Comprehensive Inorganic Chemistry II || Chelation Therapy

Co

3.35 Chelation TherapySJS Flora, Defence Research and Development Establishment, Gwalior, India

ã 2013 Elsevier Ltd. All rights reserved.

3.35.1 Introduction 9883.35.2 Metal Exposure and Health Effects 9883.35.2.1 Aluminum 9883.35.2.2 Arsenic 9893.35.2.3 Lead 9893.35.2.4 Mercury 9903.35.2.5 Cadmium 9903.35.2.6 Iron 9913.35.2.7 Chromium 9913.35.2.8 Nickel 9923.35.2.9 Manganese 9923.35.2.10 Platinum 9933.35.2.11 Thallium 9933.35.3 Chelation: Concept and Chemistry 9933.35.3.1 Chelation 9933.35.3.1.1 Advantages of chelation as a metal complexation process 9933.35.3.1.2 Thermodynamic considerations in metal chelation 9943.35.3.1.3 Kinetic considerations in metal chelation 9943.35.3.1.4 Hard and soft acids and bases principle in chelation 9953.35.3.2 Chemistry of Chelation in Biological Processes 9953.35.3.3 Conventional Chelators and Their Current Use in Metal Toxicity 9963.35.3.4 Clinical Chelators 9963.35.3.4.1 British anti-lewisite 9963.35.3.4.2 DMSA and DMPS 9963.35.3.4.3 DPA and NAPA 9983.35.3.4.4 EDTA and DTPA 9983.35.3.4.5 Triethylenetetramine 9983.35.3.4.6 DFOA 9993.35.3.4.7 L1 9993.35.3.4.8 DDTC 9993.35.3.5 Limitations of Chelating Agents 9993.35.3.5.1 Limited therapeutic efficacy 9993.35.3.5.2 Adverse effects of chelation 10003.35.3.6 Contraindications 10013.35.3.7 Recent Advancement in Chelation Therapy 10023.35.4 Development of New Chelating Agents 10023.35.4.1 Monoesters of DMSA 10023.35.4.2 Crown Ethers 10023.35.4.3 VK-28 and Its Analogues 10023.35.4.4 Indazoles 10033.35.4.5 Ellagic Acid 10043.35.4.6 b-Dicarbonyl Enolates 10043.35.5 Combination Therapy 10053.35.5.1 Use of Antioxidants and Herbal Extracts for the Removal of Toxic Metals 10053.35.6 Future for Clinical Use of Chelating Agents 10073.35.6.1 Neurological Disorders 10073.35.6.2 Wilson’s Disease 10083.35.6.3 Blood Disorders and Iron Chelation 10083.35.6.3.1 Thalassemia 10083.35.6.3.2 Myelodysplastic syndrome 10093.35.7 Conclusion 1009References 1009

mprehensive Inorganic Chemistry II http://dx.doi.org/10.1016/B978-0-08-097774-4.00340-5 987

http://dx.doi.org/10.1016/B978-0-08-097774-4.00340-5

988 Chelation Therapy

3.35.1 Introduction

Heavy metals are considered among the most dangerous and

damaging polluting substances. Exposure and toxicity of sev-

eral metals and metalloids, such as lead, cadmium, mercury,

manganese, aluminum, iron, copper, thallium, arsenic, chro-

mium, nickel, and platinum, are of major concern to human

health. In general, children and elderly persons are more sus-

ceptible than adults to the deleterious effects of metals. The

increasing industrial use of metals has led to an environment

in which chronic intoxication is common. Consequently, oc-

cupational and environmental risks for human health derived

from metal exposure are of concern. Hereditary conditions

such asWilson’s disease caused by excess copper accumulation,

or patients with secondary iron overload (thalassemia major),

require treatment because of excess accumulation of these

metals. Reduction of aluminum accumulation and toxicity

following chelation may also prove beneficial in end-stage

renal disease patients, and perhaps those suffering from neu-

rodegenerative disorders such as Alzheimer’s disease (AD).

Chelation therapy has been practiced in various forms for

more than five decades. The development of organic com-

pounds capable of reducing body toxic burden continues to

be an area of general importance. Metal complexes formed

with these metal ions and chelating agent in vivo are readily

excreted in the urine or feces leading to the reduction of toxic

metal burden. These compounds are called therapeutic chelat-

ing agents. For a molecule to function as a chelating agent, it

must have (1) at least two appropriate functional groups, the

donor atoms of which are capable of combining with a metal

by donating pairs of electrons, and (2) the donor atoms must

be situated in the molecule to allow the formation of a ring with

a metal atom as the closing member. The different toxic ele-

ments can have very distinct preferences for chelate antidotes.

There are only limited numbers of chelating agents currently

available and found suitable for use in humans. Those chelating

agents that, based on clinical data on human usage are available,

can be classified into structurally related categories such as

polyaminocarboxylic acids, chelators with vicinal –SH groups,

b-mercapto-alpha-amino acids, hydroxamic acids, ortho hydro-

xycarboxylic acids or orthodiphenols, andmiscellaneous agents.

A number of chelators available in the past for the treat-

ment of metal intoxication include calcium disodium ethyle-

nediaminetetraacetic acid (CaNa2EDTA) which has been the

mainstay of chelation therapy, particularly for lead poisoning

while 2,3-dimercaptopropanol (BAL) therapy has been a stan-

dard treatment for children with acute lead encephalopathy.

D-Penicillamine (DPA) is used in the treatment of Wilson’s

disease and for lead toxicity. In recent years, meso-2,3-

dimercaptosuccinic acid (DMSA) and the sodium salt of 2,3-

dimercaptopropanesulfonic acid (DMPS) are the most widely

accepted potential antidotes for lead, arsenic, and mercury. It

has been shown that the polyaminocarboxylic acids diethyle-

netriaminepentaacetic acid (DTPA) and cyclohexanediamine-

tetraacetic acid (CDTA) can enhance the urinary excretion of

zinc, manganese, and thorium. In turn, the hydroxycarboxylic

acid sodium catechol 3,5-disulfonate (Tiron) has been found

to be effective in increasing the urinary excretion of vanadium

and uranium. No clinical chelation treatment for cadmium

intoxication is available, although it has been reported that

systemic cadmium poisoning can be alleviated by administra-

tion of dithiocarbamates (DDCs). In addition, the chelators,

desferrioxamine (DFO) and deferiprone (L1), are used in

the removal of iron and aluminum following overload of

these elements.

In this chapter, a review is presented of the current status of

chelation therapy with particular focus on the recent results

related to advantages, disadvantages (drawbacks/limitations)

of currently used chelating agents, the current trend in finding

a safe and specific antidote to treat cases of metal intoxication,

and the future direction.

3.35.2 Metal Exposure and Health Effects

3.35.2.1 Aluminum

Aluminum is the third most abundant element belonging to

group III of the periodic table and it is used extensively in

industries such as transportation and construction facilities,

therapeutic drugs, food processing plants, cosmetics, and in

household products such as cookware and other utensils. It has

been used extensively in the automotive and aerospace indus-

tries because of its lightweight; worldwide production of alu-

minum has increased by 30% between 2001 and 2005

indicating a high demand for the metal and the chances of

human exposure. Humans can be subjected to occupational

exposure in aluminum manufacturing, welding industries,

metallurgy, etc.1 Exposure may also occur due to ingestion

and topical application of therapeutic agents such as antacids,

buffered aspirin, and cosmetics containing aluminum. Other

nonindustrial sources of exposure include food and drinking

water due to its use in processing, preservation and packaging

of food stuffs, and in purification of water.2 Clinical manifes-

tations of aluminum may include neurotoxic disorders such as

AD and Parkinson’s disease3; neurobehavioral alterations such

as memory, learning, and cognitive dysfunction; and sensory

defects such as vision and auditory loss.4 Other symptoms of

aluminum poisoning include extreme nervousness, anemia,

headache, and osteoporosis. Smelter workers in the aluminum

industry have also been reported to show asthma-like symp-

toms, known as ‘potroom asthma.’5 Aluminum enters the

human body through oral, nasal, and dermal routes with

very limited gastrointestinal (GI) absorption. Once absorbed,

it enters the blood stream, binds to transferrin and citrate,1

followed by extensive systemic distribution to the brain, liver,

lungs, kidney, bone, etc.6 It easily crosses the blood–brain

barrier (BBB) and blood–placental barrier mimicking the met-

abolic pathways of potassium and iron. Aluminum is excreted

mainly through the urine. High aluminum concentration has

been associated with oxidative stress but, in biological systems,

it does not exhibit redox activity. Aluminum-induced oxidative

stress might be due to reactive oxygen species (ROS) generation

mediated through iron.7 Aluminum at the cellular level im-

pairs mitochondrial function leading to ROS generation8,9

resulting in peroxidation of lipids10 and zwitter ionic lipids

such as phosphotidylcholine.11 Aluminum compounds are

reported to alter membrane fluidity in liposomes,12 plasma,

myelin, and synaptosomal membranes13 affecting the neuro-

transmission and release/uptake of neurotransmitters (NTs).

Chelation Therapy 989

It also downregulates neurotransmission by a variety of mech-

anisms including direct inhibition of NTs synthesizing and/or

utilizing enzymes. Aluminum also alters cell signaling path-

ways which involves binding to regulatory proteins, membrane

polyphosphoinositides, and secondary messengers (cyclic

adenosine monophosphate (cAMP)).14 Aluminum stimulates

pro-inflammatory signals and decreases the anti-inflammatory

molecules such as neurotrophils, nerve growth factors, and

neurotrophic factors derived from the brain.15

3.35.2.2 Arsenic

Arsenic (As) is one of the most widely studied metalloids in the

field of metal poisoning. Besides its toxicity, arsenic holds an

important position in traditional medicinal therapies in China,

India,16,17 Greece, and Rome.18 Recently, it has been used

as a treatment for late-stage African trypanosomiasis

(melarsoprol)19 and for acute promyelocytic leukemia, the

drug marketed as Trisenox.20 Arsenic exists in inorganic as

well as organic forms and is found in water, soil, and air

from natural and anthropogenic sources. Ground-water arse-

nic levels in many countries exceed the maximum permissible

limit (10 ppb) established by the World Health Organization

(WHO). Leading the list are Argentina (200 ppb), Mexico

(400 ppb), and the Indo-Bangladesh region (800 ppb). Arsenic

compounds are commercially used as catalysts, bactericides,

pesticides, herbicides, cotton desiccants, wood preservatives,

fungicides, animal feed additives, corrosion inhibitors, veteri-

nary medicines, and tanning agents.

The metabolism of arsenic is a two-step procedure: oxida-

tion from trivalent to pentavalent or reduction from pentava-

lent to trivalent by the enzyme arsenate reductase.21 Urine is

the primary route of elimination for both pentavalent and

trivalent inorganic arsenicals. Arsenic, particularly trivalent

forms, binds to sulfhydryl groups, disrupts essential enzyme

activity, and leads to impaired gluconeogenesis and oxidative

phosphorylation. Arsenic-induced oxidative stress is mediated

through direct and indirect ROS and reactive nitrogen species

(RNS) generation either during its metabolism (dimethylarsi-

nic peroxyl radicals ([(CH3)2AsOO]) and dimethylarsinic

Arsen

ROS/RNS

Activation ofcaspase cascade

Mitochondrialmembranepotential

Pro-apoptoticfactors

Anti-apoptoticfactors

Cyt. Crelease

Apoptosis

High arsenic acute exposure

Figure 1 Concentration-dependent toxicity mechanism of arsenic.

radical [(CH3)2As]) or indirectly through the Fenton

reaction.22,23 Arsenic is also known to induce oxidative stress

by weakening the antioxidant defense mechanism of the cell

(Figure 1).22 It is known to impair a variety of intra- and extra-

mitochondrial membrane systems and ultimately leads to

apoptosis.22 Symptoms of acute arsenic poisoning include

abnormal liver enlargement, cardiac arrhythmia, and melano-

sis; peripheral neuropathy, including sensory loss in the pe-

ripheral nervous system; GI disorders; and anemia. The

chronic effects of arsenic have been found to include various

types of cancer,24 cardiovascular disease,25 diabetes,26 neuro-

logical disorders,27 and dermal effects.28

3.35.2.3 Lead

Lead is a well-known toxin that has been a part of human life

for thousands of years. Due to its versatile uses, human expo-

sure to lead is unavoidable. Clinical symptoms in humans post

lead poisoning are time and dose dependent. Some common

symptoms appearing at various stages and doses of lead are

abdominal pain/cramping, nausea/vomiting, short-termmem-

ory loss, depression, loss of coordination, numbness and tin-

gling in extremities, constipation, inability to concentrate, and

impotence.29

The accepted toxic threshold for lead in infants, children,

and women of child-bearing age is �10 mg dl�130 approved by

the American Pediatric Association. Measurement of blood

lead concentrations is the most effective and accepted diagno-

sis for lead exposure. In chronic cases of lead poisoning, how-

ever, blood lead levels are very low since most lead

accumulates in hard tissues such as bones where it displaces

calcium. Thus, the lead mobilization test is employed where a

single shot of intravenous CaNa2EDTA is injected to mobilize

lead from bones to blood and 24-h urinary lead is measured.

The affinity of lead for sulfhydryl groups is recognized as one of

the major mechanisms of lead toxicity. Binding of lead with

sulfhydryl moieties leads to the inhibition of various cellular

enzymatic activities affecting vital cellular pathways. Lead-

induced interference in the heme-biosynthetic pathway via

inhibition of zinc-dependent sulfhydryl containing enzyme

ic

ROS/RNS

DNAdamage

P53activation

UnabledDNA repair

Carcinogenesis

Mutations

Altered signalingpathways

Altered cellcycle phases

Low arsenic prolonged exposure


d-aminolevulinic acid dehydratase (ALAD) is one such exam-

ple that is recognized as the marker for lead toxicity.

Lead toxicity results in a variety of physiological, biochem-

ical, and behavioral malfunctioning in both experimental an-

imals and humans, which are mainly associated with central

and peripheral nervous systems, hematopoietic system, cardio-

vascular system, hepatic, renal, and male and female reproduc-

tive systems. Lead inhibits ALAD and ferrochelatase, the two

regulatory enzymes in heme-biosynthesis, resulting in the ac-

cumulation of its precursor ALA.31,32 High concentrations of

ALA have been linked with the generation of ROS33 and its

final oxidation product, 4,5-dioxovaleric acid, causes DNA

alkylation leading to genotoxic effects.34 The neurotoxic effects

of lead are due to either the inhibition of Kþ stimulated release

of g-aminobutyric acid (GABA) or GABA binding to the syn-

aptic membrane.35 It has further been reported that lead expo-

sure can cause an increase in intracellular levels of Ca2þ which

is mediated through generation of ROS, causing depression

of mitochondrial potential, leading to cytochrome c-mediated

apoptosis.33 Lead also hinders the functioning of various

antioxidant enzymes such as catalase, superoxide dismutase

(SOD), glutathione peroxidase (GPx), glucose-6 phosphate

dehydrogenase, and glutathione (GSH)-like antioxidants. Other

potentially toxic effects of lead include overproduction of nNOS

and HAO, depletion of 5HT and AchE,33 upregulation of Bax

and downregulation of Bcl2,36 increased p53 expression, and

activation of caspase-3 and caspase-937 leading to apoptosis.

3.35.2.4 Mercury

At room temperature, elemental (or metallic) mercury exists as

a liquid with a high vapor pressure38 and is listed as a heavy

metal exhibiting toxicity at varied levels. Mercury is found in

nature in several forms affecting both humans and animals. All

forms of mercury, viz. elemental, inorganic, and organic, show

toxic effects in a number of organs. Emanation of mercury in

the environment occurs naturally from volcanic emission, oce-

anic sediments, degassing from geological materials, and

by forest fires,39 whereas anthropogenic sources include indus-

trial uses, burning of fossil fuels, incineration, mining, and

from degradation of mercury-containing compounds. Humans

subjected to mercury poisoning due to occupational exposure

or through contaminated food are at risk of severe disorders

such as neurological alterations affecting cognitive and motor

dysfunctions, tremor, mental disorders, ataxia, disturbance of

taste and smell, spasticity, and blindness.40

The general mechanism of toxicity involves the covalent

binding of mercury with sulfhydryl groups inactivating various

enzymes affecting cellular functioning and metabolism.41 It

also binds to primary and secondary amine, amide, carboxyl,

and phosphoryl groups. Elemental mercury vapor is highly

liposoluble, predominantly absorbed through the lung,

where it crosses the alveolar membrane and easily reaches the

systemic circulation and body tissues.42,43 Organic mercury is

liposoluble and is rapidly absorbed through inhalation, inges-

tion, or dermal exposure. More than 90% of methyl mercury

binds to erythrocytes and slowly distributes and accumulates

in the liver, kidney, brain, hair, and epidermis. It can cross the

placenta, accumulates in the fetus, and is excreted in toxic

amounts in breast milk.44 Inorganic mercury salts are usually

absorbed through the GI tract but exhibit a low bioavailability

(5–10%) as compared to organic compounds.45 After absorp-

tion, the salt dissociates into the ionic form, distributed be-

tween red blood cells (RBCs) and plasma, and then reaches the

tissues affecting the GI tract and the kidneys (see

Chapter 3.04).

3.35.2.5 Cadmium

Cadmium is a widespread metal contaminating many areas,

either naturally or as a result of anthropologic activities.46 It is

well recognized as an occupational health hazard, responsible

for the famous itai-itai (‘ouch-ouch’) disease of Japan.47

Cadmium is listed as one of the 126 priority pollutants by

the US Environmental and Pollution Agency (EPA), and is

classified as a number one category human carcinogen by the

International Agency for Research on Cancer of USA.48

Cadmium is present in all natural sources of food at varying

concentrations. High cadmium concentrations are present in

seafood such as mollusks, crustaceans, cephalopods, and crabs

as well as in oil seeds, cocoa beans, animals, and plant-derived

food. It has been estimated that more than 80% of dietary

cadmium comes from cereals, vegetables, and potatoes.49

Industrial sources of cadmium are mainly electroplating,

smelting and refining, welding, pigment production, and

battery-manufacturing industries. Human exposure to cad-

mium occurs generally through ingestion or by inhalation.

Respiratory exposure to cadmium can occur through inhala-

tion of cigarette smoke,47 indoor dust contaminated with

cadmium,50 or by working in cadmium-related industries.

Blood cadmium levels are commonly used as an indicator of

cadmium toxicity, while measurement of urinary cadmium

concentration is a biomarker of lifetime exposure. The major

symptoms associated with cadmium toxicity include pulmo-

nary edema, hemorrhage, fulminate hepatitis, and testicular

injury. At higher concentrations, its toxic symptoms include

renal damage characterized by early increase in excretion of

low-molecular-weight proteins (b2 and a1 microglobulins)

due to glomerular damage and dysfunctioning of tubular reab-

sorption, along with glycosuria and aminoaciduria.51,52

Itai-Itai disease is also a hallmark of cadmium poisoning.

After absorption, cadmium binds to albumin and is trans-

ported to the liver, where it promotes the synthesis of metal-

lothionein (MT), a cysteine-rich heavy metal-binding

protein.53 MT–cadmium complex is then released from the

liver to the plasma and eliminated in the urine. It is however

reabsorbed from glomerular filtrate by the renal tubule cells,

where it is cleaved by lysosomal action, thus releasing Cd2þ

ions that are re-excreted into the tubular fluid.53

Cadmium toxicity is mainly associated with the generation

of tumors.54 In cadmium-related carcinogenicity, various reg-

ulatory genes are activated including immediate early response

genes (IEGs). Significant cadmium-induced overexpression of

IEGs constitutes mitogenic growth signals, stimulating cell

proliferation and induction of carcinogenesis.55 Further

cadmium-induced carcinogenicity causes expression of several

stress response genes such as those for encoding MT synthesis,

heat-shock proteins (HSPs), oxidative stress response, and syn-

thesis of SH and related genes. Cadmium influences the activ-

ity of several transcription factors being a powerful inducer of


c-fos and c-jun that have suggested playing an important role

in cadmium-induced cell transformation and tumorigenesis.56

Cadmium also affects the expression of genes regulating

translation.57 Cadmium-induced oxidative stress is mediated

via indirect ROS generation including superoxide radical, hy-

droxyl radical, and nitric oxide.58 Generation of nonradical

hydrogen peroxide, which in turn may be a significant source

of radicals via Fenton chemistry, has also been reported. Other

cellular effects induced by disruption of physiological signal

transduction systems, including those mediated by Ca2þ,cAMP, NO, mitogen-activated protein (MAP)-kinase, PKB/

Akt, and nuclear factor-kappa-B.59

3.35.2.6 Iron

Iron is a physiologically essential metal with biological roles

extending from hemoglobin synthesis and function to the

respiratory chain enzymes of mitochondria serving vital func-

tions in the body.60 Iron uptake in humans may follow various

routes including intentional administration during blood

transfusion resulting in iron overload since about 500 ml of

whole blood may contain 200–250 mg of iron.61 Inhalation of

nonindustrial iron in the form of particulates, especially in the

subways,62 and active and passive smoking may cause systemic

exposure to iron. Moreover, iron is ingested as an ingredient of

all natural foods; however, overload may result from consum-

ing food items containing added iron including flour, corn

meal, farina, and rice, as well as ready-to-eat cereals.63

Iron is essential for cell survival yet excess of iron leads to

numerous malfunctions and cellular insults including endocri-

nological, GI, infectious, neoplasmic, neurodegenerative, ob-

stetric, ophthalmic, orthopedic, pulmonary, and vascular

diseases.64 It also contributes to diseases of aging – AD, Parkin-

son’s disease, and atherosclerosis – mortality, and pathogenic

Atherosclerosis

Alzheimer’s disease

Parkinson’s disease

F

Lipid peroxidation

Blood

AccumulationFe2+

Figure 2 Mechanism of toxicity in iron overload.

invasions (Figure 2). Iron toxicity is a result of two different

attributes of the metal.

The active redox form of iron (Fe2þ) reacts with cellular

H2O2 and reduced to Fe3þ generating ROS via Fenton redox

reaction:

Fe2þ þH2O2 ! Fe3þ þOH� þOH�

These generated ROS cause cellular damage and imbalances

including damage to proteins and DNA and cause lipid perox-

idation and polysaccharide depolymerization reactions.65 An-

other attribute that accounts for iron-induced toxicity is that it

serves as a potential growth-promoting agent for almost all

pathogenic organisms such as bacteria, fungi, protozoa, and

for all cancerous cells, thus causing cellular tensions.61

3.35.2.7 Chromium

Chromium (Cr) is ubiquitously present in the environment.

Chromium has versatile applications with the most highlighted

being as a component of stainless steel.66 Chromium exists in

two important stable states: trivalent [Cr(III)] and hexavalent

[Cr(VI)]. It has been suggested that Cr(III) is an essential micro-

nutrient for the biological activity of insulin, glucose, and lipid

metabolism.67 Cr(III) is found in most fresh foods, vegetables,

cereals, spices, bread, and drinking water, and its deficiency has

been associated with impaired glucose tolerance, hyperglyce-

mia, glucosuria, diabetes, cardiovascular disease, etc.68 Indus-

trial applications also include manufacturing of pigments for

metal, glass, and synthetic rubies, preservation of wood, tanning

of leather, refractory materials, super alloys for jet engines and

gas turbines, etc. In nature chromium exists in the trivalent

[Cr(III)] state which in noncarcinogenic, while the hazardous

hexavalent [Cr(VI)] form is predominantly produced by anthro-

pogenic activities. Human exposure to occupational chromium

Fe2+

Fe2+Fe2+

Fe2+Fe2+

Uptake of iron

e2+

Facilitate ROS generation

ROS

Fe3+

Fenton reaction

Protein oxidationDNA damage

Transferrin receptor


usually occurs through inhalation, and the severity depends

upon the nature and function of industries.69 Nonoccupational

chromium exposure by inhalation mainly occurs due to auto-

mobile emissions and by smoking cigarettes. Exposure to

high chromium loads may also be due to drinking of Cr-

contaminated water.

Cr(III) compounds are relatively nontoxic, noncarcinogenic,

and nonmutagenic because of its inability to pass through the

cell membrane and remain within the cells bound to macro-

molecules such as DNA. Cr(VI) is classified as a group I human

carcinogen and is transported into cells through anion channels

as chromate.70,71 Then it undergoes rapid metabolic reduction

in the presence of cellular reductants such as ascorbic acid,

reduced GSH, and cysteine to generate stable Cr(III) and unsta-

ble Cr(IV) and Cr(V) intermediates.72 During the reduction

process, Cr(VI) generates ROS by Fenton and Haber–Weiss

type of reactions which ultimately results in oxidative stress

causing DNA lesions including Cr–DNA adducts, DNA–protein

crosslink, DNA–DNA crosslink, activation of nuclear transcrip-

tion factors, upregulation of antioxidants, and activation of

enzymes responsible for Cr(VI) reduction.73

Reduction of Cr(IV) also induces cell-cycle arrest at G1

phase, S-phase, and G2 phase. Cr(IV) induces apoptosis by

activating both an intrinsic mitochondrial pathway and an

extrinsic death factor pathway.73,74 In addition to cancer, chro-

mium toxicity also causes dermatitis, hand ulcers, perforation

of the nasal septum, and renal and hepatic damage.75

3.35.2.8 Nickel

Studies suggest that nickel may be an essential trace element in

metabolism and may play a key role in maintaining certain

biological processes in animals.76 Industrially, nickel is widely

used in the production of stainless steel, coins, jewelry, metal-

lurgical processes, electrical components such as batteries,

medical devices, chemical and food processing industries, car-

bon particles, as well as in nickel refinery, plating, and welding.

Thus, with the increasing demand for nickel, its environmental

health concerns are also increasing.76

Natural sources of atmospheric nickel include dusts from

volcanic emissions, wind-blown dust, weathering of rocks and

soils, and forest fires and vegetation. The human population

may be exposed to nickel through air, food, and oral intake of

contaminants in the drinking water. Nickel exposure primarily

occurs via inhalation and ingestion which is an important

route in occupational exposure among nickel metallurgy

workers and tobacco smokers.76 Some medical implants con-

taining nickel used in the treatment of cardiac disorders and

iatrogenic administration of nickel-contaminated medications

lead to significant parenteral exposures. Moreover, nickel-

fabricated articlesmay result in cutaneous nickel absorption.76,77

The most common clinical symptom of nickel poisoning is

allergic contact dermatitis reaction producing erythema, eczema,

and lichenification of the hands and other areas of the skin in

patients sensitive tonickel.79Other symptoms include headache,

vertigo, nausea, vomiting, insomnia, and irritability, which usu-

ally last a few hours depending upon the severity and duration of

nickel exposure. Delayed symptoms include tightness of the

chest, nonproductive cough, dyspnea, cyanosis, tachycardia, pal-

pitations, sweating, visual disturbances, vertigo, weakness, and

lassitude. Symptoms of chronic nickel poisoning include respi-

ratory disorders such as asthma, bronchitis, rhinitis, sinusitis,

and pneumoconiosis.79

Nickel can be absorbed as the soluble nickel ion (Ni2þ),while sparingly soluble compounds can be phagocytized and

its absorption depends on the physicochemical form. Nickel is

poorly absorbed through the GI tract. Following inhalation

and ingestion, lungs and kidneys are the primary target organs

for nickel. Following absorption, nickel distributes in liver,

heart, lungs, peripheral nervous tissues, and brain where it

binds to specific proteins in the blood serum and distributed

in the body.80,81 Nickel generates free radicals in various tissues

leading to modifications of DNA bases including DNA meth-

ylation and loss of histone acetylation in H2A, H2B, H3, and

H4, enhanced lipid peroxidation, and altered calcium and

sulfhydryl homeostasis. Nickel is also considered to be carci-

nogenic, and the probable causes include generation of oxida-

tive stress, genetic and epigenetic changes, and inhibition of

DNA repair enzymes.80,82,83

3.35.2.9 Manganese

Manganese (Mn) exists in 11 oxidation states starting from �3

to þ7, of which þ2, þ3, þ4, þ6, and þ7 are the most com-

mon. Manganese is an essential trace element crucial for

growth and developmental processes, but when the level

exceeds the required concentrations it may cause severe

toxicity.84 Natural sources of manganese include rocks, soil,

water, air, and food; available in the form of oxides and hy-

droxides, and cycles through its various oxidation states.

Anthropological sources of manganese include mining indus-

tries, burning fossil fuels, and pesticide use. Exposure of humans

can occur from occupational, medical, and environmental

sources.85 Other sources include intake of Mn-contaminated

water and use of pesticides such as Mn ethylene-bis-

dithiocarbamate (MANEB), fertilizers, and fuels (methylcyclo-

pentadienyl manganese tricarbonyl).86 Manganese is absorbed

predominantly via inhalation, followed by ingestion, or dermal

routes.87 After absorption, it is readily distributed to brain and

liver by binding to transferrin, gamma globulin, and albumin.88

Manganese can also cross BBB and blood–placental barrier fol-

lowing metabolism similar to that of iron. Overexposure to

manganese results in severe neurotoxicity that depends on the

elemental state of the manganese. Trivalent manganese is more

toxic than divalent.89 Chronic exposure leads to clinical mani-

festations similar to that of Parkinson’s disease called as ’man-

ganism’ (Parkinsonian syndrome); the symptoms include

headache and insomnia, memory loss, emotional instability,

exaggerated tendon reflexes, hyper-myotonia, hand tremor, and

speech disturbance. Several months before the appearance of

manganism symptoms, presymptoms appear known as ‘manga-

nese madness’ which include irritability, emotional liability,

illusions, and hallucinations.90 Excess Mn was reported to be

toxic to cardiac muscle cells and tissues by blocking calcium

channels. Manganese burden also causes acute liver toxicity by

modulating enzymes required for cholesterol metabolism and

bile production. Manganese overexposure also causes decreased

fertility rate and also can cause fetal abnormalities.91Manganese-

induced neurotoxicity has been reported which can arise from

disruption of mitochondrial metabolism,92 alteration in iron


homeostasis,93 oxidative stress,94 inflammation,95 and altered

glutamate and dopamine (DA) metabolism.96,97

3.35.2.10 Platinum

Platinum belongs to the members of platinum group elements

(PGEs) and is of concern as a potential environmental and

biological hazard. Use of PGEs in various industries particu-

larly as vehicle exhaust catalysts (VECs) results in their accu-

mulation in airborne particles, road dust, soil, mud, and water

from where they activate particularly by biotransformation.98

This makes them mobile and soluble in water99 leading there-

after to entry in organisms and finally bioaccumulation mainly

in the kidney, liver, spleen, and adrenal glands.100 Human

exposure to PGEs occurs mainly while working in chemical

plants, refineries, electronic plants, jewelry production as well

as from hospital effluents,101 and their concentrations are

found to increase in tissues and bodily fluids102 resulting in

serious health problems.

Platinum compounds, especially the soluble salts, are toxic

and are responsible for the development of an allergic syn-

drome known as ‘platinosis,’ which is characterized by respi-

ratory and cutaneous hypersensitivity on chronic occupational

exposure.103 Platinum compounds are known to cause a range

of toxic effects in humans. Pt(II) binds to proteins in human

blood, particularly MT.104 Platinum is also administered as an

anticancer drug (cisplatin, carboplatin, and oxaliplatin)

against various types of cancers. Certain platinum compounds

are also known to be neurotoxic, cytotoxic, and have muta-

genic and carcinogenic effects.105 They also induce hypersensi-

tivity reactions causing severe allergy leading to rhinitis,

conjunctivitis, asthma, and urticaria.106 Platinum compounds

induce oxidative stress, which is responsible for the platinum-

induced renal, cardiac, hepatic, and gastric toxicity.107

3.35.2.11 Thallium

Thallium (Ti) is a highly toxic heavy metal. The US EPA in-

cluded thallium in the list of priority toxic pollutants owing to

the fact that thallium is responsible for a number of occupa-

tional and accidental poisonings. It is introduced into the

environment mainly as waste from the production of zinc,

cadmium, and lead and by combustion of coal. Anthropogenic

sources of thallium include gaseous emissions from cement

industries, copper smelting, petroleum refining, coal-based

power plants, and metal sewers and from ore processing

operations.108 Thallium intoxication can result from skin con-

tact, since thallium salts are easily absorbed through the

skin.109 Exposure via inhalation may also occur during extrac-

tion of the metal, in the manufacture of thallium-containing

rodenticides and thallium-containing lenses, and in the sepa-

ration of industrial diamonds.110

Clinical manifestations of thallium poisoning include an-

orexia, headache, pain in abdomen, upper arms, thighs, and

even in all over the body111 while the most significant symp-

tom of thallium poisoning is the loss of hair or alopecia. Acute

thalliumpoisoning usually results in GI symptoms, while chronic

exposure leads to neurological disorders. Other symptoms in-

clude polyneuritis, encephalopathy, tachycardia, degenerative

changes of the heart, liver, and kidney, hemorrhage, and bone

marrow depression.112

Thallium exhibits adverse effects on various organs,

which may lead to death in the case of severe thallium intox-

ication. Thallium competitively substitutes potassium ions in

(Naþ/Kþ)-adenosine triphosphatase (ATPase) as well as in

pyruvate kinase, and aldehyde dehydrogenase. Thallium

also has a strong affinity for sulfhydryl groups from proteins

and other biomolecules, thus rendering them inactive and

interrupting cellular homeostasis. Studies have shown that

thallium potentially replaces potassium involved in ribosome

stabilization, as well as in physiological functions such as

muscle contraction.111 In spite of these findings, the exact

mechanism of thallium toxicity remains unknown, although

impaired GSHmetabolism, oxidative stress, and disruption of

potassium-regulated homeostasis may play a role. There are

some indicative studies on thallium-induced cancer and other

such diseases. However, the lack of data on the mutagenic,

carcinogenic, or teratogenic effects of thallium compounds

in humans calls for further research on this very toxic

heavy metal.

3.35.3 Chelation: Concept and Chemistry

3.35.3.1 Chelation

Chelating agents are defined as ligands whose structures permit

the attachment of their two or more donor atoms (or sites) to

the same metal ion simultaneously and thus produce one or

more rings. These molecules are also called ‘chelates’ (Greek

word meaning claw of a lobster) or chelating groups and the

formation of rings is termed ‘chelation.’ These metal-binding

molecules are of interest as drugs in chelation therapy. The

resultant metal complexes often have the ability to be resolved

into optically active (right- and left-hand) forms. The stability

of metal–ligand complexes varies with the pattern of complex

formation. The complexes in which groups bonding to the

target metal are also bonded to each other forming a ring

show greater stability than their corresponding analogues

where groups are only bonded to the metal.113 The difference

in stability becomes more important in the increasingly dilute

solutions that are of higher relevance in biological systems

such as serum or tissue. Thus, the main objective of chemically

identifying a chelating agent must be higher stability constants

of metal complexes in dilute solutions, for their use in the

treatment of metal intoxication.

This section briefly summarizes the historical background,

as well as chemical and biological principles along with the

advantages and limitations of conventionally available chelat-

ing agents used in the treatment of metal intoxication.

3.35.3.1.1 Advantages of chelation as a metal complexationprocessChelation finds use in analytical chemistry as metal ions can be

identified by the formation of stable and highly colored che-

lates with these ions. Chelation finds use in water softening, in

food preservation, in solvent extraction, for example, cupfer-

ron, and in the elimination of toxic and radioactive metals

from the body.


The chemistry of chelation finds its place in various metal

toxicity mechanisms and therapy. Toxic metals exert many of

their adverse biological effects by forming complexes with

enzymes, DNA, or other biomolecules in the cell. On the

other hand, the chelation concept is extensively used in med-

ical management of metal poisoning. Chelating agents are

added exogenously into the biological system for the removal

of toxic metals that may be bound to endogenous ligands. For

a chelating agent to be successful it must effectively compete

with the in vivo binding ligand for the possession of the toxic

metal ion (Mnþ).A chemical entity that qualifies as an ideal chelator in vitro

might not prove so in vivo, either due to the toxicity consider-

ations or due to the presence of endogenous substances (he-

moglobin, cytochromes, etc.) that can also chelate metal ions

and thus offer competition. Poor in vivo selectivity is an impor-

tant factor to be considered in chelation therapy. Further, pH

also is a factor, which influences complex formation and sta-

bility. Most chelating agents are unstable at low pH, whereas at

high pH metals tend to form insoluble hydroxides, which are

less accessible to chelating agents. This feature becomes signif-

icant in pathological conditions leading to acidosis or alkalo-

sis. Despite some limiting factors, chelating agents are used as

antagonists for toxic metals and literature in this area has

grown considerably.114–117

3.35.3.1.2 Thermodynamic considerations in metalchelationThe formation of metal complexes with ligands can be repre-

sented using a simple equation:

Mþ L ! ML

Mþ iL ! MLi

where M represents metal ions and L represents ligands (Lewis

bases and acids); the expression of the stability constants is

straightforward. The expression for the stability constant can

be shown to be

KML ¼ ML

M½ � L½ �and the overall stability constant is

KMLi ¼ML

M½ � L½ �i

with KML being equal to KMLi . The stability of a complex de-

pends on

DG ¼ DH � TDS ¼ �RT lnKMLi

Due to the fully available entropy contribution from deso-

lvation, multidentate ligands formmore stable complexes than

unidentate ligands and the overall stability in general increases

with the number of rings formed.118 The chelate effect can be

defined as the logarithm of the equilibrium constant for a

displacement reaction where i independent donors are ex-

changed by i identical donors present in one ligand. Compar-

ison of two ligand exchange reaction series where the ligand

L contains i donors identical to A

Mþ iA ! MAi; KMA ¼ MAi½ �M½ � A½ �i ; DGMA ¼ �RT lnKMA

Mþ Li ! MLi; KML ¼ MLi½ �M½ � Li½ � ; DGML ¼ �RT lnKML

shows that formation of the MA complex depends much more

on the concentration of the ligand (A is in the ith power in

KMA) than does the formation of the ML complex (L is in the

first power in KML). Thus, the increased stability of the chelate

is related to the free energy of the reaction. Especially at low

ligand concentration, chelate complexes are far more stable

than the corresponding complexes with unidentate ligands.

The entropy contribution is often the primary determinant

of increased stability of metal complexes with multidentate

ligands; however, when mutual repulsive forces between

charged groups are overcome by incorporating them into one

molecule, a considerable enthalpy effect may result. Among

other factors, steric conditions, for example, ion and ring size,

considerably influence the stability, mainly through changes in

ΔH.119

3.35.3.1.3 Kinetic considerations in metal chelationThe two factors, which must be considered when designing a

ligand for application in chelation treatment, are (1) high

thermodynamic stability of the complex formed with the target

metal ion and (2) the fast rate of complexation with the metal

ion of interest (kinetics). For any metal ion to form a complex,

the reaction must be either thermodynamically or kinetically

favorable or both. The two factors that favor a reaction ther-

modynamically are large negative enthalpy of formation and a

large positive entropy change. However, in vivo changes are not

limited to the above-mentioned two factors. However, com-

plex formation is limited due to the rate effects and competi-

tion with kinetics of chelate transport in the organism.120 The

rate-limiting step (slower step) in the metal complexation

process involves breaking a pre-existing chelate ring formed

with a biological multidentate ligand. Other factors such as

steric hindrance also govern the kinetics of the complex

formation.

The concentrations of ‘free’ toxic metals are often very low

in biological systems due to the availability of numerous small

biological ligands with which the metal forms mixed aqua-

bioligand complexes. Therefore, the complexation reactions

in vivo between the toxic metal and the ‘therapeutic’ chelating

agent most often occur as a series of kinetically controlled

ligand exchange reactions as well as by metal exchange reac-

tions. In a physiological solution, when a multidentate ligand

reacts with the target metal ion competing with the numerous

endogenous ligands and functional groups in proteins, the

kinetics and thermodynamics of complex formation become

complicated. Thus, stability constant in vitro and ‘effective sta-

bility constants’ in vivo may not be the same for any given

metal–ligand pair. ‘Effective stability constants’ denoted often

by K or b govern the success of biological chelation and should

be employed for the description of complex formation in vivo.

Such constants may be several orders of magnitude smaller

than standard stability constants. Interestingly, for most practi-

cal uses, only Ca2þ needs to be taken into account for estimat-

ing the ‘effective stability constant’ in vivo.


3.35.3.1.4 Hard and soft acids and bases principlein chelationThe hard–soft acid–base (HSAB) concept put forward by

Pearson elaborated the hardness/softness (HS) characteristics

of electron donors and acceptors.121 This concept forms an

important determining factor for complex formation consid-

ered by the chemist.

Hard species tends to be those with small size and low

polarizability, whereas the soft species are larger and more

easily polarizable. Pearson’s principle for the bonding between

acids and bases is that a hard species tends to bond with a hard

species, whereas soft species would prefer a soft species. It is

only an approximate qualitative prediction of the relative sta-

bility for the adducts and is not a theory or explanation of the

observations. The HS characteristics of donor and acceptor

atoms in complexation reactions determine the chelator’s de-

gree of metal selectivity in relation to competing essential

metals present in biological fluids. On the contrary, the selec-

tivity of the toxic metal for the chelator in comparison to the

competing biological ligands often available at higher concen-

tration is also determined by HS character. Examples from

these classes of metal ions and donor groups employed in

clinically useful chelators are given in Table 1.

Metal-related characteristics determining chelatability in vivo

include ionic diameter and preferred coordination number,

tendency for participation in redox reactions (e.g., transition-

metal ions), and preferred oxidation state, which besides con-

tributing to HS character also determine bioligand preference

and limitations for chelation. Further, the chemical similarity of

toxic metal with essential metal ions and rates of ligand ex-

change reactions in biological systems also determine the spec-

ificity. Together, these characteristics also influence nature of the

toxic reactions induced in the organism by the metal, for exam-

ple, induction of oxidative stress by Fenton reactions catalyzed

by transition metals (Cu, Fe, and Ni), and aggressive local re-

actions such as chromate-induced irreversible enzyme inhibi-

tion, binding of Cd2þ or Hg2þ to SH groups, etc.

3.35.3.2 Chemistry of Chelation in Biological Processes

Metal–chelate complexes have quite different biochemical, chem-

ical, and physical properties as compared to the uncomplexed

Table 1 Important chelating agents, ordered according to their HSAB cha

HS character Metal ion Donor group in clinicall

H Be2þ R–COOHMg2þ R1R2C¼OCa2þ R1R2CH–OH, R1R2¼C–

RO–SO3�

I Ni2þ RNH2

Fe2þ R1CO–NR2

Co2þ R1N–CO–NR2, R1R2–NHMn2þ R1N¼CNR2N0, R1R2R3NZn2þ R1–NOH–CO–NH–R2

S Cd2þ R–SHCu2þ R1–SS–R2, R2N–CSSHPb2þ R1

2N–CSSR2

Hg2þ

metal ion. In a chemical system, metal cations form bonds to

ligands, which can share electron pairs with it and the forma-

tion constitutes the ‘coordination sphere’ of themetal ion. The

bonded metal atoms can form complexes with a coordination

number ranging from 2 to 8. These atoms in the coordination

sphere undergo a change when a toxic metal salt is ingested

and absorbed by a person or other organism. Metal ions in

biological system form bonds to various types of donor atoms

to achieve a more stable characteristic coordination sphere.

Donor atoms foundmost commonly in living systems include

oxygen atoms (water, carbonyl, carboxyl and phenolic groups

of amino acids, phosphate groups, etc.), nitrogen atoms

(amino acids, porphyrins, nucleotides, etc.), and sulfur

atoms (cysteine- and thiol-containing compounds such as

lipoic acid (LA) and methionine). When a toxic metal atom

coordinates with ligands of physiologically essential mole-

cules such as an enzyme, messenger molecule, or DNA, it

results in a decrease in the reactivity pattern and hence the

viability of the organisms. These changes in the reactivity

patterns of physiological molecules are the basis for most of

the biological effects of toxic metals.

Toxic metals usually react with a wide variety of molecules,

which contain appropriate donor atoms. A given toxic metal is

found to have an effect on any part of an organism where it

reacts with, and changes the reactivity of molecules, which are

critical for the normal functioning of the organism. For exam-

ple, low lead concentration may produce higher toxicity in

certain body organs such as the kidneys, bone marrow, and

nervous system whereas liver and pancreas may be less af-

fected. Pb2þ especially reacts with and inactivates enzymes

involved in the synthesis of the heme unit of hemoglobin

that may be used for diagnostic purposes.

Clinical efficacy of a chelating agent is dictated by its ability

to displace the toxic metal ion from biomolecules such as

enzymes and subsequently reactivating it. In the process, the

toxic metal–chelate complex must be excreted from the body

rendering the metal unavailable for attaching to another bio-

molecule. Moreover, the metal present in the system as a

metal–chelate complex is not toxic compared to the uncom-

plexed metal ion. For example, the toxicity of cadmium was

reduced when complexed with EDTA and some of its ana-

logues. It was further evident since the lethal dose, 50%

(LD50) of cadmium chloride in mice was 4.9 mg Cd kg�1 that

racter

y used chelators Examples

DMSA, DPA, NAPA, EDTA, DTPA1,2-Dimethyl-3-hydroxypyrid-4-one (L1)

OH BAL, L1DMPSDPA, TETA, DFOANAPATETA,EDTA, DTPA, L1DFOADMSA, DMPS, BAL, DPA, NAPADDCTTD


was altered by simultaneous EDTA administration to 18.4 mg

Cd kg�1 and was raised for the Cd–DTPA complex to 48.4 mg

Cd kg�1.122

The chelating agent-induced excretion of a toxic metal de-

pends on whether the chelator is available extracellularly or

can penetrate the intracellular space and gain access to the toxic

metal ion in the cytosol and organelles inside the cell, thus

facilitating its removal. Chelating agents may gain access to

intracellular deposits of toxic metals by four common

pathways:

1. nonpolar chelating agents can pass through the lipid por-

tion of the cellular membrane,

2. electrically neutral polar chelating agents can pass through

the cellular membrane,

3. some monoanions can pass through appropriate monoa-

nion transport systems, and

4. some dianions can pass through appropriate dianion trans-

port systems.

The use of a chelating agent restricted to the extracellular

space can cause a large reduction of the toxic metal concentra-

tion in that space. This is turn will usually favor the diffusion of

some of the toxic metal from intracellular sites to extracellular

sites. The repeated administration of such a chelating agent will

result in a gradual reduction of the total amount of toxic metal

present. The chelating agents can be categorized based on the

types of intracellular sites to which they can readily gain access.

This is summarized in Table 2.

Toxic metals frequently concentrate in the liver and the

kidneys. Using these criteria, we can categorize those chelating

agents used in the clinic (Figure 3).

3.35.3.3 Conventional Chelators and Their Current Usein Metal Toxicity

British anti-lewisite (2,3-dimercaprol; BAL) is electrically un-

charged and has a solubility in lipids, which is sufficient to

allow it to penetrate into most organs. DMPS has a single

negative charge at physiological pH values and can be trans-

ported into intracellular sites in organs, which have suitable

transport systems such as the kidneys.123 meso-DMSA has a�2

charge at physiological pH and hence can apparently be trans-

ported into renal cells via the succinate transport system. EDTA

and DTPA carry negative charges of�2 and�3 at physiological

pH and are almost completely confined to the extracellular

spaces. A small fraction of the EDTA is, however, secreted by

kidneys124 and a small fraction of DTPA is excreted in the

bile.125 D-Pencillamine (DPA) is rapidly absorbed from the

GI tract, undergoes a complex metabolism, is rapidly excreted

Table 2 Sites accessible to various types of chelating agents

Type of chelating agent Accessible sites

Neutral (uncharged) Intra- and extracellular sPolar, highly hydrophobic Extracellular sites, brain,Negative charge (single, double, and more than�3 charge)

Extracellular spaces, cell(e.g., kidneys and liver

High molecular weight Liver cells if compound i

in the urine, and in rat concentrates in organs, which have a

high collagen content.126 Triethylenetetramine dihydrochlor-

ide (TRIEN.2HCl) is absorbed up to about 20% in the gut of

the rat.127 When given intravenously (i.v.), most TRIEN passes

rapidly into the urine and a smaller fraction is presumably

excreted in the bile and appears in the feces. DFOA is largely

confined to the extracellular space and enhances the urinary

and biliary iron excretion of iron.128 Diethyldithiocarbamate

(DDTC) is a monoanion and can presumably use the mono-

anion transport systems in kidneys and liver to reach the

intracellular sites.129

3.35.3.4 Clinical Chelators

3.35.3.4.1 British anti-lewisiteBAL (dimercaprol) is indicated as a chelating agent in arsenic,

gold, and mercury poisoning. In arsenic (except for arsine gas)

toxicity, early administration of dimercaprol helps in the rever-

sal of acute and some of the chronic manifestations of poison-

ing, although polyneuropathy may be refractory.130 Chelation

therapy is recommended if urinary arsenic levels are consis-

tently above 200 mcg l�1.131 Dimercaprol therapy is most ef-

fective in acute inorganic and aryl organic mercury toxicity,

when begun within 1 or 2 h after metal ingestion, and ceases

to be effective after about 6 h. It is also effective in elemental

mercury poisoning. It is also indicated for the treatment of

acute and chronic lead poisoning when administered in con-

junction with calcium EDTA. When administered promptly,

dimercaprol complements edetate calcium disodium

(CaNa2EDTA) by more rapidly removing lead from RBCs and

the central nervous system131 than edetate calcium disodium

alone.132

BAL, due to its limited therapeutic efficacy and high toxic-

ity, is suited only for brief treatment of acute intoxications. It is

unstable, susceptible to oxidation, and therefore difficult to

store as a ready-for-use preparation. Owing to its high lipophi-

licity, it can be administered only by intramuscular injection,

which is very painful. The absorption from the site of injection

is rapid and complete, and the drug demonstrates apparently

high systemic distribution. It easily crosses most physiological

barriers and is rapidly excreted in urine as dithiols and

glucuronides.

3.35.3.4.2 DMSA and DMPSDMSA and DMPS are water-soluble chemical analogues of

dimercaprol (BAL), and have less toxicity and greater water

solubility than the parent compound. DMSA is registered in

USA as a drug for treatment of lead intoxication. DMPS is

registered in Germany for the treatment of mercury intoxica-

tion; however, it is not approved in USA.

itescells and fatty tissue, etc.s with appropriate monoanionic transport systems in their membranes)s excreted in the bile

CH2 CH CH2

SH SH OH

HOOC CH CH

SH SH

COOH H2C CH CH2 SO3Na

SHSH

DMPS

N CH2 CH2 N

HOOCCH2

HOOCCH2

CH2COOH

CH2COOH

DMSA

N CH2 CH2 N

HOOCCH2

HOOCCH2

CH2COOH

CH2COOH

N CH2 CH2

CH2COOH

DTPA

CH3 C

CH3

SH

CH

NH2

C

O

OH

DPA

H2N CH2 CH2 NH CH2 CH2 NH CH2 CH2 NH2.2HCl

N C

CH3CH2

CH3CH2

S

S-Na+

DDTC

N

O

OH

CH3

CH3

H2N N

OH

O

N

OH

O

CH3NH

N C

CH3CH2

CH3CH2

S

S S

S

N

CH2CH3

CH2CH3

TTD

DFOA

HN O

OH

O

CH3

H3C

CH3

SH

NAPA

EDTA

TETA

L1

BAL

2

Figure 3 Structure of chelating agents used in the treatment of metal intoxications.


DMSA or succimer is indicated for the treatment of lead

poisoning in children with blood lead concentrations above

45 mcg dl�1.133 It is also is indicated to treat lead toxicity in

adults. Succimer used orally forms stable, water-soluble com-

plexes with lead and consequently increases its urinary excre-

tion. It also chelates other heavy metals such as arsenic and

mercury.134,135 The metal–chelate structures of DMSA are

shown in Figure 4.136 The complexes of Cd2þ or Pb2þ are

insoluble in the pH range of 1.0–7.1, but they are solubilized

when the noncoordinated thiol and carboxylic acid groups are

dissociated. The Hg2þ complex is insoluble in the pH range of

1.0–3.0. It dissolves when one of the noncoordinated carbox-

ylic acid groups is deprotonated.

Due to the presence of highly charged carboxyl groups in its

structure meso-DMSA cannot enter a cell membrane. The sta-

bilities of the metal chelates of the disodium salt of DMSA

follow the order Cd2þ>Pb2þ>Fe2þ>Hg2þ>Zn2þ>Ni2þ.However, the major drawback of interpretation of these results

in vivo is that Cd2þ in a cell is firmly bound to MT. Since DMSA

has limited potential to enter the cell and compete with MT for

the Cd2þ, its therapeutic use is limited regardless of how stable

the chelate is.

A quantitative comparison has demonstrated that DMPS

is 28 times more effective than BAL for arsenic therapy

in mice.137 DMPS is more effective than DMSA in removing

mercurial compounds from the kidneys.138 DMPS has also

O C C C C O

-O

H H

SH O-SH

O C C C C O

O

H H

SH O-S

O C C C C O

-O

H H

S O-S

XX = Cd or Pb

Hg

Figure 4 Metal chelate structures of meso-2,3-dimercapotosuccinic acid.


been shown to protect mice against the lethal properties of Cd

compounds.139 Structures have been proposed for the soluble

As–DMPS complexes formed by DMPS and different arsenic

compounds. Classical thioarsenite ring structures having

DMPS:arsenic ratios of 3:2 have been suggested.140

DMPS is slightly more toxic than DMSA, and both com-

pounds are much less toxic than BAL.141 They are available as

tablets for oral administration, and both are suited for paren-

teral administration as well. Both these drugs are absorbed to

some degree in the intestinal tract142 and DMSA shows up to

40% urinary excretion within 16 h of an oral dose in

humans.143 The distribution of both drugs is predominantly

extracellular; however, DMPS has also some intracellular

distribution.144 The primary route of excretion for DMSA is

urinary, with an elimination half-time of less than 4 h in

humans,145 whereas DMPS follows slower excretion with a

half-life of 9–10 h.146 After an oral dose of DMSA to humans,

more than 95% of the drug available in blood is covalently

bound to proteins mainly albumin.145 More than 90% of

urinary DMSA is excreted as the DMSA–cysteine mixed

disulfide.147 As opposed to DMSA, the urinary excretion prod-

ucts after oral administration of DMPS to humans are various

acyclic and cyclic homopolymers of DMPS, whereas a mixed

disulfide with cysteine is almost completely absent.148

3.35.3.4.3 DPA and NAPAWalshe discovered that patients administered with penicillin

showed elevated levels of copper in their urine.149 The com-

pound responsible was identified as DPA, a metabolic product

of penicillin. DPA is used to remove excess copper associated

with Wilson’s disease.149 It acts by reductive chelation, viz.

reducing copper bound to proteins, which causes decrease in

the affinity of protein for copper, thus allowing the copper to

bind with DPA. The immediate and most dramatic effect of the

administration of DPA is a marked increase in urinary copper

excretion. It is also used to reduce cysteine in the urine (cystin-

uria) and to treat severe rheumatoid arthritis. DPA has also

been shown to increase the amount of 109Cd in kidneys when

compared to nontreated controls.150 Nephrotoxicity is how-

ever, the chief disadvantage when using DPA. DPA can be

administered orally as well as by i.v. infusion. The intestinal

absorption of DPA in rats and humans is about 50%. The

volume of distribution is close to that of extracellular water,

and the formation of mixed disulfides with serum albumin is

extensive. The majority of the absorbed dose is rapidly excreted

in urine as free DPA or the oxidized dimer without significant

metabolism.

N-acetyl-D-penicillamine (NAPA) is a white crystalline com-

pound sparingly soluble in water and is ninhydrin negative

owing to the lack of a reactive a-amino group. The metabolic

behavior of NAPA is similar to that of DPA. NAPA was equally

effective as DPA in Iraq in an outbreak of methyl mercury

poisoning (1971–72) due to fungicide-treated wheat.151

3.35.3.4.4 EDTA and DTPAEDTA is used to lower blood levels of calcium in case of severe

overdose. DTPA works by removing certain radioactive chemi-

cals from the body and is considered most effective in decreas-

ing the amount of Cd. DTPA and EDTA are effective in

reducing tissue Cd and increasing urinary Cd excretion. Unfor-

tunately, in clinical situations the need for antidotes is usually

at a time long after exposure to Cd, for example, in ‘itai-itai’

disease in Japan. Na2CaEDTA is effective in the treatment of

lead intoxication. The use of the calcium complex eliminated

the danger of tetany, which was found when the parent com-

pound (Na2EDTA) was administered rapidly. Na2CaEDTA is

still in use for lead poisoning in some cases though it has been

replaced by Na3ZnDTPA for the treatment of plutonium

intoxication.114

Both EDTA and DTPA are poorly absorbed in the GI tract

(<5%), and are administered by slow i.v. infusion of their

calcium or zinc complexes. Their volumes of distribution are

close to that of extracellular water, and both chelators are

rapidly excreted in the urine without significant metabolism.

EDTA and DTPA form complexes with a variety of metal ions,

including most essential metals. Accordingly, continued expo-

sure may induce trace element depletion, especially for Zn, Cu,

and Mn.152 The teratogenicity of high EDTA doses is due to Zn

depletion, which is readily reversed by co-administration of

zinc. Extensive zinc binding is most likely involved in the acute

toxicity of CaNa2EDTA, thus Zn2EDTA is more than one order

of magnitude less toxic than Ca2EDTA, which is a factor of 20

times less toxic than the tetra sodium salt. However, EDTA and

DTPA are problematic clinical chelating agents due to low

intestinal uptake necessitating slow intravenous administra-

tion, their exclusively extracellular distribution, and high sta-

bility constants with some essential metals.

3.35.3.4.5 TriethylenetetramineDue to the frequent development of DPA intolerance among

patients, triethylenetetramine (TETA) has been used recently as

an alternative for removing excess copper from the body in

Wilson’s disease. TETA is, however, a less efficient Cumobilizer

than DPA and its toxicity is not extensively studied. It is ad-

ministered orally and shows poor GI absorption as demon-

strated by Gibbs and Walsche.128 Less than 20% of the orally

administered 14CTETA was recovered in the carcass and urine of

rats.127 Kodama et al. recovered only about 1% free TETA in the


urine after an oral dose of TETA was given to human

volunteers.153 The major part was excreted as 1-acetyl-TETA.

The threshold of toxicity for TETA was established as being

close to 50 mg kg�1 day�1 in female rats and less in male

rats.154 The acute toxicity of TETA is low. However, the recom-

mended dosage in Wilson’s disease is 0.75–2 g day�1, which is

quite close to a potentially toxic dose. Based on experience with

the long-term use of TETA in Wilson’s disease patients, this

chelator is remarkably free of side effects compared to DPA.155

Recently, sodium tetrathiomolybdate (Na2MoS4) has been

used as an alternative to DPA or TETA.156

3.35.3.4.6 DFOADFOA binds to iron and removes it from the blood stream.

DFOA is used to treat iron overload caused by blood trans-

fusions in adults and children at least 3 years old. It completely

encapsulates Fe(III) during complex formation, thereby

preventing iron catalyzed free radical reactions.157 DFOA was

demonstrated to increase urinary iron excretion in thalassemia

major patients, offering treatment of infusion-related iron

toxicity in these patients for the first time.158 Also, severe

iron-poisoning cases due to ingestion of concentrated iron

supplements show corrosion of the gastric mucosa, metabolic

acidosis, coagulopathies, and multiorgan failure. In such cases,

the treatment available is the mechanical removal of residual

tablets, extensive supportive care, and chelation with DFOA.

However, clinical studies of milder cases due to multivitamin

tablets have failed to demonstrate a beneficial effect of DFOA

chelation.159

The absorption of DFOA in the GI tract is low. DFOA is

therefore administered by i.v. infusion or injection. Its distri-

bution volume is extracellular, and the protein binding in

plasma is low (<10%). It follows a biphasic renal excretion

with slow half-life being about 6 h. The acute toxicity is rather

low, and i.v. infusion is safe provided slow infusion follows

to avoid hypotension. However, a wide range of side effects

have been documented in patients undergoing prolonged

therapy.160

3.35.3.4.7 L1Deferiprone (1, 2-dimethyl-3-hydroxypyridin-4-one, also known

as L1, CP20 or Ferriprox) is a low-molecular-weight bidentate

orally active iron chelator, belonging to the 3-hydroxy-4-

pyridinone group compounds. It introduced a new chapter in

the iron overloadmanagement. L1 has a higher affinity for Fe(III)

than similar oxygen-donating bidentate ligands. Due to the aro-

matic resonance effects electron density on the oxygen donor

increases in the 4-position, making the deprotonated pyridinone

pro-ligand a double oxo donor for metal cations.161 L1 can

induce an appreciable urinary excretion of iron when given

orally.162 L1 offers an alternative to DFOA in the treatment of

transfusional Fe overload inhemoglobinopathies inpatientswho

do not tolerate DFOA; the cost effectiveness of L1 compared to

DFOA forms an added advantage. L1when administered orally is

rapidly absorbed in the GI tract. The main excretion route is via

the kidneys, with a half-life of 47–134 min.163 Recovery from

urine is close to 100%, themain species being free L1, the Fe and

Cu complexes, and glucuronide. The acute toxicity of L1 is some-

what lower than that of DFOA. Clinical experience with L1 in-

dicates various adverse effects, thus indicating the need for better

chelators, which can be used for lifelong chelation of chronic

transfusional Fe overload.

3.35.3.4.8 DDTCSodium DDTC was introduced for the treatment of nickel

carbonyl intoxication. It forms a chelate with nickel such that

nickel bisDDTC is formed and excreted through kidneys. It was

proposed that DDTC, unlike water-soluble chelating agents, is

capable of binding intracellular Ni in the lung and brain, thus

decreasing the nickel content of the principal target organs for

nickel carbonyl toxicity. The use of DDTC is not recommended

for other toxic metals because of the lipid-soluble complexes,

which it forms with many of them. These lipophilic complexes

readily pass into the central nervous system and may cause

toxicity. Disulfiram (Antabuse), a prodrug of DDTC, has

more recently been considered as an alternative chelating

agent. Disulfiram may protect against nickel carbonyl toxicity,

but there is currently insufficient evidence to confirm this.

Moreover, particularly at high disulfiram doses, brain

nickel retention is a potential hazard, and mortality may be

increased.164

3.35.3.5 Limitations of Chelating Agents

The human body with the limitation of eliminating excess

metal to achieve homeostasis has been greatly benefited with

the advent of chelation therapy in numerous pathologic man-

ifestations. However, similar to any approved regime of ther-

apy, most conventionally used chelating agents possess their

share of limited efficacy along with adverse effects and

contraindications.115,117 None of the conventionally used che-

lators has been able to fulfill the criteria to qualify as an ideal

chelating agent. In search of high metal specificity, target selec-

tivity, low or no toxic effects, newer chelating agents and their

analogues have been constantly explored. Most common

adverse effects due to chelation therapy may be generally

classified as nephrotoxicity, essential metal loss, mild hepato-

toxicity, skin reactions, etc. (see Chapter 3.08).

3.35.3.5.1 Limited therapeutic efficacyChelation therapy, unlike most prescriptions in pathologic

manifestations, follows a tailored approach specific to each

patient. Selection of drug changes with the target metal to be

eliminated, but interestingly also with the extent and duration

of metal exposure. Managing an acute case of metal poisoning

may follow a recommended protocol, but in cases of chronic

exposure, the therapeutic strategy needs to be defined. Chronic

metal exposure results in intracellular deposition of metal that

binds with the physiological ligands replacing the essential

body metals. BAL, one of the first chelating agents introduced,

is lipophilic with extensive pharmacokinetic distribution that

also dictates its adverse effects. Ironically, the hydrophilic de-

rivatives of BAL (DMSA and DMPS) thus exhibited lower tox-

icity and the advantage of oral administration, but the major

drawback of extracellular distribution rendering them ineffec-

tive in chronic metal poisoning. During clinical trials con-

ducted in Bangladesh, DMSA was found ineffective in chronic

cases of arsenic poisoning.165 Our group has repeatedly dem-

onstrated the limited efficacy of succimer during chronic metal

exposures.166–169 This also holds true for CaNa2EDTA that


cannot pass through most physiological barriers, restricting its

use to removing metals available in extracellular spaces. Neu-

rotoxicity due to metal redistribution is another important

limitation with the hard tissue (bones, hair, and nail) mobiliz-

ing chelating agents such as BAL. BAL induces redistribution of

mercury and arsenic from peripheral tissues to the brain and

increases the toxicity of cadmium and lead, and forms a com-

plex with lead (Pb–BAL complex) which is as toxic as lead

itself.116,170–172 CaNa2EDTA employed for the lead mobiliza-

tion test is also associated with lead redistribution in the brain.

3.35.3.5.2 Adverse effects of chelation3.35.3.5.2.1 Nephrotoxicity and hepatotoxicity

The coordination complexes formed following drug–metal in-

teraction are generally eliminated through the renal route.

Thus, chelation therapy increases the load of urinary metal

excretion, which allows renal tissue to be in closer proximity

with the toxic metals at higher doses. For example, CaNa2EDTA

causes severe, dose-related nephrotoxicity in patients undergo-

ing chelation therapy despite lowering the dose to below

50 mg kg�1 day�1. The symptoms reported include increased

renal function biomarkers such as creatinine along with gly-

cosuria, proteinuria, microscopic hematuria, and large epithe-

lial cells in the urinary sediment. CaNa2EDTA-induced

nephrotoxicity is attributed to high burden of lead mobilized

by treatment rather than direct adverse effect of the drug. One

such report suggests that 15000 mg of lead is eliminated in the

first 24 h; also after renal function returns to normal

CaNa2EDTA did not induce deleterious effects. Moreover, re-

ports suggestive of efficacy of EDTA in chronic renal artery

diseases are available.173,174 Succimer, another conventionally

used lead chelator, has been established as being safe with rare

or no renal adverse effects reported. DMSA- and DMPS-related

adverse effects usually are rare and mild that either subside

after continuous usage or reverse on the cessation of therapy.

The usual course or treatment is DMSA, and DMPS is short

which may be responsible for fewer side effects. Iron chelator

DFOA is experienced with renal toxicity since the drug is used

for long-term treatment.116

Hepatic transaminase activity indicative of liver function

has been found elevated during both CaNa2EDTA and succi-

mer therapy. At the recommended protocol DMSA, more than

CaNa2EDTA is associated with clinically significant hepatotox-

icity that was reversible on cessation of chelation.175,176 These

incidences may be more closely monitored in the pediatric

population due to high vulnerability.

3.35.3.5.2.2 Essential trace-metal loss

A chelating agent with an absolute specificity is yet to be

identified. Most chelating agents are known to deplete the

body’s essential metals along with reducing the desired toxic

metal burden. Stability constants of edetate complexes with the

toxic metals are similar to those with essential metals and that

of calcium complex being much lower. These pose a serious

implication on biological processes due to mobilization and

excretion of zinc, copper, and other essential elements along

with the toxic metals. Chelation of ionic Ca(II) present in

blood along with that dissociated from bones results in tetany

and even death.Modification of sodium edetate to CaNa2EDTA

was done solely to address the high calcium loss caused

by the former, leading to severe hypocalcemia and death.177

The use of CaNa2EDTA in lead poisoning does not disturb body

calcium levels since Ca(II) is readily exchanged for Pb(II) by

virtue of very different stability constants of the complexes. The

newer CaNa2EDTA has however been associated with zinc de-

pletion with an average of 11-fold increase in 24-h urinary zinc

excretion following a 20 mg kg�1 dose given intravenously.178

Similar reports in children and occupational exposures sug-

gested up to 19 and 33 times increase in mean daily urinary

zinc excretion respectively following sodium calcium edetate.

In children, this results in about a 62% decrease in plasma zinc

that later rebounded to reach 104% within 48 h of discontinu-

ing the drug. Other elements depleted are Cu and Mn.178 Zinc

depletion is established as the key mechanism responsible for

the teratogenic effects of the drug when administered during

pregnancy. Most conventional chelators thus have been associ-

ated with zinc depletion including BAL, succimer, L1, etc.

DMSA has been associated with more profound urinary copper

excretion than zinc.179–181

3.35.3.5.2.3 Allergic manifestations and skin reactions

Adverse dermatological reactions are either attributed to the

effect of drug per se or due to essential metal deficiency. For

example, zinc depletion induces the mucocutaneous lesions

reported following CaNa2EDTA treatment.178 Similarly, the

DMSA profile suggests 6% incidence of skin reactions, classi-

fied as popular rash, herpetic rash, rash, pruritus, or mucocu-

taneous eruptions, the pathogenesis of which is not well

understood.175,178,182,183 DMPS, on the other hand, needs

cautious monitoring in patients with allergic asthma symp-

toms that may develop hypersensitivity to the drug.184,185

Anaphylactic reactions following DPA administration in pa-

tients allergic to penicillin may occur. Further, prolonged use

of the drug precipitates cutaneous lesions, dermatomyosites,

adverse effects on collagen, dryness, etc. DFOA administration

has also been associated with allergic manifestations and skin

reactions, whereas L1 results in an increase in antinuclear

antibodies and rheumatoid factors in some patients of iron

overload.116,160

3.35.3.5.2.4 Miscellaneous adverse effects

Most chelating agents have been associated with unpleasant

adverse effects such as nausea and vomiting as reported for

BAL, CaNa2EDTA and succimers. BAL being highly lipophilic

exhibits the highest adverse drug events including profuse

sweating, high fever, hypertension, and tachycardia. Calcium

disodium edetate has also been associated with the onset of

malaise, fatigue, and excessive thirst followed by chills and

fever that may aggravate to result in severe myalgia, frontal

headache, nausea, vomiting, and rarely increased urinary fre-

quency and urgency.116 Other adverse reactions may include

histamine-like manifestations with nasal congestion and lacri-

mation, transitory lowering of systolic and diastolic blood

pressure, prolonged prothrombin time, inversion of the

T-wave of the electrocardiogram (ECG), and pain at the injec-

tion site. Compared to edetate, succimer is rarely associated

with incidences of nausea and vomiting or headache, but

neutropenia is suggested as a potential adverse effect.175,186

DMPA is suggested to be safer than DMSA but intravenous

infusion may result in hypotension. DPA has been considered


generally safe but has had limited use due to its adverse effects

such as hypertension, nephritic syndrome, and autoimmune

reactions.116 Compared to DPA, TETA was found to be safer

in the long-term treatment of Wilson’s disease,187 although

the recommended dose is quite close to its potentially toxic

dose. Iron chelators are generally prescribed for long-term use

that can result in various pulmonary, neuronal sensory effects

such as the ophthalmic and auditory toxicity. During pro-

longed DFO therapy, bacterial and fungal infections such

as Yersinia enterocolitica infection, sepsis, and mucormycosis

have also been reported. It is suggested that iron-dependent

pathogens that cannot synthesize siderophores such as Y.

enterocolitica and certain bacteria are thus supplied with iron

leading to their high virulence.188 Although L1 has lower side

effects compared to DFOA, the compound is associated with

adverse effects such as gastric discomfort, changes in blood

histology, transient agranulocytosis, or transient musculo-

skeletal and joins pain.116 Thus, the search for an iron chela-

tor that can be used lifelong by blood transfusion patients

is not yet over.

3.35.3.6 Contraindications

Under various physiological and pathological circumstances,

chelation therapy is either contraindicated or prescribed with

close monitoring or with a word of caution. During preg-

nancy, for instance, chelation therapy is contraindicated to

avoid possible developmental toxicity associated with most

chelating agents. Since all chelating agents share the disad-

vantage of nonspecificity to a certain extent, depletions of

essential metal, especially zinc and copper, are suggestive

mechanisms for chelation-induced developmental toxicity.

BAL demonstrated teratogenic effects in mice following sub-

cutaneous administration on gestation day 9 through 12. The

malformations were skeletal and of the extremities.189 Subse-

quently, DMSA also showed embryonic toxicity at varied

doses via parenteral and oral routes of administration. Al-

though not prescribed orally, delivery of the drug via subcu-

taneous routes established 410 mg kg�1 day�1 as the ‘no

observed teratogenic effect level’ in mice, with 820 and

1640 mg kg�1 day�1 being toxic.190 However, following de-

livery through the oral route, toxic manifestations occurred

when administered during day 6 through 15, but no effects

from day 14 to postnatal day 21 in rats. DMSA is suggested to

be toxic via direct effect of the drug on the embryo/fetus rather

than indirectly through maternal toxicity.190 Thus, along with

maternal toxicity, embryonic/fetal zinc and copper depletion

is suggested as a possible mechanism for DMSA-induced

developmental toxicity. In the late gestation period, the no

observed effect level (NOEL) for maternal and embryo toxic-

ity for DMSA orally was established at 100 mg kg�1 day�1

while the teratogenic NOEL was 1000 mg kg�1 day�1 with

no toxicity observed.191,192 When compared with DMSA its

analogue, monoisoamyl DMSA (MiADMSA), was found ef-

fective in countering the teratogenic effect of arsenite and

mercury during lactation and pregnancy. MiADMSA did not

produce any teratogenic manifestation except mild essential

metal imbalance in maternal rats.193 In recent work testing

MiADMSA using embryonic stem cell-derived embryoid

bodies, MiADMSA decreased the arsenic-induced toxicity

without any major embryo-toxic effects in vitro supported

with the corresponding in vivo model. Despite promising re-

sults, the use of MiADMSA during pregnancy for prophylactic

or therapeutic benefits is far from established.194 DMPS

although not prescribed during pregnancy has rarely been

reported to cause teratogenic or developmental toxicity in

animal experimental models. In mice, the drug was found to

be safe at levels up to 300 mg kg�1 day�1 with oral NOEL

established at 630 mg kg�1 day�1, a dose that is much higher

than that used in the treatment of humans. The teratogenic

effects of EDTA are well established as being due to zinc

depletion. Zn2EDTA has been suggested yet extensive Zn

binding during acute metal poisoning treatment is

unavoidable.116 Human reports on such subjects are usually

rare yet CaNa2EDTA infusion was documented to be well

tolerated at 75 mg kg�1 day�1 for 7 days and 1 g twice daily

for 3 days. DPA has been extensively studied with mixed

results on the safety and efficacy of the drug even in pregnant

patients under therapy.195 However, since Wilson’s disease

patients exhibit excess copper in the system, the developmen-

tal effects may be low. Thus, where human experience sup-

ports the use of DPA throughout pregnancy, certain animal

experimentation has raised concern of developmental

defects.195 Chelating agents prescribed for iron overload

show drug-specific effects. DFO in humans is reported to be

crucial for maternal survival yet risk of spontaneous abortions

cannot be ruled out. Animal experimentation with DFO

established the no observed adverse effect level (NOAEL) for

maternal toxicity at 44 mg kg�1 day�1 and for developmental

toxicity at 352 mg kg�1 day�1. Moreover, since the molecule

does not cross the placental barrier, the only possible mode

of fetal toxicity is through maternal effects at high doses.196

Interestingly, L1, which is generally considered safer than

DFO, demonstrates a higher teratogenic effect than DFO. Mat-

ernal oral NOEL established for L1 in rats was 75 mg kg�1,

whereas that for teratogenic effects is <25 mg kg�1 while in

rabbits these were 25 and 10 mg kg�1, respectively.197

Natural antioxidants with mild chelation properties, such

as N-acetylcysteine (NAC), although considered safe, may also

cause possible developmental toxic effects.198,199 NAC, when

administered orally, shows increased incidence of congenital

malformation in metal-exposed animals, whereas intravenous

administration significantly reduced the embryonic effects of

methyl mercury in mice. NAC is generally considered safe but

supportive evidence for establishing the safety profile is

needed.198,199

Several other physiological factors need to be tested before

prescribing chelation. For example, a glucose-6-phosphate de-

hydrogenase deficiency test must be conducted before BAL or

succimer therapy, and an allergy test for penicillin is required

before DPA administration due to the risk of hemolysis200 and

anaphylactic shock, respectively. DPA usage in patients with

rheumatoid arthritis also needs monitoring since thrombocy-

topenia and other common adverse effects of the drug are

reported.198 A possible mechanism suggested for these adverse

effects is due to the involvement of human lymphocyte anti-

gens that are more marked in rheumatoid patients. Similarly,

patients with anuria or severe renal disease, or those allergic to

DFO, are not prescribed the drug.198 Patients with renal and

hepatic insufficiency need close monitoring since numerous

O CH3


chelators may further worsen the situation, as discussed in the

previous section (adverse drug reactions).

OH

OHS

HS

O

CH3

Figure 5 Chemical structure of MiADMSA.

3.35.3.7 Recent Advancement in Chelation Therapy

Chelation therapy has traveled a long way from a nonestab-

lished therapeutic solution, a controversial and unapplied inter-

vention in clinical toxicology, and has moved toward better

understanding and wider application as indicated by numerous

and larger clinical trials. The science of metal chelation in ther-

apy has expanded in treating not only metal poisoning but also

other disease manifestations associated with metal overload as

in blood transfusion disorders and related cardiac complica-

tions, neurodegeneration and cancer, etc. However, as a main-

stay therapy in managing metal poisoning, the need for newer

chelating agents is being realized due to the following:

1. the unavailability of prophylactic measures or vaccines for

the populations at risk of metal exposure such as occupa-

tional or geo-environmental exposures;

2. limitations identified in the conventionally available che-

lating agents (discussed in the previous section) including

poor site selectivity, metal specificity, high toxicity, etc.; and

3. the need to establish faster and safer toxic metal elimination.

3.35.4 Development of New Chelating Agents

3.35.4.1 Monoesters of DMSA

DMSA has long been the mainstay in the treatment of lead

poisoning and alternatively recommended for chelating most

thiol-binding metals. As discussed in the previous section, the

drug was introduced as a water-soluble derivative of BAL, thus

lowering the adverse effects of the parent drug. However, its

hydrophilic nature limits its access to intracellular sites render-

ing it ineffective against chronic metal poisoning. In the 1990s

with the advent of alarming reports of arsenicosis due to

chronic arsenic exposure of millions of subjects in Asian re-

gions, the need for a newer chelating agent increased.22 The

issue was addressed by preparing slightly lipophilic analogues

of DMSA that were conceptualized, synthesized, and investi-

gated for efficacy and toxicity by our group.170 Out of all of the

analogues (methyl, ethyl, propyl, isopropyl, butyl, isobutyl,

pentyl, isopentyl, and hexyl), MiADMSA, the C5 branched

chain alkyl monoester of DMSA synthesized by controlled

esterification reaction, was found to be the most promising

(Figure 5). The potential drug shows high efficacy for toxic

metal chelation from intracellular matrix due to its lipophilic

nature and thus is effective against chronic metal poisoning

cases. The safety profile of MiADMSA was established in in vivo

and in vitro models including in human embryonic stem cell-

derived embryoid bodies. The potential drug had no effect on

the length of gestation, litter size, sex ratio, viability, and

lactation.193,194 However, Taubeneck et al. reported develop-

mental toxicity of DMSA, its parent compound by virtue of

disturbed copper metabolism and hence more critical evalua-

tion may be needed.192 MiADMSA has been developed as a

drug of choice against chronic arsenic poisoning: it is effective

in chelating lead, mercury, and other thiol-binding toxic

metals. The molecule is in its development phase and has

recently qualified to enter human clinical trials in India.

3.35.4.2 Crown Ethers

Crown ethers provide an interesting platform for the designing

of newer chelating agents by virtue of (1) their cyclic frame-

work that forms the basis of metal complexation and (2) the

possibility of enhancing the selectivity and cation-binding

ability by functionalization with pendant arm(s) containing

additional donor atom(s) (Figure 6).

Ferreiros-Martınez et al. have recently reported that pen-

dant crown ligands H2bp12c4 and H2bp15c5 show remark-

able Cd(II)/Ca(II) selectivity.201,202 Recently, newer chelating

agents from the crown derivative N0,N0-bis[(6-carboxy-2-pyridil)methyl]-4,13-diaza-18-crown-6 (H2bp18c6) for the

chelation treatment of intoxication with large metal ions

such as Pb(II) and Sr(II) have been investigated.203 The mac-

rocyclic decadentate receptor ligand bp18c62 forms com-

plexes with large metal ions such as Sr(II) and Pb(II)

providing much higher Pb(II)/Ca(II) and Pb(II)/Zn(II)

selectivity than conventionally used chelators such as

EDTA.203 Further, the ligands also offer the highest Sr(II)/Ca(II)

selectivity reported so far. These large ligands have also shown

remarkable selectivity for the lanthanide ions as the ionic radius of

the lanthanide(III) ions decreases (log KCeL–log KLuL¼6.9)

which is not observed in the case of smaller ligands.203 This

behavior is largely attributed to the decreased basicity of the two

amine nitrogen atoms as compared to EDTA. Thus, the receptor

bp18c6 shows promise for application in chelation treatment of

metal intoxication by Pb(II) and 90Sr(II).204

3.35.4.3 VK-28 and Its Analogues

The dual property of a drug to act as a brain iron chelator

and possess monoamine oxidase (MAO) inhibitory activity is a

feasible approach for neuroprotection in neurodegenerative

diseases.205 The iron chelator desferrioxamine (desferal), used

clinically in the treatment of iron overload (thalassemia major),

does not have the ability to cross the BBB and also exhibits poor

MAO inhibitory activity.205 Similarly, somemetal chelators such

as VK-28205 have been developed which cross the BBB and also

show poor MAO inhibitory activity (Figure 7).

Youdim et al. and Zheng et al. have prepared compounds

derived from the prototype iron chelator, VK-28, by combin-

ing the propargylamine MAO inhibitory moiety of the

anti-Parkinson drug, rasagiline, with the chelators at different

N

O

N

ONNHOOC COOH

OO

N N

O

NN COOHHOOC

O O

N

O

N

ONN

O O

N

O

N

O

COOH

COOH

HOOC

HOOC

O O

N

O

N

ONNHOOC COOH

O O

N

O

N

O

COOHHOOC

H2bp12c4 H2bp15c5

bpy18c6 H4oddm

H2bp18c6 H4odda

Figure 6 Chemical structures of crown ether-derived chelators.

N

N N

HO

OH

N

N N COOEt

OH

N

N N

OH

N

N N

OH

N

HN

OH

SH

OEt

O

N

OH

COOEt

NH

VK28 HLA16 M30

HLA20 M32 M31

Figure 7 VK-28 chelate analogues.


sites.206,207 These compounds retain the potent iron-chelating

property of VK-28, which is similar to desferal, and inhibit

iron-induced membrane lipid peroxidation with equal po-

tency. These newly developed drugs have been proposed to

have a greater ability to prevent the formation of ROS, oxida-

tive stress, and neuronal death than the individual drugs.

3.35.4.4 Indazoles

Poly(pyrazol-1-yl)alkanes ligands were first reported in 1960

but have only gained attention in the past decade.208 Bis(pyr-

azolyl)alkanes offer the possibility of modulating the steric and

electronic properties by introducing substituents, not only in

N

N

N

N

N

N

N

N

L1(a)

(b)

L2

N

N

N

N

Cd

N

N

N

N

Cl

Cl

N

N

N

NHg

X

X

Figure 8 (a) Chemical structure of indazole ligands. (b) Metal–chelate complexes of cadmium and mercury with indazole ligands.

O

OHO

HO OH

OH

O

O

Figure 9 Chemical structure of ellagic acid.


the pyrazole rings, but also at the central carbon atom. In a

recent expansion, bis(pyrazol-1-yl) fragments in conjunction

with several bridging spacers to generate ‘third-generation’ flex-

ible di- and polytopic ligands that are able to afford complexes

of several d-block metal acceptors with different nuclearity and

unusual magnetic properties have been introduced.208

Indazole represents a widely used medicinal chemistry

pharmacophore and is a structural element of many drugs in

themarket (Figure 8). Themetal chelation aptitude of different

regioisomers of bis(azolyl)alkanes have recently been

reported.209 The N2-donor bidentate ligands di(1H-indazol-1-

yl)methane (L1) and di(2H-indazol-2-yl)methane (L2) are other

novel chelators that have been recently synthesized, and their

coordination behavior toward the groups 12 metals Zn(II),

Cd(II), and Hg(II) salts have been studied.210

The zinc(II) halide complexes of each ligand (L1 and L2) are

1:1 adducts having tetrahedral geometries, whereas L2 reacts

with CdCl2 giving the 2:1 adduct [CdCl2(L2)2] when the reac-

tion is carried out in equimolar quantities.210 As is the case

with Zn(II), the reaction between L2 and HgX2 (X¼ I or SCN)

produced tetrahedral 1:1 adducts [HgX2(L2)] [HgI2(L

2)] and

[Hg(SCN)2(L2)], respectively. The fine-tuning of steric and

electronic properties of the ligand, R2C(pz)2 family, can pro-

vide different stoichiometry for the adducts which ultimately

can lead to more selective metal chelators.

3.35.4.5 Ellagic Acid

Ellagic acid (EA) is a dietary polyphenol known to be pres-

ent abundantly in fruits and vegetables, which possesses

antioxidant, antiproliferative, chemo-preventive, and anti-

atherogenic properties (Figure 9).

EA is the active compound in Phyllanthus urinaria that

exhibits anti-angiogenic activity and inhibits the secretion of

matrix metalloproteinase 2 (MMP-2) protein from human

umbilical vein endothelial cells (HUVECs).211 Zinc is essential

for the endopeptidase proteolytic capacity to degrade the extra-

cellular matrix (ECM). Compounds with zinc-chelating groups,

such as thiol or hydroxamate, are often used to inhibit MMP

activity by forming a stable species by d bidentate interaction

with Zn. Huang et al.211 have shown that zinc chelation is

involved in the inhibitory effect of EA on the migration of

HUVEC cells. The selectivity of EA for Zn was also established

as neither CaCl2 nor MgCl2 could reverse the inhibitory effect

of EA. In addition to this, many of the histone deacetylase

(HDAC) inhibitors possess the hydroxamate group, which can

chelate Zn and hence inhibit the Zn-dependent HDAC.212 Thus,

as a potential anticancer drug, the zinc-chelating effect of

ellagic acid may also play a role in preventing early tumor

promotion.

3.35.4.6 b-Dicarbonyl Enolates

Curcumin, phloretin, and structurally related phytopolyphe-

nols have well-described neuroprotective properties that ap-

pear to be mediated by 1,3-dicarbonyl enol substructures that

form nucleophilic enolates (Figure 10).213

Several lines of evidence suggest that metal chelation is also

a relevant mechanism of cytoprotection. A recent study by

LoPachin et al. has confirmed the metal-chelating abilities of

H3CO

HO

OCH3

OH

HO OH

OH O

OH

H3CO

HO

O OH O OH

OCH3

OH

(a)

(b)

Figure 10 (a) Keto-enol canonical forms of curcumin. (b) Chemical structure of phloretin.

O O

(a) (b)

OO

Figure 11 Chemical structure of b-dicarbonyl enolates: (a) acetylacetone (AcAc), (b) 2-acetylcyclopentanone (2-ACP).


2-acetylcyclopentanone (2-ACP) and acetyl acetone (AcAc)

which provided substantial cytoprotection in the H2O2

model (Figure 11).213

3.35.5 Combination Therapy

Despite the fact that monotherapy with single-chelating agents

is widely accepted for the removal of toxic metals from the

body, the goal of symptom-free clinical recovery including the

reversal of neuropsychological manifestations still remains a

challenge. Thus, to meet the goal of complete recovery, various

therapeutic strategies in chelation therapy have been intro-

duced. Combination therapy introducing two chelating agents

is one such example. The concept of combination chelation

therapy is that two structurally different chelating agents will

act through different mechanisms facilitating the mobilization

of toxic metal form different compartments (Figure 12). Such

synergistic effects provide the added advantage of lower toxic-

ity due to (1) dose reduction of both the chelating agents

employed, thus low ADR,214,215 and (2) a reduced possibility

of metal redistribution. Combination chelation therapy has

been found to be effective preclinically and clinically to pre-

vent metal redistribution especially in the brain to avoid

neurotoxicity.214,216–218 Flora et al. demonstrated that combi-

nation therapy employing CaNa2EDTA and DMSA in cases of

lead poisoning showed more pronounced lead elimination

and better recovery even in the clinical parameters along

with blood lead levels214 CaNa2EDTA mobilizes lead from

hard tissues such as bone making it available in the extra-

cellular circulation to form complexes with DMSA, thus

restricting redistribution to brain and other soft tissues. Co-

administration of DMSA and MiADMSA has also been

reported to provide superior benefits over monotherapy.168,169

As stated earlier, MiADMSA with higher lipophilicity, compared

to its parent compound DMSA, gains access to intracellularly

boundmetal supported by DMSA acting extracellularly. Numer-

ous animal studies supported by a few clinical investigations

suggest that combination therapy demonstrates beneficial

effects against chronic lead poisoning, especially in the brain

as measured by oxidative stress, NT alterations, memory and

neurobehavioral changes, along with reduction in toxic metal

burden.33,219 Patients with iron overload experience various

complications due to deposition of iron in organs and especially

in the heart. Tailored therapy needs to be defined based on each

case encountered. Combination therapy with deferiprone and

DFOA is one such strategy established in the case of myocardial

siderosis and is being investigated for such patients (see chela-

tion in disease manifestations).220

3.35.5.1 Use of Antioxidants and Herbal Extractsfor the Removal of Toxic Metals

A prophylactic or therapeutic strategy against metal-induced

oxidative stress can be employed to increase the antioxidant

capacity of the cell. Oxidative stress is an established mecha-

nism of metal-induced toxicity. Metal-induced free radical gen-

eration may be direct, resulting from its toxicokinetic pathway

as in the case of arsenic methylation (dimethylarsinic peroxyl

radicals (CH3)2AsOO� and dimethylarsinic radical (CH3)

2As�),221,222 or indirectly as in the case of cadmium that may

replace iron and copper from the cytoplasmic and membrane

proteins rendering them available for Fenton’s reaction,223–225

and may also lower the antioxidant function of the cell. Al-

though the beneficial effects of antioxidants per se have long

been recognized, their co-administration with the chelating

agents, especially in case of chronic metal poisoning, has

only been recently established. Antioxidant supplements such

as LA,226 NAC, melatonin,227 gossypin228, and taurine,

when co-administered with chelating agents, show beneficial

effects.229,230 Interestingly some of these compounds includ-

ing LA, NAC, and taurine are described as natural chelators and

show synergistic effects by potentiating the chelating efficacy of

the co-administered drug.231 Other antioxidants clinically

tested include vitamins C and E and these show not only

reduced oxidative stress following therapy, but also better

toxic metal elimination.232

Lipophilicchelator

Lipophobicchelator

+

Extracellular compartment

Blood

Intracellular compartment

Excretion of metal–chelator complex

M M

M

M

MM

MMM

MM

MM

M

M

M

M

M

M

MM M

M

M

Figure 12 Mechanistic representation for concept of combination chelation therapy employing lipophilic and lipophobic chelating agents.


a-LA (1,2-dithiolane-3-pentanoic acid, 1,2-dithiolane-

3-valeric acid or thioctic acid) is a thiol-containing established

antioxidant; along with its reduced form, the dihydrolipoic

acid (6,8-dimercaptooctanoic acid or 6,8-thioctic acid,

DHLA) contains two thiol groups per molecule which are

also responsible for its metal-chelating property.233 Physiolog-

ically it functions as a cofactor for multienzyme complexes and

is active in both lipid and aqueous phases.234 LA and DHLA

forma potent redox-couple with a standard reduction potential

of �0.32 V and exhibiting high antioxidant properties.235 It is

noteworthy that LA and DHLA chelate different metals in vitro

and in vivo. LA forms complexes with Mn(II), Cu(II), Zn(II),

Cd(II), Pb(II), and Hg(II) in polar but nonaqueous

solvents,235,236 whereas DHLA chelates Co(II), Ni(II), Cu(II),

Zn(II), Hg(II), and Pb(II), besides metal binding to the pair

of deprotonated thiol groups, viz. Ni(II), Co(II), Hg(II), and

Cu(I).237 LA when compared with other antioxidants provides

better protection and recovery against GaAs-induced oxidative

stress and metal burden in animal model.238 As an adjuvant in

combination with DMSA, LA has been recommended in lead

poisoning.239,240

NAC, a thiol and mucolytic agent, is a precursor of

L-cysteine and reduced glutathione. It is a sulfhydryl-

containing antioxidant with metal-chelating ability which has

been screened clinically to reveal its beneficial effects in coun-

tering oxidative stress.22,241 NAC administered intravenously

demonstrated better therapeutic efficacy than intramuscular

2,3-dimercapto-1-propanol against acute arsenic poisoning.242

NAC is capable of chelating metals such as inorganic and

organic mercury, cadmium, chromium, boron, and arsenic as

demonstrated in animal and human investigations.198 Our

group suggested that NAC administration during succimer

therapy is effective in chronic arsenic toxicity.243

Natural antioxidants present in herbal extracts have also

been recently explored as adjuvant with chelation therapy in

metal poisoning. Some leading the list include Centella asiatica,

Moringa oleifera, Hippophae rhamnoides, Aloe vera barbadensis,

Allium sativum, aloe vera, curcumin, and C. asiatica (Umbelli-

ferae), which were found to be beneficial in lowering metal-

induced oxidative stress, facilitating toxic metal elimination in

arsenic or lead-intoxicated animals individually and in combi-

nation with DMSA.25,244,245 M. oleifera seeds, owing to high

cysteine and methionine-rich protein content, show significant

protection against chronic arsenic toxicity by chelation and

antioxidant effects.246,247 In combination with DMSA and

MiADMSA it facilitates the chelation efficacy of the drug, initi-

ating better recovery in experimental animals.219 A. sativum or

garlic contains organosulfur compounds and its protection

against arsenic toxicity is attributed to thiosulfur component

and allicin present in garlic extract. Thiosulfur components of

garlic may act as Lewis acids interacting with the Lewis base

(arsenic) to form a stable product.22,248

Further, co-administration of dietary nutrients, amino acids,

and essential metals along with chelation therapy has also been

recommended. Nutritional supplements such as thiamine and

folic acid provide antioxidant benefits with mild chelating po-

tential, as well as provide better biochemical recoveries and

mobilization of heavy metals.249,250 Similarly, trace elements

such as iron and zinc, either alone or in combination with

thiol-chelating agents during and after arsenic exposure, demon-

strate more pronounced elimination of arsenic in mice.22 Simi-

larly, during lead and cadmium chelation, zinc co-

administration is suggested to have beneficial effects.251,252

Since chelation therapy results in essential metal loss, it is to be

expected that supplementation of the same will attenuate drug-

induced adverse effects. Polyphenols are another class of com-

pounds that exhibit antioxidant effects by chelating redox-active

metal ions and have been evaluated as adjuvant with chelation

therapy.QuercetinwithMiADMSA tested for therapeutic efficacy

provided better arsenic chelation with ameliorating oxidant

levels. Similarly, NAC and mannitol co-administered with

MiADMSA result in better recovery in cadmium toxicity.253


3.35.6 Future for Clinical Use of Chelating Agents

Chelation therapy in various diseased manifestations is sum-

marized in the following.

3.35.6.1 Neurological Disorders

Neurodegenerative disorders are a group of heterogeneous

diseases characterized by protein deposition within neurons

or brain parenchyma and oxidative stress. Although most neu-

rodegenerative disorders have distinct and characteristic etiol-

ogies, they share striking similarity with respect to metal

discrepancies. These have been repeatedly correlated with

metal dyshomeostasis with underlying mechanisms being ex-

tensively researched.254 Since most of these disorders are also

associated with aging, metal-based hypotheses include deteri-

orating metal metabolism, which may be enhanced under

certain neuropathological conditions, causing increased oxida-

tive stress and favoring abnormal metal–protein interaction.255

Ineffective treatment of neurodegenerative diseases is attrib-

uted to insufficient understanding of underlying etiology.

Among other etiological factors, the role of metal ions such

as aluminum, copper, zinc, and iron have been repeatedly

advocated and other chronic toxic metal exposures have also

been suggested. It is noteworthy here that other than Al metals

such as Cu, Fe, or Zn are essential for basic physiology. Thus,

the causal factor may not simply be the overexposure to a

specific metal, but a more complicated dysfunction of the

metal homeostasis machinery found in the brain. Metals may

play roles as adaptors in neurodegeneration in connecting

oxidative stress, protein misfolding and aggregation, and the

downstream events leading to neuronal cell death.255

Humans are readily exposed to Al, due to its widespread

environmental distribution. Experimental studies in animals

show intra-cerebral Al administration results in neurofibrillary

tangle formation.256 High concentrations of Al have been iden-

tified in AD amyloid deposits. This metal has been suggested as

being a contributing factor rather than a causative agent in the

etiology of neurodegenerative disease. Interestingly, brain metal

accumulation at various stages during disease progression has

also been reported.3 It was shown that divalent cations increase

in the early phase of AD, while trivalent metal ions start increas-

ing significantly in the later phase of AD, mainly in the frontal

cortex and hippocampus. Thus, Al displacing the divalentmetals

in severe AD may be a result of differences in the metal–ligand

exchange rates.255 Copper has been most accepted as the metal

involved in ND etiological factors. Extensive research has identi-

fied high concentrations of Cu in amyloid plaque (�400 mM)

compared to the normal extracellular Cu concentration in the

brain (0.2–1.7 mM l�1). Cu binds with Ab peptides and possibly

modulates their aggregation, which is a characteristic feature

of AD. This Ab-bound copper has also been reported to be

involved in oxidative stress leading to neuronal death.257 Similar

to Cu, Zn also affects Ab aggregation as evident in various in vitro

studies. Interestingly, Zn shows a neuroprotective profile by

competing with Cu in Ab binding and thus reducing possible

oxidative stress and a cascade of events.255

Recognizing the metal-based hypothesis of neurodegenera-

tive disease, chelation therapy has been proposed to break the

metal–Ab interaction and reduce neurotoxicity along with pos-

sible restoration of metal homeostasis. However, since any

drug to be effective in the brain faces the challenge of reaching

the site, chelating agents to be employed in neurodegenerative

disease must fulfill certain properties (see properties of ideal

chelators). The molecule must be of low molecular weight,

remain unionized to permeate the BBB, and be stable. Speci-

ficity to target the appropriate metal ion is highly desired to

avoid depletion of essential metal ions such as those from

metalloenzymes. After gaining entry into the brain, the most

critical property is target site specificity, that is, the chelator

should complex with the metal ions bound to the aggregated

proteins leading to their dissociation from the protein and

finally elimination. Thus, in the case of metal overload, a

‘direct chelation approach’ is pursued with the aim of remov-

ing excess metal. In contrast, in the case of ND, chelation is

employed with the objective of favorable modulation of metal

ion homeostasis and metal–Ab interactions. Conventionally

known drugs clinically tested in ND include DFO, rasagiline,

and clioquinol (CQ) (5-chloro-7-iodo-8-hydroxyquinoline).

Deferriooxamine, established as a drug of choice for the treat-

ment of Fe overload and also used in Al overloading in chronic

dialysis treatment is no longer in clinical trials for AD.257

Although the drug has behavioral and cognitive benefits in

AD patients,257 associated disadvantages included inadequate

BBB permeability, rapid degradation in vivo, and adverse effects

such as anemia due to strong affinity for Fe(III). CQ, an anti-

biotic banned for internal use in the USA, has completed a

phase II clinical trial, but was recently withdrawn from clinical

studies owing to controversial results.258–260 It is a small

(molecular weight, 305.5), lipophilic, bioavailable metal che-

lator that freely crosses the BBB and selectively binds Cu and

Zn with far higher affinity than for Ca and Mg [k1(Zn)¼7.0,

k1(Cu)¼8.9, k1(Ca)¼4.9, and k1(Mg)¼5.0], and thus was

suggested to possess the properties of a potential drug. CQ

reverses Cu- and Zn-induced Ab aggregates and solubilizes,

postmortem, Ab deposits in AD-affected brain tissue,261 sup-

ported by the observation that CQ complexes with Zn in the

brain.262 In an in vivomodel of AD (APP2576 transgenic mice),

CQ significantly inhibited amyloid-b deposits without any

neurotoxicity. The mechanism of action however remains

unclear: unlike high-affinity chelators, CQ does not cause

metal excretion, but modulates metal metabolism. Animals

studies also demonstrated significant elevation of Zn(II),

Fe(II)/(III), and Cu(II) levels in different brain regions sup-

porting CQ as an ionophore-facilitating metal uptake in the

brain. Another hydroquinolone derivative (PBT2) was intro-

duced to provide cognitive benefits and Ab oligomer inhibi-

tion in mouse models of AD and in a phase IIa double-blind

trial.263 The exact mechanisms of action of CQ and PBT2 are

unknown; thus, further investigations and larger clinical trials

are required to establish the efficacy and safety of these mole-

cules in the treatment of AD.263

Other chelating agents have been evaluated based on either

ex vivo investigations or limited in vitro and in vivo studies.

Figure 9 shows some of the metal chelators screened with the

potential to be used in the treatment of neurodegenerative

diseases. A series of 8-hydroquonoline analogues (VK-28,

HLA-20, and MA-30) other than CQ (PBT1) have also shown

potential for treatment of ND diseases. DP-109 is a lipophilic


metal chelator that reduces amyloid b protein precursor in

mice and amyloid b pathology in humans. A bifunctional

metal chelator, XH-1, containing a DTPA-based binding unit

and 4-benzothiazole-2-yl-phenylamine-amyloid binding

units, based on novel ‘pharmacophore conjugation’ concept,

has shown significantly positive results in some studies. The

compound reduced cerebral Ab amyloid pathology in trans-

genic mice model without adverse effects and specifically low-

ered the APP protein expression in in vitro cultured human

neuroblastoma cells. Derivatives of the 14-membered satu-

rated tetraamine macrocycle, including the bicyclam derivative

JKL169 (1,10-xylyl bis-1,4,8,11 tetra-azacyclotetradecane), have

generated recent interest. The compound was effective in mod-

ulating Cu levels in the brain cortex, blood, cerebrospinal fluid

(CSF), and corpus callossum in rats.264–266

In spite of the conspicuous theoretical basis for chelation

therapy in AD, there is still a substantial lack of relevant

and reliable data as well as definitive conclusions regarding

the clinical advantages of chelation in neurodegenerative

conditions.

3.35.6.2 Wilson’s Disease

Wilson’s disease, also known as hepatolenticular degeneration,

is an autosomal recessive genetic disorder in which copper

accumulates in tissues. This manifests as neurological or psy-

chiatric symptoms and liver disease. The condition is due to

mutations in the Wilson’s disease protein (ATP7B) gene. The

disease progresses as copper accumulates in the hepatocytes

and at high levels moves to lysosomes for Cu/metallothionine

complex formation. Ceruloplasmin, the Cu transporter pro-

duced in Wilson’s disease, is defective, lacks copper (termed

apoceruloplasmin), and is rapidly degraded in the bloodstream.

Copper levels in liver then overwhelm the proteins binding it

and oxidative stress results in hepatocyte necrosis leading to liver

fibrosis or cirrhosis. Excess copper is released from the liver into

the blood stream where it damages the erythrocytes causing

hemolytic anemia. At final stage of the disease, free copper

available in circulation precipitates throughout the body, espe-

cially in the kidney, brain, and eyes. In the brain, copper accu-

mulation occurs mainly in the basal ganglia and areas involved

in coordination of movements. Thus, the neurological symp-

toms associated with Wilson’s disease include deteriorating

coordination, rigidity, tremors, dementia, personality changes,

slurred speech, and behavioral problems.267

Chelation therapy and zinc supplementation is the sole

therapy available for the treatment of Wilson’s disease. BAL

introduced for the first time to chelate copper was later

substituted with DPA due to better efficacy and lower toxicity.

The therapeutic efficacy of DPA has been explained as reductive

chelation. It is suggested that possibly unstable Cu(II) com-

plexes are formed that later yield Cu(I) and the oxidized

chelator.268 Under the same hypothesis, the formation of

mixed valence Cu(I)/Cu(II) complexes that may be responsible

for copper elimination has been proposed with mixed

valence cluster complexes of stoichiometry [Cu(II)6Cu(I)8

(penicillamine)12Cl]5� being isolated.237,268 Thus, the mech-

anisms elucidated reveal that Cu(II) is in equilibrium with the

aqueous medium and is strongly coordinated by N and S

atoms, while Cu(I) is removed from equilibrium; CH3-groups

of the chelator (DPA) are essential in preventing Cu(I) oxida-

tion; high aqueous solubility of the complex derives from the

12 negatively charged –COO� groups on the cluster ‘surface’;

chloride is essential for the formation of the complex, playing

an important structural role. The cluster thus formed is very

stable along with dual role of DPA as chelating and reducing

agents increase its efficacy for copper elimination.237 However,

owing to DPA-induced adverse effects, safer new analogues or

therapeutic strategies are being explored.269 Zinc supplemen-

tation in Wilson’s disease has been advocated both as a com-

plimentary therapy replacing DPA and as a follow-up

treatment post copper chelation and achievement of a normal

range. Zinc(II) salts are used in the treatment of Wilson’s

disease with the rationale that it is a better inducer of metal-

lothionine than copper, yet binds with lower affinity. Thus, by

inducing intestinal metallothionine, Zn(II) blocks GI copper

absorption causing its excretion in the feces. Moreover, in-

creased blood and hepatic metallothionine complexes with

the systemic free copper, rendering it unavailable for brain

deposition and hepatic damage.270,271

Tetrathiomolybdate, administered as (NH4)2MoS4, is an-

other copper chelator developed for the treatment of Wilson’s

disease. The drug acts dually by first complexing with copper

and food in the GI tract blocking its absorption when admin-

istered orally and second, in the blood, it forms complexes

with copper to render it unavailable for cellular uptake. Thus,

the compound has been recommended for the initial treat-

ment of the disease yet owing to its possible toxicity the com-

pound must be administered with caution.272–274 Reports

suggesting the use of DMSA and DMPS as potential treatments

for Wilson’s disease are available.270,275

3.35.6.3 Blood Disorders and Iron Chelation

Any diseased manifestations or pathologic conditions that re-

quire repeated blood transfusions generally result in iron

overload as a secondary manifestation. These include b-thalas-semia, sickle-cell disease, myelodysplastic syndrome (MDS),

etc. Thus,long-term use of chelation therapy in such subjects

remains the mainstay as a supplementary treatment. Without

chelation iron accumulates in the organs causing progressive

damage especially to the liver, heart, and joints leading to

organ failure and even death (see Chapter 3.02).237,270,276,277

3.35.6.3.1 Thalassemia

Patients with b-thalassemia major are dependent on chronic

blood transfusions and rapidly develop potentially damaging

levels of iron overload that initially distributes to macrophages

and to the liver followed by the heart, pancreas and gonads,

pituitary, thyroid, and hypothalamus glands, if not controlled.

The most common cause of death in thalassemia major pa-

tients is cardiac failure.

Chelation therapy in thalassemia patients may aim to re-

duce the accumulated iron in patients with long-term inade-

quate control or to maintain the body’s iron at safe levels by

striking a balance between transfusion rate, iron intake, and

chelation therapy. Desferrioxamine B, a hexadentate trivalent

metal ion chelator, is a fungally derived siderophore isolated

from Streptomyces pilosus and is employed to treat iron overload.


However, owing to its poor oral bioavailability and toxicity

profile the drug was soon replaced by other orally active com-

pounds. Although polyaminopolicarboxylic acid (EDTA,

DTPA) may be effective due to the hard nature of Fe(III), they

share the limitation of having no orally available dosage form

and lack of specificity for the target metal. Defriprone was later

found to be successful in treating transfusion-induced iron

overload, and has the advantage of removing cardiac iron.

However, despite good chelation properties, the drug faced

differing opinions over its prescription to patients owing to

possible adverse effects, making it only a second-line drug. The

drug is not yet approved by the US Food and Drug Adminis-

tration (FDA).278 The iron chelator most recently introduced,

diferasirox (4-[3,5-bis-(2-hydroxyphenly)-1,2,4,-triazol-1-yl]-

benzoic acid), shows high iron specificity, oral availability,

and tolerability. The compound has been approved by FDA.278

Iron chelation therapy also aims at reducing exposure to

labile forms of iron such as the extracellular non-transferrin-

bound iron (NTBI) and labile plasma iron (LPI), as well as the

intracellular labile iron pool (LIP). DFOA and deferiprone

monotherapy at standard doses does not provide 24-h protec-

tion since LPI rebounds between doses. The latter was achieved

by introducing combination therapy with deferiprone (multi-

ple doses) and DFOA, but the same 24-h coverage was

achieved with deferasirox monotherapy once daily.279,280

Optimizing the iron chelation therapy for thalassemia major

patients is the prime challenge that needs to be addressed.

Combination therapy with DFOA and deferiprone is being

practiced following numerous protocols that may be tailored

for a specific case. These drugs may be given simultaneously or

in a sequential fashion, the latter being preferred, since when

both drugs are present in the cell, iron may shuttle between

them. Thus, sequential administration during combination

therapy results in alternate presence of the drug in the system,

achieved within 24 h since the half-life of both compounds is

short. DFOA and deferiprone combination therapy also dem-

onstrates a superior benefit in lowering the serum ferritin levels

and cardiac iron levels compared to their respective standard

monotherapies.281–286 Moreover, both the drugs in monother-

apy have low tolerability in the case of low ferritin or iron

levels. Deferasirox is gaining highest recent interest with prom-

ising results for long-term oral iron chelation therapy. The

compound has been extensively investigated in humans and

recent studies suggest long-term efficacy and a low adverse

effect profile. Inference from the pooled analysis of the ongo-

ing clinical studies with 4.5 years of follow-up indicates signif-

icant control over serum ferritin level that continuously

decreased in a dose-dependent manner. Most importantly, no

new safety concerns have been raised and continuous use of

the drug was associated with improved tolerability. It was well

tolerated even in patients with serum ferritin levels below

1000 mg 1�1. Similar results were shown in pediatric popula-

tion. Further, with the added advantage to remove cardiac iron

deferasirox improves cardiac function and prevents further

iron accumulation.287,288

3.35.6.3.2 Myelodysplastic syndrome

The MDS comprises a diverse group of hematopoietic stem cell

disorders characterized by abnormal differentiation and

maturation of blood cells, bone marrow failure, and a genetic

instability with an enhanced risk of transformation to leuke-

mia. Clinically, the condition is diagnosed by peripheral cyto-

penia, refractory anemia, and acute myeloid leukemia

generally in patients in the age range of 65–75 years. Low

and intermediate-1 risk group patients are recommended to

have only supportive therapy including RBC transfusions for

symptomatic anemia. Regular RBC transfusions in combina-

tion with prolonged dyserythropoiesis and increased iron ab-

sorption contribute to the accumulation of iron resulting in

secondary iron overload. This can ultimately lead to organ

dysfunction affecting the liver, endocrine glands, and the

heart resulting in reduced life expectancy. Thus, iron chelation

therapy is usually recommended since the human body has no

natural means of getting rid of excess iron.

DFOA ((4-[3,5-bis(2-hydroxyphenyl)-1H-1,2,4-triazol-1-yl]-

benzoic acid))289 is an established drug of choice for iron over-

load. It is approved for the treatment of secondary iron overload

by the US FDA (Food and Drug Administration (FDA) 2010)

and the European Medicines Agency (EMEA) (European Medi-

cines Agency 2010). DFO has proven (mono or in combina-

tion) to be the only chelating agent with beneficial effects in

large cohorts of patients with thalassemia. However, the rec-

ommendations cannot be directly transferred to MDS patients

based on the beneficial evidence of iron chelation therapy in

thalassemia patients alone. It is recommended that strategies

for the treatment of MDS patients need to be tested by exten-

sive investigation, especially in long-term studies as were

carried out for other transfusion-dependent anemias such as

thalassemia.285,287–291

3.35.7 Conclusion

Metals, on one hand, serve as essential components of the

normal health physiology yet, on the other hand, can cause

serious toxic manifestations. It is evident that complexation of

metal allows for removal of excess or toxic metal from the

system rendering it immediately nontoxic and reducing the

consequent effects. Although a range of metal chelators are

now available for chelation therapy, the development of mol-

ecules that may be categorized as ideal chelators is far from

reality. Most chelators have the disadvantages of numerous

adverse effects, nonspecific binding, and administration incon-

venience resulting in poor patient compliance. The increasing

acceptance of drugs such as DMSA and DMPS and the utiliza-

tion of DMSA esters have notably widened the possibilities of

these therapies. In addition, the use of a combination of che-

lating agents is useful in many cases and is an aspect, which

requires further exploration. Employing combination therapy

with more than one chelating agent and/or prescribing antiox-

idants or nutraceuticals requires serious consideration as one

of the ways forward for chelation therapy.

References

1. Yokel, R. A.; Hicks, C. L.; Florence, R. L. Food Chem. Toxicol. 2008, 46,2261–2266.

2. Krewski, D.; Yokel, R. A.; Nieboer, E.; Borchelt, D.; Cohen, J.; Harry, J.; Kacew, S.;Lindsay, J.; Mahfouz, A. M.; Rondeau, V. J. Toxicol. Environ. Health B Crit. Rev.2007, 10, 1–269.

http://refhub.elsevier.com/B978-0-08-097774-4.00340-5/rf0010






3. Domingo, J. L. J. Alzheimers Dis. 2006, 10, 331–341.4. Chu, P. L.; Wu, C. C.; Hsu, C. J.; Wang, Y. T.; Wu, K. D. Laryngoscope 2007,

117, 137–141.5. Barnard, C. G.; McBride, D. I.; Firth, H. M.; Herbison, G. P. Occup. Environ. Med.

2004, 61, 604–608.6. Aremu, D. A.; Meshitsuka, S. Brain Res. 2005, 1031, 284–296.7. Savory, J.; Herman, M. M.; Ghribi, O. J. Inorg. Biochem. 2003, 97, 151–154.8. Mailloux, R.; Lemire, J.; Appanna, V. Cell. Physiol. Biochem. 2007, 20, 627–638.9. Kumar, V.; Bal, A.; Gill, K. D. Toxicology 2009, 255, 117–123.10. Kaizer, R. R.; Correa, M. C.; Spanevello, R. M.; Morsch, V. M.; Mazzanti, C. M.;

Goncalves, J. F.; Schetinger, M. R. J. Inorg. Biochem. 2005, 99, 1865–1870.11. Ohyashiki, T.; Suzuki, S.; Satoh, E.; Uemori, Y. Biochim. Biophys. Acta 1998,

1389, 141–149.12. Kaneko, N.; Sugioka, T.; Sakurai, H. J. Inorg. Biochem. 2007, 101, 967–975.13. Silva, V. S.; Cordeiro, J. M.; Matos, M. J.; Oliveira, C. R.; Goncalves, P. P.

Neurosci. Res. 2002, 44, 181–193.14. Verstraeten, S. V.; Villaverde, M. S.; Oteiza, P. I. Chem. Phys. Lipids 2003, 122,

159–163.15. Johnson, V. J.; Sharma, R. P. Neurotoxicology 2003, 24, 261–268.16. Lynch, E.; Braithwaite, R. Expert Opin. Drug Saf. 2005, 4, 769–778.17. Wong, S. T.; Chan, H. L.; Teo, S. K. Singapore Med. J. 1998, 39, 171–173.18. Hunt, E.; Hader, S. L.; Files, D.; Corey, G. R. N. C. Med. J. 1999, 60, 70–74.19. Rusyniak, D. E.; Arroyo, A.; Acciani, J.; Froberg, B.; Kao, L.; Furbee, B.

InMolecular, Clinical and Environmental Toxicology Clinical Toxicology; Luch, A.Ed.; Birkhauser Verlag: Switzerland, 2010; Vol. 2.

20. FDA. Trisenox Consumer Information Sheet. U.S. Food and Drug AdministrationCenter for Drug Evaluation and Research, 2001. http://www.fda.gov/.

21. Radabaugh, T. R.; Sampayo-Reyes, A.; Zakharyan, R. A.; Aposhian, H. V. Chem.Res. Toxicol. 2002, 15, 692–698.

22. Flora, S. J. S. Free Radic. Biol. Med. 2011, 51, 257–281.23. Valko, M.; Morris, H.; Cronin, M. T. D. Curr. Med. Chem. 2005, 12, 1161–1208.24. Miller, W. H., Jr.; Schipper, H. M.; Lee, J. S.; Singer, J.; Waxman, S. Cancer Res.

2002, 62, 3893–3903.25. Navas-Acien, A.; Sharrett, A. R.; Silbergeld, E. K.; Schwartz, B. S.; Nachman, K. E.;

Burke, T. A.; Guallar, E. Am. J. Epidemiol. 2005, 162, 1037–1049.26. Diaz-Villasenor, A.; Burns, A. L.; Hiriart, M.; Cebrian, M. E.; Ostrosky-Wegman, P.

Toxicol. Appl. Pharmacol. 2007, 225, 123–133.27. Vahidnia,A.; Van der Voet,G.B.; deWolff, F. A.Hum. Exp. Toxicol.2007,26, 823–832.28. Cohen, S. M.; Arnold, L. L.; Eldan, M.; Lewis, A. S.; Beck, B. D. Crit. Rev. Toxicol.

2006, 36, 99–133.29. Lyn Patrick, N. D. Altern. Med. Rev. 2006, 11, 114–127.30. Flora, S. J. S.; Flora, G.; Saxena, G. Lead Chemistry, Analytical Aspects,

Environmental Impact and Health Effects. Elsevier: UK, 2006; pp 158–228.31. Saxena, G.; Pathak, U.; Flora, S. J. S. Toxicology 2005, 214, 39–56.32. Chia, S. E.; Yap, E.; Chia, K. S. Neurotoxicology 2004, 25, 1041–1047.33. Flora, S. J. S.; Saxena, G.; Mehta, A. J. Pharmacol. Exp. Ther. 2007, 322,

108–116.34. Fuchs, J.; Weber, S.; Kaufmann, R. Free Radic. Biol. Med. 2000, 28, 537–548.35. Brennan, P. A.; Kendrick, K. M.; Keverne, E. B. Neuroscience 1995, 69,

1075–1086.36. Xu, J.; Ji, L. D.; Xu, L. H. Toxicol. Lett. 2006, 166, 160–167.37. Moreira, E. G.; Vassilieff, I.; Vassilieff, V. S. Neurotoxicol. Teratol. 2001, 23,

489–495.38. Flora, S. J. S.; Mittal, M.; Mehta, A. Indian J. Med. Res. 2008, 128, 501–523.39. Lamborg, C. H.; Fitzgerald, W. F.; O’Donnell, J.; Torgerson, T. Geochim.

Cosmochim. Acta 2002, 66, 1105–1118.40. Ceccatelli, S.; Dare, E.; Moors, M. Chem. Biol. Interact. 2010, 188, 301–308.41. Rooney, J. P. K. Toxicology 2007, 238, 216.42. Tchounwou, P. B.; Ayensu, W. K.; Ninashvili, N.; Sutton, D. Environ. Toxicol.

2003, 18, 149–175.43. Clarkson, T. W.; Magos, L. Crit. Rev. Toxicol. 2006, 36, 609–662.44. Sinicropi, M. S.; Amantea, D.; Caruso, A.; Saturnino, C. Arch. Toxicol. 2010, 84,

501–520.45. Berlin, M.; Zalup, R. K.; Fowler, B. A. In Handbook on the Toxicology of Metals;

Nordberg, G. F. Fowler, B. A. Nordberg, M. Friberg, L. Eds.; Academic Press:New York, 2005; pp 675–730.

46. Jarup, L.; Berglund, M.; Elinder, C. G.; Nordberg, G.; Vahter, M. Scand. J. WorkEnviron. Health 1998, 24, 1–51.

47. IARC, International Agency for Research on Cancer, In InternationalAgency for Research on Cancer Monographs on the Evaluation of CarcinogenicRisks to Humans; IARC Scientific Publications: Lyon, 1993; Vol. 58, pp 119–237.

48. Olsson, I. M.; Bensryd, I.; Lundh, T.; Ottosson, H.; Skerfving, S.; Oskarsson, A.Environ. Health Perspect. 2002, 110, 1185–1190.

49. Hogervorst, J.; Plusquin, M.; Vangronsveld, J.; Nawrot, T.; Cuypers, A.; Van, H. E.;Roels, H. A.; Carleer, R.; Staessen, J. A. Environ. Res. 2007, 103, 30–37.

50. Bernard, A. J. Toxicol. Environ. Health A 2008, 71, 1259–1265.51. Kobayashi, E.; Suwazono, Y.; Honda, R.; Dochi, M.; Nishijo, M.; Kido, T.;

Nakagawa, H. Biol. Trace Elem. Res. 2008, 124, 164–172.52. Nordberg, M.; Jin, T.; Nordberg, G. F. IARC Sci. Publ. 1992, 118, 293–297.53. Cohen, S. M. Drug Metab. Rev. 1998, 30, 339–357.54. Waalkes, M. P. Mutat. Res. Fund Mol. Mech. Mutagen. 2003, 533, 107–120.55. Smirnova, I. V.; Bittel, D. C.; Ravindra, R.; Jiang, H.; Andrews, G. K. J. Biol. Chem.

2000, 275, 9377–9384.56. Joseph, P.; Lei, Y. X.; Whong, W. Z.; Ong, T. M. Cancer Res. 2002, 62, 703–707.57. Galan, A.; Garcia-Bermejo, L.; Troyano, A.; Vilaboa, N. E.; Fernandez, C.;

de Blas, E.; Aller, P. Eur. J. Cell Biol. 2001, 80, 312–320.58. Watanabe, M.; Henmi, K.; Ogawa, K.; Suzuki, T. Comp. Biochem. Physiol. C:

Toxicol. Pharmacol. 2003, 134, 227–234.59. Thevenod, F. Toxicol. Appl. Pharmacol. 2009, 238, 221–239.60. Crichton, R. R.; Danielsson, B. G.; Geisser, P. Iron Therapy with Special Emphasis

on Intravenous Administration. UNI-Med Verlag AG: Bremen, 2008.61. Weinberg, E. D. Metallomics 2010, 2, 732–740.62. Karlsson, H. L.; Nilson, L.; Moller, L. Chem. Res. Toxicol. 2005, 18, 19–23.63. Whitaker, P.; Tufaro, P. R.; Rader, J. I. J. Am. Coll. Nutr. 2001, 20, 247–254.64. Ganz, T. Blood 2003, 102, 783–788.65. Galaris, D.; Mantzaris, M.; Amorgianiotis, C. Int. J. Endocrinol. Metab. 2008, 7,

114–122.66. Emsly, J. Nature’s Building Blocks: An A–Z Guide to the Elements. Oxford

University Press: Oxford, 2001; pp 495–498.67. O’Brien, T. J.; Ceryak, S.; Patierno, S. R. Mutat. Res. 2003, 533, 3–36.68. Codd, R.; Dillon, C. T.; Levina, A.; Lay, P. A. Coord. Chem. Rev. 2001, 216,

537–582.69. Cieslak-Golonka, M. Polyhedron 1996, 15, 3667–3689.70. The Agency for Toxic Substances and Disease Registry (ATSDR), 2000. see:

http://www.atsdr.cdc.gov/HEC/CSEM/chromium/exposure_pathways.html.71. Costa, M.; Klein, C. B. Crit. Rev. Toxicol. 2006, 36, 155–163.72. Shi, X.; Chiu, A.; Chen, C. T.; Halliwell, B.; Castranova, V.; Vallyathan, V.

J. Toxicol. Environ. Health B Crit. Rev. 1999, 2, 87–104.73. Chiu, A.; Shi, X. L.; Lee, W. K. P.; Hill, R.; Wakeman, T. P.; Katz, A.; Xu, B.;

Dalal, N. S.; Robertson, J. D.; Chen, C.; Chiu, N.; Donehower, L. J. Environ. Sci.Health C 2010, 28, 188–230.

74. Ding, M.; Shi, X. Mol. Cell. Biochem. 2002, 234/235, 293–300.75. Katz, S. A.; Salem, H. J. Appl. Toxicol. 1993, 13, 217–224.76. Cempel, M.; Nikel, G. Pol. J. Environ. Stud. 2006, 15, 375–382.77. Diagomanolin, V.; Farhang, M.; Ghazi-khansari, M.; Jafarzadeh, N. Toxicol. Lett.

2004, 151, 63.78. Gordon, P. M.; White, M. I.; Scotland, T. R. Contact Dermatitis 1994, 30, 181–182.79. Denkhausa, E.; Salnikowb, K. Crit. Rev. Oncol. Hematol. 2002, 42, 35–56.80. Salnikow, K.; Zhitkovich, A. Chem. Res. Toxicol. 2008, 21, 28–44.81. Das, K. K.; Das, S. N.; Dhundasi, S. A. Indian J. Med. Res. 2008, 128, 412–425.82. Kasprzak, K. S.; Sunderman, F. W.; Salnikow, K. Mutat. Res. 2003, 533, 67.83. Arita, A.; Costa, M. Metallomics 2009, 1, 222–228.84. Keen, C. L.; Ensunsa, J. L.; Watson, M. H. Neurotoxicology 1999, 20, 213–223.85. Keen, C. L.; Ensunsa, J. L.; Clegg, M. S. In Manganese and Its Role in Biological

Processes; Sigel, A., Sigel, H. Eds.; Marcel Dekker: New York, 2000; pp 89–121.86. Ono, K.; Komai, K.; Yamada, M. J. Neurol. Sci. 2002, 199, 93–96.87. Hudnell, H. K. Neurotoxicology 1999, 20, 379–397.88. ATSDR, Toxicological Profile for Manganese. US Department of Health and

Human Services, Agency for Toxic Substances and Disease Registry: Atlanta, GA,2000; pp 1–466.

89. Hardy, G. Gastroenterology 2009, 137, S29–S35.90. Crossgrove, J.; Zheng, W. NMR Biomed. 2004, 17, 544–553.91. Hamai, D.; Bond, S. Ann. N. Y. Acad. Sci. 2004, 1012, 129–141.92. Zhang, F.; Xu, Z.; Gao, J.; Xu, B. Environ. Toxicol. Pharmacol. 2008, 26, 232–236.93. Kwik-Uribe, C.; Smith, D. R. J. Neurosci. Res. 2006, 83, 1601–1610.94. Milatovic, D.; Gupta, R.; Sidoryk, M. Toxicol. Sci. 2007, 98, 198–205.95. Zhao, F.; Cai, T.; Liu, M.; Zheng, G. Toxicol. Sci. 2009, 107, 156–164.96. Erikson, K. M.; Aschner, M. Neurochem. Int. 2003, 43, 475–480.97. Gwiazda, R.; Lucchini, R.; Smith, D. J. J. Toxicol. Environ. Health A 2007, 70,

594–605.98. Artelt, S.; Creutzenberg, O.; Kock, H.; Levsen, K.; Nachtigall, D.; Heinrich, U. Sci.

Total Environ. 1999, 228, 219–242.99. Lustig, S.; Zang, S.; Michalke, B.; Schramel, P.; Beck, W. Sci. Total Environ.

1996, 188, 195–204.100. Uozumi, J.; Ueda, T.; Yasumasu, T.; Koikawa, Y.; Naito, S.; Kumazawa, J.;

Sueishi, K. Int. Urol. Nephrol. 1993, 25, 215–220.


























http://www.fda.gov/











































































http://www.atsdr.cdc.gov/HEC/CSEM/chromium/exposure_pathways.html











































101. Bernardis, F. L.; Grant, R. A.; Sherrington, D. C. React. Funct. Polym. 2005, 65,205–217.

102. Minakata, K.; Suzuki, M.; Nozawa, H.; Gonmori, K.; Watanabe, K.; Suzuki, O.Forensic Toxicol. 2006, 24, 83–87.

103. Ravindra, K.; Bencs, L.; Van Grieken, R. Sci. Total Environ. 2004, 318, 1–43.104. Ek, K. H.; Morrison, G. M.; Rauch, S. Sci. Total Environ. 2004, 334–335, 21–38.105. Gebel, T.; Lantzsch, H.; Pleßow, K.; Dunkelberg, K. Mutat. Res. 1997, 389,

183–190.106. Rosner, G.; Merget, R. In Immunotoxicology and Immunotoxicology of Metals;

Dayan, A. Hertel, R. F. Hesseltine, E. Kazautzis, G. Smith, E. H. Eds.; PlenumPress: New York, 1990; pp 93–104.

107. Hartmann, J. T.; Lipp, H. P. Expert Opin. Pharmacother. 2003, 4, 889–901.108. US Environmental Protection Agency (US EPA). Consumer Factsheet on Thallium,

2009. http://www.epa.gov/safewater/pdfs/factsheets/ioc/thallium.pdf.109. Ewers, U. Sci. Total Environ. 1988, 71, 285–292.110. Kazantzis, G. Environ. Geochem. Health 2000, 22, 275–280.111. John Peter, A. L.; Viraraghavan, T. Environ. Int. 2005, 31, 493–501.112. Saddique, A.; Peterson, C. D. Vet. Hum. Toxicol. 1983, 25, 16–22.113. Morgan, G. T.; Drew, H. D. K. J. Chem. Soc.Trans. 1920, 117, 1456–1465.114. Martell, A. E.; Hancock, R. D.; Motekaitis, R. J. Coord. Chem. Rev. 1994, 133,

39–65.115. Catsch, A.; Harmuth-Hoene, A. E. In The Chelation of Heavy Metals; Levine, W. G.

Ed.; Pergamon Press: Oxford, 1979; pp 107–224.116. Angle, C. R. In Toxicology of Metals; Chang, L. W. Ed.; CRC Press: Boca Raton,

FL, 1996; pp 487–504.117. Andersen, O. Mini Rev. Med. Chem. 2004, 4, 11–21.118. Flora, S. J. S.; Pachauri, V. Int. J. Environ. Res. Public Health 2010, 7, 2745–2788.119. Andersen, O. Chem. Rev. 1999, 99, 2683–2710.120. Anderegg, G. In Coordination ChemistryACS Monograph 168; ; Martell, A. E. Ed.;

ACS Monograph 168; Van Nostrand-Reinhold: New York, 1971; Vol. 1, pp 427–490.121. Margerum, D. W.; Cayley, G. R.; Weatherburn, D. C.; Pagenkopf, G. K.

In Coordination Chemistry; ACS Monograph 174; Martell, A. E. Ed.; AmericanChemical Society: Washington, DC, 1978; Vol. 2.

122. Pearson, R. G. J. Am. Chem. Soc. 1963, 85, 3533–3539.123. Eybl, V.; Kotyzova, D.; Koutensky, J.; Mickova, V.; Jones, M. M.; Singh, P. K.

Analyst 1998, 123, 25–26.124. Zalups, R. K.; Parks, L. D.; Cannon, V. T.; Barfuss, D. W. Mol. Pharmacol. 1998,

54, 353–363.125. Aposhian, H. V.; Maiorino, R. M.; Gonzalez-Ramirez, D.; Zuniga-Charles, M.;

Xu, Z.; Hurlbut, K. M.; Junco-Munoze, P.; Dart, R. C.; Aposhian, M. M.Toxicology 1995, 97, 23–38.

126. Jones, M. M. Comment Inorg. Chem. 1992, 13, 91–110.127. Rakela, J.; Vargas, H.; Arenas, J. Liver Transpl. 2002, 8, 502–503.128. Gibbs, K. R.; Walsche, J. M. InOrphan Diseases and Orphan Drugs; Scheinberg, I. H.

Walsche, J. M. Eds.; Manchester University Press: Manchester, UK, 1986;Chapter 6, p 33.

129. Weinberg, E. D. J. Pharm. Pharmacol. 2006, 58, 575–584.130. Gale, G. R.; Smith, A. B.; Jones, M. M.; Singh, P. K. Pharmacol. Toxicol. 1992,

71, 452–456.131. Mandal, B. K.; Suzuki, K. T. Talanta 2002, 58, 201–235.132. Ellenhorn, M. J.; Barceloux, D. G. Medical Toxicology: Diagnosis and Treatment

of Human Poisoning. Elsevier: New York, 1988; pp 77, 1015, 1037–1038.133. Silbergeld, E. K. Annu. Rev. Public Health 1997, 18, 187–210.134. Gordon, J. N.; Taylor, A.; Bennett, P. N. Br. J. Pharmacol. 2002, 53, 451–458.135. Flora, S. J. S. Clin. Exp. Pharmacol. Physiol. 1999, 26, 865–869.136. George, G. N.; Prince, R. C.; Gailer, J.; Buttigieg, G. A.; Denton, M. B.;

Harris, H. H.; Pickering, I. J. Chem. Res. Toxicol. 2004, 17, 999–1006.137. Maiorino, R. M.; Aposhian, M. M.; Xu, Z. F.; Li, Y.; Polt, R. L.; Aposhian, H. V.

J. Pharmacol. Exp. Ther. 1993, 267, 1221–1226.138. Flora, S. J. S.; Dube, S. N.; Arora, U.; Kannan, G. M.; Shukla, M. K.;

Malhotra, P. R. Biometals 1995, 8, 111–116.139. Pingree, S. D.; Simmonds, P. L.; Woods, J. S. Toxicol. Sci. 2001, 61, 224–233.140. Yan, H.; Carter, C. E.; Xu, C.; Singh, P. K.; Jones, M. M.; Johnson, J. E.;

Dietrich, M. S. J. Toxicol. Environ. Health 1997, 52, 149–168.141. Aposhian, H. V. Annu. Rev. Pharmacol. Toxicol. 1983, 23, 193–215.142. Goyer, R. A.; Cherian, M. G.; Jones, M. M.; Reigart, J. R. Environ. Health Perspect.

1995, 103, 1048–1052.143. Wiedemann, P.; Fichtl, B.; Szinicz, L. Biopharm. Drug Dispos. 1982, 3, 267–274.144. Dart, R. C.; Hurlbut, K. M.; Maiorino, R. M.; Mayersohn, M.; Aposhian, H. V.;

Hassen, L. V. J. Pediatr. 1994, 125, 309–316.145. Aposhian, H. V.; Maiorino, R. M.; Rivera, M.; Bruce, D. C.; Dart, R. C.;

Hurlbut, K. M.; Levine, D. J.; Zheng, W.; Fernando, Q.; Carter, D.;Aposhian, M. M. J. Toxicol. Clin. Toxicol. 1992, 30, 505–528.

146. Maiorino, R. M.; Atkins, J. M.; Blaha, K.; Carter, D. E.; Aposhian, H. V.J. Pharmacol. Exp. Ther. 1990, 254, 570–577.

147. Maiorino, R. M.; Dart, R. C.; Carter, D. E.; Aposhian, H. V. J. Pharmacol. Exp.Ther. 1991, 259, 808–814.

148. Maiorino, R. M.; Bruce, D. C.; Aposhian, H. V. Toxicol. Appl. Pharmacol. 1989,97, 338–349.

149. Maiorino, R. M.; Xu, Z. F.; Aposhian, H. V. J. Pharmacol. Exp. Ther. 1996, 277,375–384.

150. Gaffney, D.; Fell, G. S.; O’Reilly, D. S. J. Clin. Pathol. 2000, 53, 807–812.151. Bakka, A.; Aaseth, J.; Rugstad, H. E. Acta Pharmacol. Toxicol. 1981, 49, 432–437.152. Clarkson, T. W. Environ. Health Perspect. 2002, 110, 11–23.153. Ibim, S. E.; Trotman, J.; Musey, P. I.; Semafuko, W. E. Toxicology 1992, 73,

229–237.154. Kodama, H.; Murata, Y.; Iitsuka, T.; Abe, T. Life Sci. 1997, 61, 899–907.155. Yanagisawa, T.; Maemura, S.; Sasaki, H.; Endo, T.; Okada, M.; East, P. W.;

Virgo, D. M.; Creasy, D. M. J. Toxicol. Sci. 1998, 23, 619–642.156. Walshe, J. M. Q. J. Med. 1996, 89, 553–555.157. Redman, B. G.; Esper, P.; Pan, Q.; Dunn, R. L.; Hussain, H. K.; Chenevert, T.;

Brewer, G. J.; Merajver, S. D. Clin. Cancer Res. 2003, 9, 1666–1672.158. Tilbrook, G. S.; Hider, R. C. Met. Ions Biol. Syst. 1998, 35, 691–730.159. Olivieri, N. F.; Brittenham, G. M. Blood 1997, 89, 739–761.160. Mills, K. C.; Curry, S. C. Emerg. Med. Clin. North Am. 1994, 12, 397–413.161. Kontoghiorghes, G. J. Toxicol. Lett. 1995, 80, 1–18.162. Dobbin, P. S.; Hider, R. C.; Hall, A. D.; Taylor, P. D.; Sarpong, P.; Porter, J. B.;

Xiao, G.; van der Helm, D. J. Med. Chem. 1993, 36, 2448–2458.163. Berdoukas, V.; Farmaki, K.; Wood, J. C.; Coates, T. Expert Rev. Hematol. 2011, 4,

17–26.164. Kontoghiorghes, G. J.; Bartlett, A. N.; Hoffbrand, A. V.; Goddard, J. G.;

Sheppard, L.; Barr, J.; Nortey, P. Br. J. Haematol. 1990, 76, 295–300.165. Bradberry, S. M.; Vale, J. A. J. Toxicol. Clin. Toxicol. 1999, 37, 259–264.166. Guha Mazumder, D. N.; Ghoshal, U. C.; Saha, J.; Santra, A.; De, B. K.;

Chatterjee, A.; Dutta, S.; Angle, C. R.; Centeno, J. A. Clin. Toxicol. 1998, 36,683–690.

167. Flora, S. J. S.; Bhatt, K.; Dwivedi, N.; Pachauri, V.; Kushwaha, P. Clin. Exp.Pharmacol. Physiol. 2011, 38, 373–379.

168. Flora, S. J. S.; Pant, B. P.; Tripathi, N.; Kannan, G. M.; Jaiswal, D. K. J. Occup.Health 1997, 39, 119–123.

169. Bhadauria, S.; Flora, S. J. S. Cell Biol. Toxicol. 2007, 23, 91–104.170. Mishra, D.; Mehta, A.; Flora, S. J. S. Chem. Res. Toxicol. 2008, 21, 400–407.171. Nogueira, C. W.; Soares, F. A.; Bolzan, R. C.; Jacques-Silva, M. C.; Souza, D. O.;

Rocha, J. B. T. Neurochem. Res. 2000, 25, 1553–1558.172. Nogueira, C. W.; Rotta, L. N.; Tavares, R. G.; Diogo, O.; Rocha, J. B. T.

Neuroreport 2001, 12, 511–514.173. Santos, F. W.; Rocha, J. B. T.; Nogueira, C. W. Toxicol. In Vitro 2006, 20,

317–323.174. Lin-Tan, D. T.; Lin, J. L.; Yen, T. H.; Chen, K. H.; Huang, Y. L. Nephrol. Dial.

Transplant. 2007, 22, 2924–2931.175. Lin, J. L.; Ho, H. H.; Yu, C. C. Ann. Intern. Med. 1999, 130, 7–13.176. Bradberry, S.; Sheehan, T.; Vale, A. QJM 2009, 102, 721–732.177. Liebelt, E. L.; Shannon, M.; Graef, J. W. J. Pediatr. 1994, 124, 313–317.178. Brown, M. J.; Willis, T.; Omalu, B.; Leiker, R. Pediatrics 2006, 118, e534–e536.179. Bradberry, S.; Vale, A. Clin. Toxicol. 2009, 47, 841–858.180. Graziano, J. H.; Siris, E. S.; LoIacono, N.; Silverberg, S. J.; Turgeon, L. Clin.

Pharmacol. Ther. 1985, 37, 431–438.181. Chisolm, J. J. J. Toxicol. Clin. Toxicol. 2000, 38, 365–375.182. Torres-Alanis, O.; Garza-Ocanas, L.; Pineyro-Lopez, A. Hum. Exp. Toxicol. 2002,

21, 573–577.183. Grandjean, P.; Jacobsen, I. A.; J�rgensen, P. J. Pharmacol. Toxicol. 1991, 68,

266–269.184. Lee, A. Adverse Drug Reactions, 2nd ed.; Pharmaceutical Press: London, 2006;

pp 125–156.185. McNeill Consumer Products Company, Chemet Product Information. McNeill

Consumer Products Company: Fort Washington, PA, 1994.186. Ruprecht, J. Scientific Monograph Dimaval (DMPS). Heyltex Corporation:

Houston, TX, 1997.187. Besunder, J. B.; Anderson, R. L.; Super, D. M. Pediatrics 1995, 96, 683–687.188. Walshe, J. M. J. Q. Med. 1996, 89, 553.189. Nishimura, H.; Takagaki, S. Anat. Rec. 1959, 135, 261–267.190. Domingo, J. L.; Paternain, J. L.; Llobet, J. M.; Corbella, J. Fundam. Appl. Toxicol.

1988, 11, 715–722.191. Domingo, J. L. J. Toxicol. Environ. Health 1990, 30, 181–190.192. Taubeneck, M. W.; Domingo, J. L.; Llobet, J. M.; Keen, C. L. Toxicology 1992,

72, 27–40.













http://www.epa.gov/safewater/pdfs/factsheets/ioc/thallium.pdf







































































































































193. Mehta, A.; Pant, S. C.; Flora, S. J. S. Reprod. Toxicol. 2006, 21, 94–103.194. Flora, S. J. S.; Mehta, A. Biochem. Pharmacol. 2009, 78, 1340–1349.195. Domingo, O. L. Reprod. Toxicol. 1998, 12, 499–510.196. Bosque, M. A.; Domingo, J. L.; Corbella, J. Arch. Toxicol. 1995, 69, 467–471.197. Schnebli, H.P. Preclinical evaluation of CGP 37 391 (L1). Biology Report ERS

62/93. Ciba-Geigy Ltd, Basle, 1993.198. Blanusa, M.; Varnai, V. M.; Piasek, M.; Kostial, K. Curr. Med. Chem. 2005, 12,

2771–2794.199. Ornaghi, F.; Ferrini, S.; Prati, M.; Giavini, E. Fundam. Appl. Toxicol. 1993, 20,

437.200. Gerr, F.; Frumkin, H.; Hodgins, P. J. Toxicol. Clin. Toxicol. 1994, 32, 569–575.201. Ferreiros-Martinez, R.; Esteban-Gomez, D.; de Blas, A.; Platas-Iglesias, C.;

Rodriguez-Blas, T. Inorg. Chem. 2009, 48, 11821–11831.202. Ferreiros-Martinez, R.; Platas-Iglesias, C.; de Blas, A.; Esteban-Gomez, D.;

Rodriguez-Blas, T. Eur. J. Inorg. Chem. 2010, 17, 2495–2503.203. Roca-Sabio, A.; Mato-Iglesias, M.; Esteban-Gomez, D.; Toth, E.; de Blas, A.;

Platas-Iglesias, C.; Rodriguez-Blas, T. J. Am. Chem. Soc. 2009, 131,3331–3341.

204. Ferreiros-Martinez, R.; Esteban-Gomez, D.; Toth, E.; de Blas, A.;Platas-Iglesias, C.; Rodriguez-Blas, T. Inorg. Chem. 2011, 50, 3772–3784.

205. Ben Shachar, D. B.; Kahana, N.; Kampel, V.; Warshawsky, A.; Youdim, M. B.Neuropharmacology 2004, 46, 254–263.

206. Youdim, M. B.; Fridkin, M.; Zheng, H. J. Neural Transm. 2004, 111,1455–1471.

207. Zheng, H.; Weiner, L. M.; Bar-Am, O.; Epsztejn, S.; Cabantchik, Z.;Warshawsky, A.; Youdim, M. B.; Fridkin, M. Bioorg. Med. Chem. 2005, 13,773–783.

208. Reger, D. L.; Foley, E. A.; Smith, M. D. Inorg. Chem. 2010, 49, 234–242.209. Abbotto, A.; Bradamante, S.; Facchetti, A.; Pagani, G. A. J. Org. Chem. 2002, 67,

5753–5772.210. Pettinari, C.; Marinelli, A.; Marchetti, F.; Ngoune, J.; Galindo, A.; Alvarez, E.;

Gomez, M. Inorg. Chem. 2010, 49, 10543–10556.211. Huang, S. T.; Wang, C. Y.; Yang, R. C.; Wu, H. T.; Yang, S. H.; Cheng, Y. C.;

Pang, J. H. Evid. Based Complement. Alternat. Med. 2011, 2011, 215035.212. Huang, S. T.; Yang, R. C.; Wu, H. T.; Wang, C. N.; Pang, J. W. S. PLoS One

2011, 6, e18986 1–11.213. LoPachin, R. M.; Gavin, T.; Geohagen, B. C.; Zhang, L.; Casper, D.; Lekhraj, R.;

Barber, D. S. J. Neurochem. 2011, 116, 132–143.214. Flora, S. J. S.; Bhattacharya, R.; Vijayaraghavan, R. Fundam. Appl. Toxicol. 1995,

25, 233–240.215. Kostial, K.; Blanusa, M.; Plasek, L. J.; Samarzila, M.; Jones, M. M.; Singh, P. K.

J. Appl. Toxicol. 2001, 15, 201–206.216. Cory-Slechta, D. A. J. Pharmacol. Exp. Ther. 1988, 246, 84–91.217. Flora, S. J. S.; Saxena, G. J. Biochem. Mol. Toxicol. 2004, 18, 221–233.218. Besunder, J. B.; Super, D. M.; Anderson, R. L. J. Pediatr. 1997, 130, 966–971.219. Flora, S. J. S.; Saxena, G.; Gautam, P.; Kaur, P.; Gill, K. D. Chem. Biol. Interact.

2007, 170, 209–220.220. Tanner, M. A.; Galanello, R.; Dessi, C.; Smith, G. C.; Westwood, M. A.; Agus, A.;

Pibiri, M.; Nair, S. V.; Walker, J. M.; Pennell, D. J. J. Cardiovasc. Magn. Reson.2008, 10, 1–9.

221. Li, Y.; Qu, X.; Qu, J.; Zhang, Y.; Liu, J.; Teng, Y.; Hu, X.; Hou, K.; Liu, Y. CancerLett. 2009, 284, 208–215.

222. Hengartner, M. O. Nature 2000, 407, 770–776.223. Casalino, E.; Calzaretti, G.; Sblano, C.; Landriscina, C. Toxicology 2002, 30,

37–50.224. Waisberg, M.; Joseph, P.; Hale, B.; Beyersmann, D. Toxicology 2003, 192,

95–117.225. Watjen, W.; Beyersmann, D. Biometals 2004, 17, 65–78.226. Pande, M.; Flora, S. J. S. Toxicology 2002, 177, 187–196.227. Flora, S. J. S.; Pande, M.; Kannan, G. M.; Mehta, A. Cell. Mol. Biol. 2004, 50,

543–551.228. Gautam, P.; Flora, S. J. S. Nutrition 2010, 26, 563–570.229. Flora, S. J. S.; Pande, M.; Bhadauria, S.; Kannan, G. M. Hum. Exp. Toxicol. 2004,

23, 157–166.230. Flora, S. J. S.; Chouhan, S.; Kannan, G. M.; Mittal, M.; Swarnakar, H. Oxid. Med.

Cell. Longev. 2008, 1, 39–45.231. Pande, M.; Mehta, A.; Pant, B. P.; Flora, S. J. S. Environ. Toxicol. Pharmacol.

2001, 9, 173–184.232. Flora, S. J. S. J. Nutr. Environ. Med. 2002, 12, 51–65.233. Shila, S.; Ramanathan, K.; Tamilselvan, J.; Panneerselvam, C. Toxicol. Lett. 2005,

155, 27–34.234. Biewenga, G. P.; Haenen, G. R. M. M.; Bast, A. Gen. Pharmacol. Vasc. Syst. 1997,

29, 315–331.

235. Petersen Shay, K.; Moreau, R. F.; Smith, E. J.; Smith, A. R.; Hagen, T. M. Biochim.Biophys. Acta 2009, 1790, 1149–1160.

236. Rooney, J. P. K. Toxicology 2007, 234, 145–156.237. Baran, E. J. Curr. Med. Chem. 2010, 17, 3658–3672.238. Bhatt, K.; Flora, S. J. S. Environ. Toxicol. Pharmacol. 2009, 28, 140–146.239. Gurer, H.; Ozgunes, H.; Oztezcan, S.; Ercal, N. Free Radic. Biol. Med. 1999,

27, 75–81.240. Gurer, H.; Ercal, N. Free Radic. Biol. Med. 2000, 29, 927–945.241. Ziment, I. Respiration 1986, 50, 26–30.242. Martin, D. S.; Willis, S. E.; Cline, D. M. J. Am. Board Fam. Pract. 1990, 3,

293–296.243. Kannan, G. M.; Flora, S. J. S. Biol. Trace Elem. Res. 2006, 110, 43–59.244. Gupta, R.; Flora, S. J. S. Phytother. Res. 2005, 19, 23–28.245. Gupta, R.; Flora, S. J. S. J. Appl. Toxicol. 2006, 26, 213–222.246. Gupta, R.; Kannan, G. M.; Sharma, M.; Flora, S. J. S. Environ. Toxicol. Pharmacol.

2005, 20, 456–464.247. Mishra, D.; Gupta, R.; Pant, S. C.; Kushwaha, P.; Satish, H. T.; Flora, S. J. S.

Toxicol. Mech. Methods 2009, 19, 169–182.248. Flora, S. J. S.; Mehta, A.; Gupta, R. Chem. Biol. Interact. 2009, 177, 227–233.249. Flora, S. J. S.; Tandon, S. K. Toxicology 1990, 64, 129–139.250. Flora, S. J. S.; Singh, S.; Tandon, S. K. Life Sci. 1986, 38, 67–71.251. Flora, S. J. S.; Gubrelay, U.; Kannan, G. M.; Mathur, R. J. Appl. Toxicol. 1998,

18, 357–362.252. Jones, M. M.; Singh, P. K.; Gale, G. R.; Atkins, L. M.; Smith, A. B. Toxicol. Appl.

Pharmacol. 1988, 95, 507–514.253. Mishra, D.; Flora, S. J. S. Biol. Trace Elem. Res. 2008, 122, 137–147.254. Bolognin, S.; Messori, L.; Zatta, P. Neuromol. Med. 2009, 11, 223–238.255. Good, P. F.; Perl, D. P.; Bierer, L. M.; Schmeidler, J. Ann. Neurol. 1992, 31,

286–292.256. Butterworth, R. F. Neurotox. Res. 2010, 18, 100–105.257. Ritchie, C. W.; Bush, A. I.; Mackinnon, A.; Macfarlane, S.; Mastwyk, M.;

MacGregor, L.; Kiers, L.; Cherny, R.; Li, Q. X.; Tammer, A.; Carrington, D.;Mavros, C.; Volitakis, I.; Xilinas, M.; Ames, D.; Davis, S.; Beyreuther, K.;Tanzi, R. E.; Masters, C. L. Arch. Neurol. 2003, 60, 1685–1691.

258. Treiber, C.; Simons, A.; Strauss, M.; Hafner, M.; Cappai, R.; Bayer, T. A.;Multhaup, G. J. Biol. Chem. 2004, 279, 1958–51964.

259. Richardson, D. R. Ann. N. Y. Acad. Sci. 2004, 1012, 326–341.260. Cherny, R. A.; Atwood, C. S.; Xilinas, M. E.; Gray, D. N.; Jones, W. D.;

McLean, C. A.; Barnham, K. J.; Volitakis, I.; Fraser, F. W.; Kim, Y.; Huang, X.;Goldstein, L. E.; Moir, R. D.; Lim, J. T.; Beyreuther, K.; Zheng, H.; Tanzi, R. E.;Master, C. L.; Bush, A. I. Neuron 2001, 30, 665–676.

261. Padmanabhan, G.; Becue, I.; Smith, J. A. In Analytical Profiles of DrugSubstances; Klauss, E. Florey, E. Eds.; Academic Press: New York, 1989;pp 57–90.

262. Lannfelt, L.; Blennow, K.; Zetterberg, H.; Batsman, S.; Ames, D.; Harrison, J.;Masters, C. L.; Targum, S.; Bush, A. I.; Murdoch, R.; Wilson, J.; Ritchie, C. W.Lancet Neurol. 2008, 7, 779–786.

263. Ana, B. Acta Pharm. 2011, 61, 1–14.264. Muralidhar, L.; Hegde, P.; Bharathi, A. S.; Venugopal, C.; Jagannathan, R.;

Poddar, P.; Srinivas, P.; Sambamurti, K.; Rao, K. J.; Scancar, J.; Messori, L.;Zecca, L.; Zattah, P. J. Alzheimers Dis. 2009, 17, 457–468.

265. Bush, A. I.; Tanzi, R. E. Neurotherapeutics 2008, 5, 421–432.266. DiDonato, M.; Sarkar, B. Biochim. Biophys. Acta 1997, 1360, 3–16.267. Crisponi, G.; Nurchi, V. M.; Fanni, D.; Gerosa, C.; Nemolato, S.; Faa, G. Coord.

Chem. Rev. 2010, 254, 876–889.268. Chvapil, M.; Kielar, F.; Liska, F.; Sihankova, A.; Brendel, K. Connect. Tissue Res.

2005, 46, 242–250.269. Scott, L. E.; Orvig, C. Chem. Rev. 2009, 109, 4885–4910.270. Hoogenraad, T. U. In Handbook of Metal–Ligand Interactions in Biological

Fluids; Berthon, G. Ed.; Marcel Dekker: New York, 1995; Vol. 2,pp 1176–1181.

271. Brewer, G. J.; Dick, R. D.; Yuzbasiyan-Gurkin, V.; Tankanow, R.; Young, A. B.;Kluin, K. J. Arch. Neurol. 1991, 48, 42–47.

272. Quagraine, E. K.; Georgakaki, I.; Coucouvanis, D. J. Inorg. Biochem. 2009, 103,143–155.

273. Reedijk, J. Ed.; In Bioinorganic Catalysis; Marcel Dekker: New York, 1993.274. Ruprecht, J. Dimaval. Wissenschaftliche Produktmonographie, 7th ed.; Heyl

GmbH & Co. K.G: Berlin, 2008.275. Dobbin, P. S.; Hider, R. C. Chem. Brit. 1990, 26, 565–568.276. Nick, H. Curr. Opin. Chem. Biol. 2007, 11, 419–423.277. Kontoghiorghes, G. J. Hemoglobin 2009, 33, 332–338.278. Cabantchik, Z. I.; Breuer, W.; Zanninelli, G. Best Pract. Res. Clin. Haematol. 2005,

18, 277–287.


















































































































































279. Daar, S.; Pathare, A.; Nick, H. Eur. J. Haematol. 2009, 82, 454–457.280. Mourad, F. H.; Hoffbrand, A. V.; Sheikh-Taha, M. Br. J. Haematol. 2003, 121,

187–189.281. Gomber, S.; Saxena, R.; Madan, N. Indian J. Pediatr. 2004, 41, 21–27.282. Galanello, R.; Kattamis, A.; Piga, A. Haematologica 2006, 91, 1241–1243.283. Aydinok, Y.; Ulger, Z.; Nart, D. Haematologica 2007, 92, 1599–1606.284. Tanner, M. A.; Galanello, R.; Dessi, C. Circulation 2007, 115, 1876–1884.285. Tanner, M. A.; Galanello, R.; Dessi, C. J. Cardiovasc. Magn. Reson. 2008, 25, 12.

286. Porter, John B. Blood Rev. 2009, 1, S3–S7.287. Brittenham, G. M. N. Eng. J. Med. 2011, 364, 146–156.288. Roberts, D. J.; Brunskill, S. J.; Doree, C.; Williams, S.; Howard, J.; Hyde, C. J.

Cochrane Database Syst. Rev. 2007, 18, CD004839.289. Roberts, D. J.; Rees, D.; Howard, J.; Hyde, C.; Alderson, P.; Brunskill, S. Cochrane

Database Syst. Rev. 2005, 19, CD004450.290. Leitch, H. A. Leuk. Res. 2007, 31, S7–S9.291. Greenberg, P. L. J. Natl. Compr. Canc. Netw. 2006, 4, 91–96.















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