Novel Oral Iron Therapies for Iron Deficiency Anemia in Chronic Kidney Disease

Anemia is a common feature of CKD associated with poor outcomes. The current management of patients with anemia in CKD is controversial, with recent clinical trials demonstrating increased morbidity and mortality related to erythropoiesis stimulating agents. Here, we examine recent insights into the molecular mechanisms underlying anemia of CKD. These insights hold promise for the development of new diagnostic tests and therapies that directly target the pathophysiologic processes underlying this form of anemia.

The increasing prevalence of multiple comorbidities among anemic patients with chronic kidney disease has made the use of serum ferritin and transferrin saturation more challenging in diagnosing iron deficiency. Because serum ferritin is an acute-phase reactant and because the inflammatory state may inhibit the mobilization of iron from reticuloendothelial stores, the scenario of patients with serum ferritin >800 ng/ml, suggesting iron overload, and transferrin saturation <20%, suggesting iron deficiency, has become more common. This article revisits the basis for the Kidney Disease Outcomes Quality Initiative recommendations regarding the use of serum ferritin and transferrin saturation in guiding iron therapy, then explores some of the newer alternative markers for iron status that may be useful when serum ferritin and transferrin saturation are insufficient. These newer tests include reticulocyte hemoglobin content, percentage of hypochromic red cells, and soluble transferrin receptor, all of which have shown some promise in limited studies. Finally, the role of hepcidin, a hepatic polypeptide, in the pathophysiology of iron mobilization is reviewed briefly.

Despite fluctuations in dietary iron intake and intermittent losses through bleeding, the plasma iron concentrations in humans remain stable at 10-30 μM. While most of the iron entering blood plasma comes from recycling, appropriate amount of iron is absorbed from the diet to compensate for losses and maintain nontoxic amounts in stores. Plasma iron concentration and iron distribution are similarly regulated in laboratory rodents. The hepatic peptide hepcidin was identified as the systemic iron-regulatory hormone. In the efferent arc, hepcidin regulates intestinal iron absorption, plasma iron concentrations, and tissue iron distribution by inducing degradation of its receptor, the cellular iron exporter ferroportin. Ferroportin exports iron into plasma from absorptive enterocytes, from macrophages that recycle the iron of senescent erythrocytes, and from hepatocytes that store iron. In the more complex and less well understood afferent arc, hepatic hepcidin synthesis is transcriptionally regulated by extracellular and intracellular iron concentrations through a molecular complex of bone morphogenetic protein receptors and their iron-specific ligands, modulators and iron sensors. Through as yet undefined pathways, hepcidin is also homeostatically regulated by the iron requirements of erythroid precursors for hemoglobin synthesis. In accordance with the role of hepcidin-mediated iron redistribution in host defense, hepcidin production is regulated by inflammation as well. Increased hepcidin concentrations in plasma are pathogenic in iron-restrictive anemias including anemias associated with inflammation, chronic kidney disease and some cancers. Hepcidin deficiency causes iron overload in hereditary hemochromatosis and ineffective erythropoiesis. Hepcidin, ferroportin and their regulators represent potential targets for the diagnosis and treatment of iron disorders and anemias. This article is part of a Special Issue entitled: Cell Biology of Metals. Copyright © 2012 Elsevier B.V. All rights reserved.