The differential effect of modern intravenous iron on fibroblast growth factor 23 and phosphate in non-dialysis dependent CKD – the exploratory randomized controlled double-blind ExplorIRON-CKD study
Authors: Kassianides Xenophon , Bhandari Sunil 
Published: February 2024

Abstract
Background
Intravenous iron is commonly used in patients with non-dialysis-dependent chronic kidney disease (CKD). Modern intravenous iron compounds (e.g. ferric derisomaltose (FDI), ferric carboxymaltose (FCM)) are increasingly utilized with similar efficacy. A differential effect in terms of hypophosphatemia has been noted following administration of FCM, which may be related to fibroblast growth factor 23 (FGF23). This study was designed to examine the comparative effects of FDI and FCM on FGF23, phosphate and other markers of bone turnover.

Methods
The single-center double-blind randomized controlled trial “Iron and Phosphaturia – ExplorIRON-CKD” primarily assessed the effects of FCM and FDI on intact FGF23 and phosphate, whilst also studying the impact on vitamin D, parathyroid hormone and phosphaturia. Bone markers including alkaline phosphatase, bone-specific alkaline phosphatase, procollagen type 1 N-terminal propeptide and carboxy-terminal collagen cross-linked telopeptide were monitored. Non-dialysis-dependent CKD patients (stage 3a-5) with iron deficiency with/without anemia (serum ferritin < 200 µg/L or transferrin saturation = 20% and serum ferritin 200-299 µg/L) were randomized to receive FDI or FCM in a 1:1 ratio. At baseline 1000 mg of intravenous iron was administered followed by 500-1000 mg at 1 month to achieve replenishment. Measurements were performed at baseline, 1–2 days following iron administration, 2 weeks, 1 month (second iron administration), 1–2 days following second administration, 2 months and 3 months following initial infusion.

Results
Twenty-six patients participated in the trial; 14 randomized to FDI and 12 to FCM. Intact FGF23 increased following administration of iron, and the increase was significantly higher with FCM compared to FDI (Baseline to 1–2 days following 1st administration: FDI: 3.0 (IQR: - 15.1 - 13.8) % vs. FCM: 146.1 (IQR: 108.1–203.1) %; p < 0.001 and Baseline to 1–2 days following 2nd administration: FDI: 3.2 (IQR: - 3.5 – 25.4) % vs. FCM: 235.1 (138.5–434.6) %; p = 0.001). Phosphate levels decreased in the FCM group, causing a significant difference versus FDI 2 weeks following administration of the first dose. A significantly greater decrease in 1,25 (OH)2 Vitamin D was noted with FCM. Several markers of bone turnover significantly changed following administration of FCM but not FDI.

Conclusions
The study suggests a differential effect on FGF23 following administration of FCM compared to FDI in non-dialysis-dependent CKD patients, similar to other patient groups. This may lead to changes consistent with hypovitaminosis D and alterations in bone turnover with potential clinical consequences. Further definitive studies are required to understand these differences of intravenous iron compounds.

Trial registration
European Union Drug Regulating Authorities Clinical Trials Database (EudraCT) number: 2019–004370-26 (https://www.clinicaltrialsregister.eu/ctr-search/trial/2019-004370-26/GB) (First date of trial registration: 03/12/2019).