Iron deficiency, the commonest cause of anaemia, is treated by iron supplementation. Oral iron, introduced by the French physician Pierre Blaud in the 19th century is the mainstay of therapy of iron deficiency. Oral iron is inexpensive and safe but is poorly absorbed and causes abdominal adverse effects in 35-59% of the patients. Injectable iron preparations were initially developed for the treatment of iron deficiency in patients intolerant to iron. Their use became more prevalent when it became clear that iron deficiency was a common cause of failure of erythropoiesis stimulating agent therapy and that oral iron was insufficient for this indication. The acceptance of intravenous iron was accelerate by the improved safety profile of the recently introduced parental iron preparation. The threshold for opting for intravenous iron is much lower than it was about two decades ago.
Is it possible to inject an iron salt, say ferric chloride, for iron deficiency, like it is to inject calcium chloride for hypocalcaemia? Iron, unlike calcium or potassium, produces free radicals (oxyradicals) that can damage macromolecules and result in cellular injury. Ferric hydroxide, the first injectable iron preparation used, caused an immediate release of iron in circulation resulting in severe reactions. When an iron oxide, hydroxide or an iron salt is used, only a small dose can be safely administered. One estimate calculated the maximum iron permissible as 8mg/day (this is equivalent of the unbound iron binding capacity of transferrin). It would take about 6 months to bring up haemoglobin at this dose.
About 25mg of iron is delivered each day to the erythroid precursors for haemoglobin synthesis. Iron is a poorly absorbed micronutrient and stores are needed to provide for sudden increase in demand as may be seen in patients with acute blood loss. Given the propensity of iron to cause free radical induced cellular injury, transport and storage systems capable of protecting the body from iron have evolved. Iron is transported bound to transferrin and stored as ferritin and haemosidin. In both the situations iron is bound to apoproteins (apotrasferrin and apoferritin) and this binding does not allow free radical generation.
Erythrocyte destruction takes place in the macrophages and macrophages have systems for the handling, storage and recycling of iron. Injectable iron preparations have a carbohydrate shell that prevents exposure of the plasma to free iron. Injectable iron preparations are taken up by macrophages of the reticuloendothelial system by endocytosis. The iron is released and enters the macrophages iron pool. This iron has a fate similar to iron reaching the macrophage from other sources viz. intestinal absorption and erythrocyte destruction. Binding iron to carbohydrate shell has made it possible to administer as high as dose as 1000mg over as short a time as 15 minutes. This is a 125 fold increase over the amount of iron that can be administered without the carbohydrate shell.
Injectable iron preparations share a common structure. They have a core of iron oxide/hydroxide that is associated with a carbohydrate shell. The carbohydrate shell keeps free iron from entering the plasma and in a way performs the same function as transferrin. The antigenicity of the shell and the strength of binding between the iron core and the carbohydrate shell determines the side effects and the maximum dose per injection.
Preparations of Injectable Iron
The preperations of injectable iron include
- Preparations with a strong association between iron core and the carbohydrate shell
- High-Molecular weight iron dextran
- Low -molecular weight iron dextran
- Ferric carboxymaltose
- Perperations with a weak association between iron core and carbohydrate shell
- Iron Sucrose
- Preperations that have a labile low molecular weight components
- Ferric Gluconate
- Iron-Sorbitol-Citric Acid Complex, Dextrin-Stabilized
Table 1 – parentral iron preparations
Dose and administration
||High moleculer weight dextran
||Should be administered 1 hr after an intravenous test dose (0.5ml over at least 5 minutes) Maximum dose 20mg/kg. Administered as intravenous bolus 100mg/day slowly. A total dose infusion may be given over a prolonged period.
|| Low molecular weight dextan
||Should be administered 1 hr after an intravenous test dose (0.5ml over at least 30 seconds). It may be administered as daily iv bolus of 100mg over 2 mins or a a total dose infusion over 3-4 hours may be administered 20mg/kg
||up to 300mg in a single dose, higher doses have been administered over as a prolonged infusion. May be administered undiluted as an iv bolus 200mg over at least 10 mins or as an infusion in 0.9% saline over 15-60 minutes
||125mg, iv bolus at 12.5mg/min or as a 1 hr infusion in 0.9% saline
||The maximum dose is 20mg/kg (max 1000mg). Doses of 200-500mg should be administered at 100mg/min, doses between 500mg and 1000mg should be administered over 15 minutes. It may be administered as an iv infusion diluted in 0.9% saline
||May be administered as an intravenous bolus dose of 500mg un to three times a week at the rate of 50mg/min or 20mg/kg (maximum 1000mg) in 0.9% saline over 1 hour
||510mg as a iv bolus at the rate of 30mg/sec. A second dose may be administered after 3-8 days
The parental iron preparations differ in the carbohydrate that surrounds the iron core. The therapeutic implications of these differences are:
- Antigenicity: Dextran is antigenic. Patinets may have performed antibodies to dextran or may develop antibodies during therapy. All iron preparations containing dextran can give rise to anaphylaxis. The risk is greatest with high molecular weight iron dextran. Anaphylaxis may also be seen with low molecular weight iron dextran though the risk is lower and has also been reported with ferumoxytol that contains carboxymethyl dextran.
- Maximum dose per injection: All intravenous iron preparations tend to release labile iron. There is an inverse relationship between the amount of labile iron released and the strength of association between the iron core and carbohydrate shell. Higher the molecular weight of the iron preparation, stronger is the association. Labile iron is taken up by transferrin. Non-transferrin bound iron (NTBI) appears when the binding capacity of transferrin is overwhelmed. NTBI is responsible for tissue damage and restricts the dose of a injectable preparation that can be given in a single infusion. Iron preparations may be classified on the basis of strength of the associations between iron core and the carbohydrate shell. Iron dextran, ferric carboxymaltose and ferumoxytol, that have a high molecular weights have a tight binding between iron core and the carbohydrate shell, release up to 2% labile iron. These can be given in a large single dose and are suitable for total dose infusion. Iron sucrose has a weaker association between iron core and carbohydrate shell and has 4-5% labile iron. This limits the amount of drug that can be administered at one time to 300mg. Ferric gluconate is a large molecule but has two types of polymers. Lower weight polymers (about 18,000) have a weaker association between iron and carbohydrate limiting the dose to 125mg per dose.
Dose and Administration
The total dose of parenteral iron is calculated as follows
Total iron dose (mg) = weight (kg) X Hemoglobin deficit X 0.24 + 500
Haemoglobin deficit (g/L) = Target Haemoglobin (g/L) – Actual Haemoglobin (g/L)
- Infusion Related Events: the risk of infusion related events (per million) is 0.6 for iron sucrose, 0.9 for ferric gluconate, 3.3 for low molecular weight dextran iron and 11.3 for high molecular weight iron dextran. Iron sucrose has been safely administered to patients who having sensitivity to iron dextran. Despite the fact that some studies suggest that low molecular weight iron dextran in as safe as iron sucrose, a test dose must be administered before any iron dextran preparation.
- Delayed Reactions to Intravenous Iron: A syndrome charecterized by fever, arthralgia and lymphadenopathy may be seen as a delayed consequence of intravenous iron therapy. Premedication with steroids may reduce the risk of this manifestation.
- Rodolfo Delfini Cançado, Manuel Muñoz. Intarvenous iron therapy: How far have we come? Rev Bras Hematol Hemoter. 2011; 33(6): 461–469.
- Hayat A. Safety Issues With Intravenous Iron Products in the Management of Anemia in Chronic Kidney Disease. Clin Med Res. Dec 2008; 6(3-4): 93–102.
- Peter Geisser and Susanna Burckhardt The Pharmacokinetics and Pharmacodynamics of Iron Preparations. Pharmaceutics. Mar 2011; 3(1): 12–33.