Parenteral Iron: Biochemical Basis of Drug Safety

Iron is essential for almost all organisms, but it can react with oxygen to generate toxic radicals, and so can be harmful. In the cells iron is bound to functional centers of enzymes or stored as ferritin, in plasma it is bound to transferrin. In the storage and transport form iron is not redox--reactive. Aberration of the regulated iron status to both sides can have severe consequences, namely iron overload and iron deficiency. Non transferrin bound iron (NTBI) occurs, among other pathological conditions, in the case of iron overload such as hemochromatosis, but it has also been reported in patiens with anemia of chronic renal failure (end stage renal disease, ESDR) who are under recombinant human erythropoetin(rhEPO) therapy in combination with parenteral iron administration. In this project we will establish an adequate method to exactly determine NTBI in plasma of healthy and ESDR patients which can routinely be used in the clinical laboratory. The data will be collected and evaluated in coordination with other centers of a BIOMED-study. There is scarce information regarding the metabolism and ultimate fate of parenteral iron preparations following infusion. In this project we will study transport and ultimate fate of iron in cultivated cells under conditions of iron therapy. This will include quantitative measurements of the expression, activity and localisation of proteins relevant in iron metabolism (DMT-l, SFT, ferritin, transferrin-receptor and iron regulatory protein) following iron deprivation and iron loading by parenteral. iron preparations. These effects will be compared with effects generated by oxidative stress. In parallel experiments the labile iron pool (LIP) will be measured by fluorescence quenching using calcein as a probe. The expected findings will open the clinical laboratory a new parameter to monitor patients during parenteral iron administration, to find safer dose regiments and we will get more information regarding metabolism of parenteral iron preparations.

Iron is absolutely essential for life, but its uptake and cycling must be tightly regulated. This is important since iron is obviously redox-active and can initiate free-radical reactions, particulary when it is in contact with reactive oxygen metabolites. In the cells, iron is bound to functional centers of enzymes or stored as ferritin, in the plasma it is bound to transferrin. The storage and transport forms are redox-inactive. Abberations of the regulated iron status to either side or within the continous cycle can have severe consequences, namely iron overload or iron deficiency, respectively. Non-transferrin bound iron (NTBI) typically accompanies conditions of iron overload, such as hereditary hemochromatosis, but is also found in the plasma of patients with anemia of endstage renal disease (ESRD) who were treated with recombinant human erythropoietin (rhuEPO) and intravenous iron. The general accepted dogma was that intravenous iron preparations can only be ingested by macrophages and other cells of the reticulo-endothelial system, where they are degraded enzymatically and the iron can be either stored in ferritin, and either via degradation of this storage protein, or directly the iron is loaded on plasma transferrin. In this form it is mainly transported to the erythron where it is used for hemoglobinsynthesis during erythropoiesis. Our experiments with human HepG2-cells showed that not only macrophages of the reticuloendothelial system can take up intravenous iron preparations, and that the liver is a potential target for iv. iron. We could show that all iv. iron preparation in clinical use increase the chelatable labile iron pool (LIP) in HepG2-cells, and that there exists a biochemical correlation between the concentration of intravenous iron and the occurrence of potential toxic intracellular iron (LIP). Using radioactive labelled iv. iron (iron sucrose) and using several methods of native (gradient)electrophoresis we could show that binding to plasmaproteins highly influences iron uptake.