2. Parenteral iron therapy—Parenteral therapy should be reserved for patients with documented iron deficiency who are unable to tolerate or absorb oral iron and for patients with extensive chronic anemia who cannot be maintained with oral iron alone. This includes patients with advanced chronic renal disease requiring hemodialysis and treatment with erythropoietin, various postgastrectomy conditions and previous small bowel resection, inflammatory bowel disease involving the proximal small bowel, and malabsorption syndromes.
The challenge with parenteral iron therapy is that parenteral administration of inorganic free ferric iron produces serious dose-dependent toxicity, which severely limits the dose that can be administered. However, when the ferric iron is formulated as a colloid containing particles with a core of iron oxyhydroxide surrounded by a core of carbohydrate, bioactive iron is released slowly from the stable colloid particles. In the United States, the three traditional forms of parenteral iron are iron dextran, sodium ferric gluconate complex, and iron sucrose. Two newer preparations are available (see below).
Iron dextran is a stable complex of ferric oxyhydroxide and dextran polymers containing 50 mg of elemental iron per milliliter of solution. It can be given by deep intramuscular injection or by intravenous infusion, although the intravenous route is used most commonly. Intravenous administration eliminates the local pain and tissue staining that often occur with the intramuscular route and allows delivery of the entire dose of iron necessary to correct the iron deficiency at one time. Adverse effects of intravenous iron dextran therapy include headache, light-headedness, fever, arthralgias, nausea and vomiting, back pain, flushing, urticaria, bronchospasm, and, rarely, anaphylaxis and death. Owing to the risk of a hypersensitivity reaction, a small test dose of iron dextran should always be given before full intramuscular or intravenous doses are given. Patients with a strong history of allergy and patients who have previously received parenteral iron dextran are more likely to have hypersensitivity reactions after treatment with parenteral iron dextran. The iron dextran formulations used clinically are distinguishable as high-molecular-weight and low-molecular-weight forms. In the United States, the INFeD preparation is a low-molecular-weight form while Dexferrum is a high-molecular-weight form. Clinical data—primarily from observational studies—indicate that the risk of anaphylaxis is largely associated with high-molecular-weight formulations.
Sodium ferric gluconate complex and iron-sucrose complex are alternative parenteral iron preparations. Ferric carboxymaltose is a colloidal iron preparation embedded within a carbohydrate polymer. Ferumoxytol is a superparamagnetic iron oxide nanoparticle coated with carbohydrate. The carbohydrate shell is removed in the reticuloendothelial system, allowing the iron to be stored as ferritin, or released to transferrin. Ferumoxytol may interfere with magnetic resonance imaging (MRI) studies. Thus if imaging is needed, MRI should be performed prior to ferumoxytol therapy or alternative imaging modality used if needed soon after dosing. The U.S. Food and Drug Administration (FDA) has issued a black box warning about risk of potentially fatal allergic reactions associated with the use of ferumoxytol.
For patients treated chronically with parenteral iron, it is important to monitor iron storage levels to avoid the serious toxicity associated with iron overload. Unlike oral iron therapy, which is subject to the regulatory mechanism provided by the intestinal uptake system, parenteral administration—which bypasses this regulatory system—can deliver more iron than can be safely stored. Iron stores can be estimated on the basis of serum concentrations of ferritin and the transferrin saturation, which is the ratio of the total serum iron concentration to the total iron-binding capacity (TIBC).
A. Acute Iron Toxicity