus none of the ferumoxytol-treated patients (P < 0.001). Ferric carboxymaltose, but not ferumoxytol, increased circulating concentrations of biologically active FGF23 (mean within-patient percentage change from baseline to week 2 peak: +302.8 ± 326.2% vs. +10.1 ± 61.0%; P < 0.001), which was significantly associated with contemporaneous hypophosphatemia, renal phosphate wasting, and decreased serum calcitriol and calcium, and increased PTH concentrations.CONCLUSIONS: Ferric carboxymaltose rapidly increases biologically active FGF23 in patients with iron deficiency anemia. Paralleling hereditary and other acquired syndromes of hypophosphatemic rickets/osteomalacia, ferric carboxymaltose-induced FGF23 elevation triggers a pathophysiological cascade of renal phosphate wasting, calcitriol deficiency, and secondary hyperparathyroidism that frequently culminates in hypophosphatemia.TRIAL REGISTRATION: ClinicalTrials.gov, NCT02694978FUNDING. AMAG Pharmaceuticals, Inc.Role of the funding source: This study was supported by AMAG Pharmaceuticals, Inc. The academic investigators designed the clinical trial, performed the analyses, and authored the manuscript with input from the coauthors from AMAG Pharmaceuticals, Inc.
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