Iron in PET studies
Iron is essential cofactor in enzymes of electron transport chain and DNA synthesis and repair, and in transport of oxygen. This is due to its transition between ferrous (Fe2+) and ferric (Fe3+) redox states. Iron-containing enzymes are needed for reactive oxygen species (ROS) production in granulocytes, but free iron ions would cause excessive ROS production. Free Fe3+ hydrolyses rapidly to insoluble hydroxides in aqueous solutions. Therefore iron in the body is kept bound to transporter molecules, such as transferrin, and its amount in circulation is tightly regulated (Evstatiev & Gasche, 2012; Finberg, 2013; Duck & Connor, 2016). Ceruloplasmin is a plasma protein that transports Cu2+, but also has two iron-binding sites and functions as ferroxidase, converting Fe2+ to Fe3+. Tight sequestering of iron also has a role in the innate immune system called iron withholding: it creates an environment low in free iron, for example in mucosa, thus preventing bacterial and fungal growth. Only members of the Lactobacillus and Bacillus families can survive without iron.
Labile plasma iron (LPI) includes exchangeable and redox active iron that causes oxidative stress and does not bind to transferrin. Soluble iron-phosphate complex is one form of LPI. Inflammatory conditions may lead to LPI even when transferrin is not saturated and plasma iron levels are low (Matias et al., 2017). Citrate binds iron ions, and albumin binds both iron ions and citrate, which facilitates Fe3+ binding to transferrin (Matias et al., 2017).
Most of the iron is delivered to bone marrow and incorporated into new erythrocytes. Reticuloendothelial cells in the liver collect iron mainly by phagocytosis of senescent erythrocytes. Ferritin protein complexes act as intracellular Fe3+ carrier and storage. Ferritin tends to aggregate over time, and the aggregates are degraded by lysosomes into hemosiderin, which is an agglomerate of denatured protein, lipids, and iron oxide. Hemosiderin functions as a long-term iron storage, mainly in the liver, spleen, and bone marrow.
52Fe is a cyclotron produced positron emitter with half-life of 8.28 h. Branching ratio is 0.561, that is, β+ particle is produced in 56.1% of decay events of this isotope. It has a positron-emitting daughter isotope, 52mMn, with half-life of ∼21 min, which complicates the interpretation and quantification of PET scans; in PET studies the isotope is often referred to as 52Fe/52mMn.
For blood and ex vivo tissue samples, the radioactivity concentration can be corrected for the impact of 52mMn by following the decay of the sample. For intracellularly trapped 52Fe also the daughter isotope can be assumed to stay trapped, and activity can be corrected by the physical equilibrium between 52Fe and 52mMn (Beshara et al., 1999; Lubberink et al., 1999). 52mMn is rapidly cleared from blood, with highest uptake in the liver and myocardium (Buck et al., 1996). Calonder et al (1999) showed that the influence of 52mMn on PET data can be neglected, except for the first minutes after injection.
Other iron isotopes
Hepcidin is a peptide hormone that regulates iron balance in the body. Hepcidin inhibits absorption of iron from the GI system and the release of iron from the liver and spleen, thus reducing the concentration of iron in the circulation (Evstatiev & Gasche, 2012). Intestinal absorption of iron is an important step in regulation of iron balance, because there is no regulated excretory pathways for iron. Inflammation increases the level of hepcidin, and chronic inflammatory diseases may lead to anaemia. (Finberg, 2013; Reichert et al., 2017). Fe2+ enters into duodenal enterocytes via SLC11A2 (divalent metal/cation transporter 1, DMT1). Ferroportin (Fpn) on the basal sides of enterocytes transports iron into blood (Bonaccorsi di Patti et al., 2018).
After intravenous administration of [52Fe]citrate, 52Fe is bound to transferrin in the plasma. In the brain tumours, 52Fe uptake (administered as citrate) reflects the status of the BBB, and not the number of transferrin receptors (Roelcke et al., 1996). Brain uptake data can be analyzed using MTGA for irreversible uptake (Patlak plot) or irreversible 1-tissue compartmental model where only K1 and vascular volume fraction are fitted (Roelcke et al., 1996; Calonder et al., 1999; Bruehlmeyer et al., 2000). Plasma elimination half-life is 89±17 min (Leenders et al., 1993). [52Fe]citrate was used in a clinical study to delineate differences in brain iron metabolism between healthy and Wilson’s disease patients (Bruehlmeyer et al., 2000). Brain uptake is very low, consisting mostly of vascular activity and [52Fe]transferrin bound to endothelial wall transferrin receptors. Yet, a slow irreversible uptake component was also observed, and assumed to represent receptor-mediated transcytosis (Bruehlmeyer et al., 2000).
Distribution of 52Fe-labelled Fe-hydroxide sucrose and polysaccharides, that are used for anaemia treatment, have been studied by PET in animals and humans (Beshara et al., 1999a, 1999b, and 2003). Liver and bone marrow uptake was analysed using Patlak plot.
Many bacteria produce and secrete siderophores, molecules with high affinity for Fe3+, to acquire iron from their environment. Due to their Fe3+-chelating properties, Citrate and iso-citrate are in the core of many bacterial siderophores (Banerjee et al., 2016). Fe3+-siderophore complex is recognized and transported into the bacteria by specific TonB-dependent receptor proteins (also by other bacteria than the ones that secreted the siderophores). Bacterial siderophores and siderophore transporters facilitate bacterial uptake of also other trivalent metal ions, including Al3+, Sc3+, In3+, and Ga3+, as well as divalent metal ions such as Mg2+ Some pathogenic bacteria express transferrin or lactoferrin binding proteins (Banerjee et al., 2016).
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Updated at: 2018-10-04
Created at: 2018-09-30
Written by: Vesa Oikonen