Intravenous iron preparations transiently generate non-transferrin-bound iron from two proposed pathways

Intravenous iron-carbohydrate complex preparations (IVIP) are noninterchangeable pro-drugs: their pharmacokinetics (PK) varies determined by semi-crystalline iron core and carbohydrate shell structures, influences pharmacodynamics (PD) and thus efficacy and safety. Examining PK/PD relationships of three IVIP we identify a two-pathway model of transient non-transferrin-bound iron (NTBI) generation following single dose administration. Twenty-eight hypoferremic non-anemic patients randomized to 200 mg iron as ferric carboxymaltose (Fe-carboxymaltose), iron sucrose (Fe-sucrose), iron isomaltoside 1000 (Fe-isomaltoside- 1000), n=8/arm, or placebo, n=4, on a 2-week PK/PD study, had samples analysed for total serum iron, IVIP-iron, transferrin-bound iron (TBI) by high-performance liquid chromatography in combination with inductively coupled plasma mass spectrometry (HPLC-ICP-MS), transferrin saturation (TSAT), serum ferritin (s-Ferritin) by standard methods, NTBI and hepcidin as published before. IVIP-dependent increases in these parameters returned to baseline in 48-150 hours (h), except for s-Ferritin and TSAT. NTBI was low with Fe-isomaltoside-1000 (0.13 μM at 8 h), rapidly increased with Fe-sucrose (0.8 μM at 2 h, 1.25 μM at 4 h), and delayed for Fe-carboxymaltose (0.57 μM at 24 h). NTBI area-under-curve (AUC) were 7-fold greater for Fe-carboxymaltose and Fe-sucrose than for Fe-isomaltoside-1000. Hepcidin peak time varied, but not AUC or mean levels. s-Ferritin levels and AUC were highest for Fe-carboxymaltose and greater than placebo for all IVIP. We propose two mechanisms for the observed NTBI kinetics: rapid and delayed NTBI appearance consistent with direct (circulating IVIP-to-plasma) and indirect (IVIP-to-macrophage-to-plasma) iron release based on IVIP plasma half-life and s-Ferritin dynamics. IVIP generate different, broadly stability- and PK-dependent, NTBI and s-Ferritin signatures, which may influence iron bioavailability, efficacy and safety. Longer-term studies should link NTBI exposure to subsequent safety and efficacy parameters and potential clinical consequences.