Current Medicinal Chemistry - Volume 12, Issue 23, 2005

Effective new therapies and mechanisms have been developed for the targeting and prevention of iron overload and toxicity in thalassaemia and idiopathic haemochromatosis patients. A new era in the development of chelating drugs began with the introduction of deferiprone or L1, which as a monotherapy or in combination with deferoxamine can be used universally for effective chelation treatments, rapid iron removal, maintenance of low iron stores and prevention of heart and other organ damage caused by iron overload. Several experimental iron chelators such as deferasirox (4-[3,5-bis (2-hydroxyphenyl)-1,2,4-triazol-1-yl]-benzoic acid) or ICL670, deferitrin (4,5-dihydro-2- (2,4-dihydroxyphenyl)-4-methylthiazole-4 (S)-carboxylic acid) or GT56- 252, 1-allyl-2-methyl-3-hydroxypyrid-4-one or L1NAll and starch deferoxamine polymers have reached different stages of clinical development. The lipophilic ICL670, which can only be administered once daily is generally ineffective in causing negative iron balance but is effective in reducing liver iron. It is suspected that it may increase iron absorption and the redistribution of iron from the liver to the heart and other organs. The experimental iron chelators do not appear to have significant advantages in efficacy and toxicity by comparison to deferiprone, deferoxamine or their combination. However, the prospect of combination therapies using deferiprone, deferoxamine and new chelators will provide new mechanisms of chelator interactions, which may lead to higher efficacy and lower toxicity by comparison to monotherapies. A major disadvantage of the experimental chelators is that even if they are approved for clinical use, they are unlikely to be as inexpensive as deferiprone and become available to the vast majority of thalassaemia patients, who live in developing countries.

Iron, the major trace element in the body, is an essential component of many proteins and enzymes. As low-molecular-weight iron is potentially toxic to cells, higher organisms express a number of proteins for the transport and storage of iron. We review our current understanding of the intestinal absorption of iron in the light of recently identified membrane proteins, namely the ferrric reductase, Dcytb, the two iron(II) transport proteins, DMT1 and ferroportin/Ireg1, and hephaestin, the membrane-bound homologue of the ferroxidase ceruloplasmin. Two types of mammalian transferrin receptor, TfR1 and TfR2, are now known to exist. The structure of TfR1 and its role in the process of receptor-mediated cellular uptake of iron are presented together with structural information on the iron storage protein ferritin. Mechanisms for the regulation of levels of TfR1 and ferritin, as well as other proteins involved in iron homeostasis, are discussed. Our current knowledge and understanding of the structure of members of the transferrin family of iron-binding proteins and the nature of the iron-binding centres in transferrins is presented, together with information on the processes of iron-uptake and iron-release by transferrin and a summary of the elements that have been found to bind to transferrins.

The maintenance of iron and other essential metal ion balance in humans is based on the presence of homeostatic mechanisms of regulatory absorption, storage, re-utilisation and excretion. There are a number of factors and mechanisms that can affect the level of iron excretion or absorption and overall body iron stores. Net iron loss due to increased iron excretion by comparison to dietary iron absorption is considered as one of the causes of iron deficiency anaemia. Body iron loss greater than normal has been shown in many other conditions. These include the increase in urinary iron excretion observed in iron loaded patients, the substantial reduction in serum ferritin and liver iron of ex-thalassaemia patients several years following bone marrow transplantation and the increase in iron excretion in normal individuals following long term sport activities. There are differences in the metabolism, mode of action, interactions with the iron pools and routes of iron excretion, of the iron chelating drugs deferiprone (L1), deferoxamine and other experimental chelators such as ICL670 in iron-loaded patients. Naturally occurring chelators and some synthetic drugs are known to bind iron and affect iron absorption and excretion. The molecular characteristics of naturally occurring or synthetic chelators can influence other aspects of iron metabolism in addition to iron absorption or excretion. Similar mechanisms and factors can affect the metabolism of other essential metals. The understanding of the mechanisms involved in iron excretion and their overall effects on body iron levels can facilitate the design of new chelators and improved therapeutic protocols for the treatment of conditions of iron and other metal metabolic imbalance and toxicity.

The field of iron (Fe) metabolism has been invigorated in the past 10 years with the discovery of a variety of new molecules involved in the homeostatic control of this critical nutrient. These proteins include the transferrin receptor 2, frataxin, hephaestin, hepcidin, hemojuvelin and others. Basic understanding of the metabolism of Fe in cells is vital in order to develop Fe chelators for the treatment of a variety of disease states. In addition, examination of the role of Fe in the regulation of cell cycle progression and angiogenesis has led to investigations of the use of novel Fe chelators as anti-proliferative agents. These studies have resulted in the identification of new ligands that show selective and potent anti-tumor activity in vitro and in vivo. Moreover, the ability of these chelators to inhibit growth is not only limited to the inhibition of DNA synthesis. In fact, there is a range of targets that are affected by Fe-depletion, such as molecules involved in cell cycle control, angiogenesis and metastasis suppression. These include hypoxia-inducible factor-1α (HIF-1a), vascular endothelial growth factor-1 (VEGF1), p21CIP1/WAF1, cyclin D1 and the protein product of the N-myc downstream regulated gene-1 (Ndrg1). As such, Fe chelators can now be designed to target molecules to induce specific effects, for instance, angiogenesis or metastasis suppression.

Free radicals are a one of damaging factors in diseases associated with iron overload. This review considers two principal questions: the mechanisms of free radical-mediated damage in cells and tissue and findings concerning the discovery of iron-stimulated free radical cascades in thalassemia and Fanconi anemia. There are two major precursors of all reactive oxygen and nitrogen species formed in living organism - superoxide (O2.-) and nitric oxide (NO). However, it has been shown that in addition to well-known mechanisms of the formation of reactive hydroxyl radicals and peroxynitrite from superoxide and NO, there are signal pathways by which these "physiological" radicals directly induce apoptosis, proton leak in mitochondria and an increase in oxygen consumption leading to cell death. In present review the mechanisms of free radical damage are considered with the particular emphasis of iron-induced free radical formation in thalassemia and Fanconi anemia. Furthermore free radical reactions leading to lipid peroxidation, LDL oxidation, the stimulation of apoptosis and other damaging processes are discussed. An importance of the chelating and antioxidant treatments of thalassemic and Fanconi anemia patients is also considered within the context of free radical damage and its prevention.

The heme based respiratory proteins myoglobin and hemoglobin can, under certain conditions, exhibit a peroxidase-like enzymic activity, in which a catalytic cycle, driven by peroxides, leads to oxidation of bio molecules. These heme proteins are implicated in what is termed "oxidative stress" as this catalytic cycle, when it occurs in vivo, generates cytotoxic product that are implicated in the pathology of a number of disease states. Here we review the evidence that such reactions occur in vivo, in particular in animal models and human patients and examine the underlying chemical mechanism. This mechanism involves the production of ferryl heme (FeIV=O2-) and it is this and associated radicals that initiate processes such as lipid peroxidation and the generation of bioactive molecules such as isoprostanes. The reactivity of the high oxidation state of the heme also allows us to identify unambiguous biomarkers for its presence in vivo in such conditions as rhabdomyolysis and brain hemorrhage. Ways to inhibit the peroxidatic cycle are discussed and the role of iron chelators such as desferrioxamine is discussed in terms of their often neglected properties as reducing agents. Suppression of the peroxidatic activity of hemoglobin is discussed in the context of the development of blood substitutes.

Both zinc and copper are essential minerals that are required for various cellular functions. Although these metals are essential, they can be toxic at excess amounts, especially in certain genetic disorders. Zinc and copper homeostasis results from a coordinated regulation by different proteins involved in uptake, excretion and intracellular storage/trafficking of these metals. Apart from zinc transporters (ZnT) families and Cu-ATPase, metallothionein is an important storage protein for zinc and copper. Metallothioneins are intracellular polypeptides with a remarkable ability to bind metallic ions. These proteins bind both essential metals indispensable for the organism and also toxic metals (e.g. cadmium or lead). Metallothioneins play a critical role to maintain zinc and copper homeostasis. In this review, we summarize the toxicity of zinc and copper and the potential treatment for zinc or copper toxicity by zinc- or copper-specific chelators as well as strategy to upregulate metallothionein.

The prospects of using chelating agents for increasing the excretion of actinides are reviewed. The removal of plutonium by chelating agents is of great importance because plutonium is extremely dangerous and induces cancer due to radiation toxicity. Similarly, uranium is a radionuclide, which causes severe renal dysfunction within a short time period due to chemical toxicity. It may also induce cancers such as leukemia and osteosarcoma in cases of long-term internal radiation exposure. Investigations on chelating agents for the removal of plutonium were initiated in the 1960's and 1970's. Diethylenetriaminepentaacetic acid (DTPA) is recognized as a chelating agent that accelerates the excretion of plutonium in early treatment after an accident. Thereafter, there has long been an interest in finding new chelating agents with radionuclide removal properties for use in therapy, and many chelating agents such as 3,4,3-LIHOPO and CBMIDA have been studied for their ability to remove plutonium and uranium. Recently, the focus has turned to drugs that have been used successfully in the treatment of a variety of other diseases, for example the iron chelating drug deferiprone or 1,2-dimethyl-3-hydroxypyrid-4-one (L1), which is used in thalassaemia and ethane-1-hydroxy-1,1- bisphosphonate (EHBP), which is used in osteoporosis. Within this context, it is important to examine the clinical use of these two drugs as well as the properties of the experimental chelators 3,4,3-LIHOPO and CBMIDA in order to identify possible uses in the treatment of radiation workers contaminated with plutonium and uranium.

The effects of chelating drugs used clinically as antidotes to metal toxicity are reviewed. Human exposure to a number of metals such as lead, cadmium, mercury, manganese, aluminum, iron, copper, thallium, arsenic, chromium, nickel and platinum may lead to toxic effects, which are different for each metal. Similarly the pharmacokinetic data, clinical use and adverse effects of most of the chelating drugs used in human metal poisoning are also different for each chelating drug. The chelating drugs with worldwide application are dimercaprol (BAL), succimer (meso-DMSA), unithiol (DMPS), D-penicillamine (DPA), N-acetyl-Dpenicillamine (NAPA), calcium disodium ethylenediaminetetraacetate (CaNa2EDTA), calcium trisodium or zinc trisodium diethylenetriaminepentaacetate (CaNa3DTPA, ZnNa3DTPA), deferoxamine (DFO), deferiprone (L1), triethylenetetraamine (trientine), N-acetylcysteine (NAC), and Prussian blue (PB). Several new synthetic homologues and experimental chelating agents have been designed and tested in vivo for their metal binding effects. These include three groups of synthetic chelators, namely the polyaminopolycarboxylic acids (EDTA and DTPA), the derivatives of BAL (DMPS, DMSA and mono- and dialkylesters of DMSA) and the carbodithioates. Many factors have been shown to affect the efficacy of the chelation treatment in metal poisoning. Within this context it has been shown in experiments using young and adult animals that metal toxicity and chelation effects could be influenced by age. These findings may have a bearing in the design of new therapeutic chelation protocols for metal toxicity.

The development of contrast agents shortening the relaxation times of protons began more than 20 years ago in order to improve the capability of diagnosing disease by means of magnetic resonance imaging (MRI). A variety of extracellular and tissue specific contrast agents were developed based on two types of molecules. One type was related to soluble paramagnetic chelates and the other type to stabilized colloidal particle solutions of iron oxides. The chelate or metal complex of gadopentetate dimeglumine was the pioneering magnetic resonance (MR) contrast agent used in 1988. Chemical modifications of this chelate and the design of new chelates led to tissue or blood pool specificity in MRI. Similarly, modifications in coating materials and variations in size of iron oxide particles allowed for tissue specificity or blood pool properties in MRI. Both types of contrast agents offer excellent perspectives for clinical MRI and for molecular imaging.