Chris W. Olsen, DVM, MS; Lars Lykke Thomsen, MD, PhD, DMSc
Pharmacosmos Inc, Watchung, New Jersey.

Regarding the research article by Morales et al1 in the July and August 2018 issue of the Journal of Swine Health and Production, we challenge the relevance of the pharmacokinetic (PK) measures used in the study and their basis for drawing any conclusion to the relative bioavailability of injectable iron products for swine.

Based on the observation that gleptoferron had a higher serum iron area under the curve (AUC) in comparison to iron dextran, the authors have concluded that gleptoferron is more bioavailable. However, as reported in human literature,

Standard approaches are not appropriate when assessing pharmacokinetics of iron supplements due to the ubiquity of endogenous iron, its compartmentalized sites of action, and the complexity of the iron metabolism. The primary site of action of iron is the erythrocyte, and, in contrast to conventional drugs, no drug-receptor interaction takes place. Notably, the process of erythropoiesis, i.e., formation of new erythrocytes, takes 3−4 weeks (in humans). Accordingly, serum iron concentration and area under the curve (AUC) are clinically irrelevant for assessing iron utilization.2

To understand why this is the case and consequently why the conclusions by Morales et al are invalid, we must understand the basic principles of iron absorption and transport as described by Crichton3 and Geisser and Burckhardt.2 Iron-carbohydrate complexes injected intramuscularly are initially taken up from the injection site by macrophages of the reticuloendothelial system where the iron molecule is gradually separated from the carbohydrate carrier. Within the macrophage, iron is either bound in the iron-carbohydrate complex or by the iron-storage protein ferritin for transient storage; or it is exported to the blood where it is bound to transferrin, the protein responsible for transportation of iron molecules through the serum to their site of utilization or storage. The iron binding capacity of transferrin is very limited, on the order of approximately 0.1% of the total body iron content, and if overwhelmed, can lead to labile iron in the serum. Free or labile iron is highly toxic hence the importance of iron transport and storage being tightly controlled.4 Upon complete absorption and incorporation into the body’s iron storage, iron functions as a reserve that is gradually transported to the bone marrow for use in hemoglobin synthesis in a process that takes several days to weeks. It is the availability of storage iron that is critical to the rise in hemoglobin over several weeks observed in this and essentially all other studies on injectable iron products, not the short-term peak in serum iron observed within the first day by Morales et al.

We contend that bioavailability measured as serum iron concentration is not a relevant measure based on the early history of parenteral iron therapy dating back to the 1930s.5 Researchers at Harvard Medical School injected adult humans with iron salts, ie, free iron not bound by a complex. The authors concluded that

Iron in doses of 16 to 32 mgm (mg) a day, given parenterally, is very close to the maximum amount of iron tolerated by man. It is attended by disagreeable symptoms, sometimes severe and possibly dangerous.

This illustrates that even low doses of free iron is toxic and, in a sense, too bioavailable. The ideal parenteral iron preparation ensures slow gradual release of the iron so as not to overwhelm the natural transport and storage mechanisms.

The basic flaw made by Morales et al is failure to consider the pharmacodynamics of the products they studied in determining which PK variables to measure and how to interpret them. Specifically, their conclusion that a larger AUC of serum iron for gleptoferron equates to a higher bioavailability is unfounded based on the previously stated principles of iron storage and transport. The higher serum iron concentrations for gleptoferron than for iron dextran should be considered from a safety stand point as it could suggest a higher risk of saturating the transferrin binding capacity leading to free iron toxicity. Further, Morales et al did not report whether they measured free- or total-serum iron, which is important when considering the toxic effects of free iron.

When considering the clinically relevant hematologic parameters such as red blood cell count and hemoglobin concentration, the levels reported in the iron dextran group were at or above those of the gleptoferron product. We contend that the values at the end of the study are of primary interest and emphasize that no differences were found for any hematologic parameter between the two products.

Thus, a reasonable interpretation of the study results would be that the two products show some differences in their PK profile, but that they result in similar efficacy. If any more substantial conclusion were to be made, it would be that iron dextran is more available to ferritin, the body’s natural iron stores, and that the higher serum iron concentrations associated with gleptoferron could be associated with increased risk of free or labile iron reactions, which should be further investigated.

References

1. Morales J, Manso A, Martin-Jimenez T, Karembe H, Sperling D. Comparison of the pharmacokinetics and efficacy of two different iron supplementation products in suckling piglets. J Swine Health Prod. 2018;26(4):200-207.

2. Geisser P, Burckhardt S. The pharmacokinetics and pharmacodynamics of iron preparations. Pharmaceutics. 2011;3(1):12-33

3. Crichton R. Mammalian iron metabolism and dietary iron absorption. In: Crichton R. Iron Metabolism: From Molecular Mechanisms to Clinical Consequences. 4th ed. Chichester, United Kingdom: John Wiley & Sons, Ltd; 2016:247-264.

*4. Albretsen, J. Toxicology brief: The toxicity of iron, an essential element. Dvm360 Web site. http://veterinarymedicine.dvm360.com/toxicology-brief-toxicity-iron-essential-element. Published February 1, 2006. Accessed July 8, 2018.

5. Heath CW, Strauss MB, Castle WB. Quantitative aspects of iron deficiency in hypochromic anemia (The parenteral administration of iron). J Clin Invest. 1932;11(6):1293-1312.

* Non-refereed reference.

Author response: Comparison of the pharmacokinetics and efficacy of two different iron supplementation products in suckling piglets

Daniel Sperling, DVM, PhD; Hamadi Karembe, DVM, MSc; Joaquin Morales, DVM, PhD; Albert Manso, DVM; Tomás Martín-Jiménez, DVM, PhD, DACVCP, DipECVPT
DS, HK: Ceva Sante Animale, Libourne, France. JM, AM: PigCHAMP Pro Europa SL, Segovia, Spain. TM-J: University of Tennessee, Knoxville, Tennessee.

We thank the authors of the Letter to the Editor (Olsen and Thomsen) for their interest in our article published recently in this journal.1 We would like to address the questions they have raised.

Relevance of selected method

The bioavailability of the intravenous route is 100% per definition and the intestinal absorption of iron and its retention is tightly regulated via homeostasis. Therefore, pharmacokinetics or bioavailability assessment is less relevant for oral and intravenous iron, the main routes of application used in humans.2 Note that the human medicine literature2 quoted in Olsen and Thomsen’s letter to the Editor focuses on oral and intravenous application, not intramuscular (IM) application.

Serum or plasma iron and area under the curve are relevant for assessing the iron utilization, or pharmacodynamics, following IM application. This is highlighted in the guidance documents issued by the regulatory authorities in the United States3 (Food and Drug Administration) and Europe4 (European Medicines Agency).

Increase from the baseline is commonly used for assessment of endogenous compounds as it gives the real effect of the hematinic product as the primary outcomes (eg, hemoglobin (Hb) and hematocrit) are linked to the initial values. In the case of the Morales et al study,1 all hematological parameters, including Hb values, were not significantly different at study day 0 (P = .15), therefore the increases can be compared. Only red blood cell counts were significantly different (P = .01), but remained in the normal range during the entire study.

Piglets are born with low iron storage and without early and efficient iron supplementation they become anemic within the first week of life.5 It is therefore important that plasma iron is rapidly incorporated into the immature red cells5 and that iron stores are replenished (bone marrow, liver, and spleen). Rapid absorption and utilization of IM iron are both important for the prevention of the iron deficiency in piglets. It is also known that the iron remaining at the injected site 72 hours after injection will remain trapped and not available for Hb synthesis.6,7 This non-absorbed iron will deposit in the connective tissue stroma and associated macrophages, resulting in unacceptable muscle or skin staining.7 Slow and incomplete absorption of iron might be one of the frequently discussed reasons of the sub-anemic status of piglets at weaning and consequently the need for a second iron injection.8

The importance of absorption, or pharmacokinetics, on the efficacy, or pharmacodynamics, of different IM iron formulations is well documented in early literature dealing with the discovery and development of proper iron complex formulations for IM application.6,7,9,10

Studies of the metabolic fate of iron dextran in animals and humans have shown that the material follows a distinct pathway after intravenous or IM injection.11 After IM injection, the iron complex is first cleared into the reticulo-endothelial system (RES). Within phagocytes, iron is released from the iron-carbohydrate compound and either incorporated by ferritin into intracellular iron stores or released from the cell to be taken up by the extracellular iron-binding protein, transferrin. Transferrin delivers iron to transferrin receptors on the surface of erythroid precursors for Hb synthesis and maturation of the red blood cell.

The well-established safety of gleptoferron

High molecular weight iron polysaccharide complexes are very stable and well tolerated allowing, for the first time, delivery of large intravenous doses without saturation of transferrin and toxicity.12 In addition, the IM route is less prone to iron overload than intravenous iron, as the injected iron complex is taken up and processed by the RES and transferred gradually to the erythroid precursor and storage organs.

Gleptoferron-based products have good safety records. Intramuscular application of the standard iron dose (200 mg/piglet) showed optimal transferrin saturation, which remains within the normal physiological range.13,14

References

1. Morales J, Manso A, Martín-Jiménez T, Karembe H, Sperling D. Comparison of the pharmacokinetics and efficacy of two different iron supplementation products in suckling piglets. J Swine Health Prod. 2018;26(4):200-207.

2. Geisser P, Burckhardt S. The pharmacokinetics and pharmacodynamics of iron preparations. Pharmaceutics. 2011;3(1):12-33.

*3. US Food and Drug Administration. Draft Guidance on Iron Dextran. https://www.fda.gov/downloads/Drugs/GuidanceComplianceRegulatoryInformation/Guidances/UCM520240.pdf. Published October 2016. Accessed October 2018.

*4. European Medicines Agency. Reflection paper on the data requirements for intravenous iron-based nano-colloidal products developed with reference to an innovator medicinal product. EMA/CHMP/SWP/620008/2012. https://www.ema.europa.eu/documents/scientific-guideline/reflection-paper-data-requirements-intravenous-iron-based-nano-colloidal-products-developed_en.pdf. Published March 26, 2015. Accessed October 2018.

5. Venn JA, Mccance RA, Widdowson EM. Iron metabolism in piglet anaemia. J Comp Pathol Ther. 1947;57(4):314-325.

6. Beresford CR, Golberg L, Smith JP. Local effects and mechanism of absorption of iron preparations administered intramuscularly. Br J Pharmacol Chemother. 1957;12(1):107-114.

7. Bossaller W. Absorption and local residues of Myofer 100 in the piglet. Blue Book for Veterinary Profession. 1971;9:18-21.

8. Bhattarai S, Nielsen JP. Early indicators of iron deficiency in large piglets at weaning. J Swine Health Prod. 2015;23:10-17.

9. Furugouri K. Kinetics in iron metabolism in piglets. J Anim Sci. 1974;38(6):1249-1256.

10. Braude R, Chamberlain AG, Kotarbinska M, Mitchell KG. The metabolism of iron in piglets given labelled iron either orally or by injection. Br J Nutr. 1962;16:427-449.

11. Henderson PA, Hillman RS. Characteristics of iron dextran utilization in man. Blood. 1969;34(3):357-375.

12. Avni T, Bieber A, Grossman A, Green H, Leibovici L, Gafter-Gvili A. The safety of intravenous iron preparations: systematic review and meta-analysis. Mayo Clin Proc. 2015;90(1):12-23.

*13. Sperling D, Lorencova A, Leva L, Trckova M, Faldyna M, Krejci R. Effectiveness of iron-dextrans and gleptoferron (Gleptosil) on iron serum biochemistry in piglets. Proc IPVS and ESPHM. Dublin, Ireland. 2016:360.

14. Pollmann DS, Smith JE, Stevenson JS, Schoneweis DA, Hines RH. Comparison of gleptoferron with iron dextran for anemia prevention in young pigs. J Anim Sci. 1983; 56:640-644.

* Non-refereed references.