A comparative evaluation of iron clearance models

Is Chelation Therapy a Potential Treatment for Parkinson's Disease?

Iron loading in some brain regions occurs in Parkinson’s Disease (PD), and it has been considered that its removal by iron chelators could be an appropriate therapeutic approach. Since neuroinflammation with microgliosis is also a common feature of PD, it is possible that iron is sequestered within cells as a result of the “anaemia of chronic disease” and remains unavailable to the chelator. In this review, the extent of neuroinflammation in PD is discussed together with the role played by glia cells, specifically microglia and astrocytes, in controlling iron metabolism during inflammation, together with the results of MRI studies. The current use of chelators in clinical medicine is presented together with a discussion of two clinical trials of PD patients where an iron chelator was administered and showed encouraging results. It is proposed that the use of anti-inflammatory drugs combined with an iron chelator might be a better approach to increase chelator efficacy.

The Efficacy of Iron Chelators for Removing Iron from Specific Brain Regions and the Pituitary-Ironing out the Brain

Iron chelation therapy, either subcutaneous or orally administered, has been used successfully in various clinical conditions. The removal of excess iron from various tissues, e.g., the liver spleen, heart, and the pituitary, in beta thalassemia patients, has become an essential therapy to prolong life. More recently, the use of deferiprone to chelate iron from various brain regions in Parkinson’s Disease and Friederich’s Ataxia has yielded encouraging results, although the side effects, in <2% of Parkinson’s Disease(PD) patients, have limited its long-term use. A new class of hydroxpyridinones has recently been synthesised, which showed no adverse effects in preliminary trials. A vital question remaining is whether inflammation may influence chelation efficacy, with a recent study suggesting that high levels of inflammation may diminish the ability of the chelator to bind the excess iron.

Scaffold Based Search on the Desferithiocin Archetype

Iron overload disorder and diseases where iron mismanagement plays a crucial role require orally available iron chelators with favourable pharmacokinetic and toxicity profile. Desferrithiocin (DFT), a tridentate and orally available iron chelator has a favourable pharmacokinetic profile but its use has been clinically restricted due to its nephrotoxic potential. The chemical architecture of the DFT has been naturally well optimized for better iron chelation and iron clearance from human biological system. Equally they are also responsible for its toxicity. Hence, subsequent research has been devoted to develop a non-nephrotoxic analogue of DFT without losing its iron clearance ability. The review has been designed to classify the compounds reported till date and to discuss the structure activity relationship with reference to modifications attempted at different positions over pyridine and thiazoline ring of DFT. Compounds are clustered under two major classes: (i) Pyridine analogues and (ii) phenyl analogue and further each class has been further subdivided based on the presence or absence and the number of hydroxy functional groups present over pyridine or phenyl ring of the DFT analogues. Finally a summary and few insights into the development of newer analogues are provided.

Oxidative Injury and Iron Redistribution Are Pathological Hallmarks of Marmoset Experimental Autoimmune Encephalomyelitis

Oxidative damage and iron redistribution are associated with the pathogenesis and progression of multiple sclerosis (MS), but these aspects are not entirely replicated in rodent experimental autoimmune encephalomyelitis (EAE) models. Here, we report that oxidative burst and injury as well as redistribution of iron are hallmarks of the MS-like pathology in the EAE model in the common marmoset. Active lesions in the marmoset EAE brain display increased expression of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (p22phox, p47phox, and gp91phox) and inducible nitric oxide synthase immunoreactivity within lesions with active inflammation and demyelination, coinciding with enhanced expression of mitochondrial heat-shock protein 70 and superoxide dismutase 1 and 2. The EAE lesion-associated liberation of iron (due to loss of iron-containing myelin) was associated with altered expression of the iron metabolic markers FtH1, lactoferrin, hephaestin, and ceruloplasmin. The enhanced expression of oxidative damage markers in inflammatory lesions indicates that the enhanced antioxidant enzyme expression could not counteract reactive oxygen and nitrogen species-induced cellular damage, as is also observed in MS brains. This study demonstrates that oxidative injury and aberrant iron distribution are prominent pathological hallmarks of marmoset EAE thus making this model suitable for therapeutic intervention studies aimed at reducing oxidative stress and associated iron dysmetabolism.

Chemical features of in use and in progress chelators for iron overload

An excessive amount of iron may become extremely toxic both for its ability to generate reactive oxygen species, and for the lack of regulatory mechanisms for iron excretion in humans. Chelation therapy has been introduced in clinical practice in the 1970's to defend thalassemia patients from the effects of iron overload and it has dramatically changed both life expectancy and quality of life. The disadvantages of the drugs in clinical use make the research for new, more suitable iron chelating agents, urgent. This review defines the requirements of an iron chelator, then points out the principal chemical features of the iron chelators in use. Finally, a survey on the last ten years of the literature relative to iron chelators is done, and the most interesting ligands are presented, with particular emphasis to those that reached clinical trials.

Metabolically programmed iron chelators

Extensive structure activity relationship (SAR) studies focused on the desferrithiocin [DFT, (S)-4,5-dihydro-2-(3-hydroxy-2-pyridinyl)-4-methyl-4-thiazolecarboxylic acid] pharmacophore have led to three different DFT analogs being evaluated clinically for the treatment of iron overload diseases, for example, thalassemia. The SAR work revealed that the lipophilicity of a ligand, as determined by its partition between octanol and water, logP(app), could have a profound effect on the drug's iron clearing efficiency (ICE), organ distribution, and toxicity profile. While within a given structural family the more lipophilic a chelator the better the ICE, unfortunately, the more lipophilic ligands are often more toxic. Thus, a balance between lipophilicity, ICE, and toxicity must be achieved. In the current study, we introduce the concept of 'metabolically programmed' iron chelators, that is, highly lipophilic, orally absorbable, effective deferration agents which, once absorbed, are quickly converted to their nontoxic, hydrophilic counterparts.

Desferrithiocin: a search for clinically effective iron chelators

The successful search for orally active iron chelators to treat transfusional iron-overload diseases, e.g., thalassemia, is overviewed. The critical role of iron in nature as a redox engine is first described, as well as how primitive life forms and humans manage the metal. The problems that derive when iron homeostasis in humans is disrupted and the mechanism of the ensuing damage, uncontrolled Fenton chemistry, are discussed. The solution to the problem, chelator-mediated iron removal, is clear. Design options for the assembly of ligands that sequester and decorporate iron are reviewed, along with the shortcomings of the currently available therapeutics. The rationale for choosing desferrithiocin, a natural product iron chelator (a siderophore), as a platform for structure-activity relationship studies in the search for an orally active iron chelator is thoroughly developed. The study provides an excellent example of how to systematically reengineer a pharmacophore in order to overcome toxicological problems while maintaining iron clearing efficacy and has led to three ligands being evaluated in human clinical trials.

Substituent effects on desferrithiocin and desferrithiocin analogue iron-clearing and toxicity profiles

Desferrithiocin (DFT, 1) is a very efficient iron chelator when given orally. However, it is severely nephrotoxic. Structure-activity studies with 1 demonstrated that removal of the aromatic nitrogen to provide desazadesferrithiocin (DADFT, 2) and introduction of either a hydroxyl group or a polyether fragment onto the aromatic ring resulted in orally active iron chelators that were much less toxic than 1. The purpose of the current study was to determine if a comparable reduction in renal toxicity could be achieved by performing the same structural manipulations on 1 itself. Accordingly, three DFT analogues were synthesized. The iron-clearing efficiency and ferrokinetics were evaluated in rats and primates; toxicity assessments were carried out in rodents. The resulting DFT ligands demonstrated a reduction in toxicity that was equivalent to that of the DADFT analogues and presented with excellent iron-clearing properties.

Desferrithiocin analogue iron chelators: iron clearing efficiency, tissue distribution, and renal toxicity

The current solution to iron-mediated damage in transfusional iron overload disorders is decorporation of excess unmanaged metal, chelation therapy. The clinical development of the tridentate chelator deferitrin (1, Table 1) was halted due to nephrotoxicity. It was then shown by replacing the 4'-(HO) of 1 with a 3,6,9-trioxadecyloxy group, the nephrotoxicity could be ameliorated. Further structure-activity relationship studies have established that the length and the position of the polyether backbone controlled: (1) the ligand's iron clearing efficiency (ICE), (2) chelator tissue distribution, (3) biliary ferrokinetics, and (4) tissue iron reduction. The current investigation compares the ICE and tissue distribution of a series of (S)-4,5-dihydro-2-[2-hydroxy-4-(polyether)phenyl]-4-methyl-4-thiazolecarboxylic acids (Table 1, 3-5) and the (S)-4,5-dihydro-2-[2-hydroxy-3-(polyether)phenyl]-4-methyl-4-thiazolecarboxylic acids (Table 1, 8-10). The three most effective polyether analogues, in terms of performance ratio (PR), defined as mean ICE(primate)/ICE(rodent), are 3 (PR 1.1), 8, (PR 1.5), and 9, now in human trials, (PR 2.2). At the onset of the clinical trial on 9, no data were available for ligand 3 or 8. This is unfortunate, as 3 has many advantages over 9, e.g., the ICE of 3 in rats is 2.5-fold greater than that of 9 and analogue 3 achieves very high levels in the liver, pancreas, and heart, the organs most affected by iron overload. Finally, the impact of 3 on the urinary excretion of kidney injury molecule-1 (Kim-1), an early diagnostic biomarker for monitoring acute kidney toxicity, has been carried out in rats; no evidence of nephrotoxicity was found. Overall, the results suggest that 3 would be a far superior clinical candidate to 9.

The impact of polyether chain length on the iron clearing efficiency and physiochemical properties of desferrithiocin analogues

(S)-2-(2,4-Dihydroxyphenyl)-4,5-dihydro-4-methyl-4-thiazolecarboxylic acid (2) was abandoned in clinical trials as an iron chelator for the treatment of iron overload disease because of its nephrotoxicity. However, subsequent investigations revealed that replacing the 4'-(HO) of 2 with a 3,6,9-trioxadecyloxy group, ligand 4, increased iron clearing efficiency (ICE) and ameliorated the renal toxicity of 2. This compelled a closer look at additional polyether analogues, the subject of this work. The 3,6,9,12-tetraoxatridecyloxy analogue of 4, chelator 5, an oil, had twice the ICE in rodents of 4, although its ICE in primates was reduced relative to 4. The corresponding 3,6-dioxaheptyloxy analogue of 2, 6 (a crystalline solid), had high ICEs in both the rodent and primate models. It significantly decorporated hepatic, renal, and cardiac iron, with no obvious histopathologies. These findings suggest that polyether chain length has a profound effect on ICE, tissue iron decorporation, and ligand physiochemical properties.

Complexation of thorium(IV) with desmethyldesferrithiocin

The complexation of Th(IV) with desmethyldesferrithiocin (H2DMDFT), a derivative of the siderophore desferrithiocin (H2DFT), was studied by potentiometry, spectrophotometry and NMR. Three protonation constants of the ligand were determined by potentiometric titrations and1H-NMR and assigned to the phenolate group, the nitrogen at the hydroxypyridine ring, and the carboxylate group. The formation constant of the 1:2 complex, Th(DMDFT)2, was determined by absorption spectrophotometry with oxalate as a competing ligand. A tridentate complex with Th(IV) involving the phenolate oxygen, the thiazoline nitrogen and the carboxylate group of H2DMDFT was proposed.

Vibriobactin antibodies: a vaccine strategy

A new target strategy in the development of bacterial vaccines, the induction of antibodies to microbial outer membrane ferrisiderophore complexes, is explored. A vibriobactin (VIB) analogue, with a thiol tether, 1-(2,3-dihydroxybenzoyl)-5,9-bis[[(4S,5R)-2-(2,3-dihydroxyphenyl)-4,5-dihydro-5-methyl-4-oxazolyl]carbonyl]-14-(3-mercaptopropanoyl)-1,5,9,14-tetraazatetradecane, was synthesized and linked to ovalbumin (OVA) and bovine serum albumin (BSA). The antigenicity of the VIB microbial iron chelator conjugates and their iron complexes was evaluated. When mice were immunized with the resulting OVA-VIB conjugate, a selective and unequivocal antigenic response to the VIB hapten was observed; IgG monoclonal antibodies specific to the vibriobactin fragment of the BSA and OVA conjugates were isolated. The results are consistent with the idea that the isolated adducts of siderophores covalently linked to their bacterial outer membrane receptors represent a credible target for vaccine development.

Desferrithiocin analogue uranium decorporation agents

PURPOSE

Previous systematic structure-activity studies of the desferrithiocin (DFT) platform have allowed the design and synthesis of analogues and derivatives of DFT that retain the exceptional iron-clearing activity of the parent, while eliminating its adverse effects. We hypothesized that a similar approach could be adopted to identify DFT-related analogues that could effectively decorporate uranium.

MATERIALS AND METHODS

The decorporation properties of nine DFT-related analogues were determined in a bile duct-cannulated rat model. Diethylenetriaminepentaacetic acid (DTPA) served as a positive control. Selected ligands also underwent multiple and delayed dosing regimens. Uranium excretion in urine and bile or stool was determined by inductively coupled plasma mass spectroscopy (ICP-MS); tissue levels of uranium were also assessed.

RESULTS

The two best clinical candidates are (S)-4,5-dihydro-2-[2-hydroxy-4-(3,6,9-trioxadecyloxy)phenyl]-4-methyl-4-thiazolecarboxylic acid [(S)-4'-(HO)-DADFT-PE (9)], with a 57% reduction in kidney uranium levels on oral (p.o.) administration and (S)-4,5-dihydro-2-[2-hydroxy-3-(3,6,9-trioxadecyloxy)phenyl]-4-methyl-4-thiazolecarboxylic acid [(S)-3'-(HO)-DADFT-PE (10)], with a 62% renal reduction on p.o. administration. The majority of the metal excretion promoted by these analogues is in the bile, thus further reducing kidney actinide exposure.

CONCLUSIONS

While 9 administered p.o. or subcutaneously (s.c.) immediately post-metal is an effective decorporation agent, withholding the dose (s.c.) until 4 h reduced the activity of the compound. Conversion of 9 to its isopropyl ester may circumvent this issue.

Desferrithiocin analogues and nephrotoxicity

The syntheses of a series of 4'-O-alkylated ( S)-4,5-dihydro-2-(2,4-dihydroxyphenyl)-4-methyl-4-thiazole-carboxylic acid and 5'-O-alkylated ( S)-4,5-dihydro-2-(2,5-dihydroxyphenyl)-4-methyl-4-thiazolecarboxylic acid ligands are described. Their partition between octanol and water, log P(app), is determined, along with their iron-clearing efficiency (ICE) in both non-iron-overloaded, bile duct-cannulated rodents and in iron-overloaded primates. The ligand-promoted biliary ferrokinetics in rats are described for each of the chelators. Plots of log P(app) versus ICE in a rodent model for both the 4'-O-alkylated 2,4-dihydroxy and 5'-O-alkylated 2,5-dihydroxy series produced an inverse parabola plot with r(2) values of 0.97 and 0.81, respectively. The plots indicate an optimum log P(app)/ICE relationship. Because of the nature of the data spread in the 4'-O-alkylated 2,4-dihydroxy series, it will be used to help assess the origin of nephrotoxicity in desferrithiocin analogues: is toxicity simply related to lipophilicity, ICE, or a combination of these properties?

Design, synthesis, and testing of non-nephrotoxic desazadesferrithiocin polyether analogues

A series of iron-clearing efficiencies (ICEs), ferrokinetics, and toxicity studies for ( S)-2-(2,4-dihydroxyphenyl)-4,5-dihydro-4-methyl-4-thiazolecarboxylic acid (deferitrin, 1), ( S)-4,5-dihydro-2-[2-hydroxy-4-(3,6,9-trioxadecyloxy)phenyl]-4-methyl-4-thiazolecarboxylic acid ( 2), and (S)-4,5-dihydro-2-[2-hydroxy-3-(3,6,9-trioxadecyloxy)phenyl]-4-methyl-4-thiazolecarboxylic acid ( 3) are reported. The ICEs in rodents are shown to be dose-dependent and saturable for ligands 2 and 3 and superior to 1. Both polyether analogues in subcutaneous (sc) versus oral (po) administration in rodents and primates demonstrated excellent bioavailability. Finally, in a series of toxicity studies of ligands 1- 3, the dosing regimen was shown to have a profound effect in animals treated with ligand 1. When ligand 1 was given at doses of 237 micromol/kg/day twice a day (b.i.d.), there was serious proximal tubule damage versus 474 micromol/kg/day once daily (s.i.d.). With 2 and 3, in iron-overloaded and/or non-iron-loaded rodents, kidney histopathologies remained normal.

Phase Ib clinical trial of starch-conjugated deferoxamine (40SD02): a novel long-acting iron chelator

The most widely used drug for iron chelation is deferoxamine (DFO) mesylate. While effective in promoting iron excretion, it requires prolonged daily infusions, often resulting in poor compliance. A clinical trial was conducted using starch-conjugated DFO (S-DFO; 40SD02), a high-molecular-weight iron chelator possessing prolonged vascular retention. Single doses of S-DFO were infused intravenously into groups of four transfusion-dependent patients with beta-thalassaemia at doses of 150, 300, 600 and 900 mg/kg. Urinary iron excretion and various pharmacologic parameters were evaluated for 1 week and safety for 3 weeks. No drug-related effects were observed on clinical chemistries, haematological and coagulation parameters, urinalyses, vital signs or electrocardiograms. Drug-related adverse events were limited to four urticarial reactions, none requiring termination of the infusion. The drug stimulated clinically significant urinary iron excretion, with the highest dose (900 mg/kg) inducing excretion of 1.31 mg of iron/kg (range 0.79-1.90 mg/kg) over 1 week, with residual iron-binding capacity present in the plasma for over 6 d. In summary, treatment with S-DFO, administered weekly, has the potential to achieve iron balance in the poorly compliant patient.

Impact of the 3,6,9-trioxadecyloxy group on desazadesferrithiocin analogue iron clearance and organ distribution

The impact of introducing a 3,6,9-trioxadecyloxyl group at various positions of the desazadesferrithiocin (DADFT) aromatic ring on iron clearance and organ distribution is described. Three DADFT polyethers are evaluated: (S)-4,5-dihydro-2-[2-hydroxy-4-(3,6,9-trioxadecyloxy)phenyl]-4-methyl-4-thiazolecarboxylic acid [(S)-4'-(HO)-DADFT-PE, 3], (S)-4,5-dihydro-2-[2-hydroxy-5-(3,6,9-trioxadecyloxy)phenyl]-4-methyl-4-thiazolecarboxylic acid [(S)-5'-(HO)-DADFT-PE, 6], and (S)-4,5-dihydro-2-[2-hydroxy-3-(3,6,9-trioxadecyloxy)phenyl]-4-methyl-4-thiazolecarboxylic acid [(S)-3'-(HO)-DADFT-PE, 9]. The iron-clearing efficiency (ICE) in rodents and primates is shown to be very sensitive to which positional isomer is evaluated, as is the organ distribution in rodents. The polyethers had uniformly higher ICEs than their corresponding parent ligands in rodents, consistent with in vivo ligand-serum albumin binding studies. Ligand 9 is the most active polyether analogue in rodents and is also very effective in primates, suggesting a higher index of success in humans. In addition, this analogue is also shown to clear more iron in the urine of the primates than many of the other chelators. If this trend were also observed in patients, it would facilitate iron-balance studies in a clinical setting.

The design, synthesis, and evaluation of organ-specific iron chelators

A series of iron chelators, three (S)-4,5-dihydro-2-(2-hydroxyphenyl)-4-methyl-4-thiazolecarboxylic acid (DADFT) and three (S)-4,5-dihydro-2-(2-hydroxyphenyl)-4-thiazolecarboxylic acid (DADMDFT) analogues are synthesized and assessed for their lipophilicity (log Papp), iron-clearing efficiency (ICE) in rodents and iron-loaded primates (Cebus apella), toxicity in rodents, and organ distribution in rodents. The results lead to a number of generalizations useful in chelator design strategies. In rodents, while log Papp is a good predictor of a chelator's ICE, chelator liver concentration is a better tool. In primates, log Papp is a good predictor of ICE, but only when comparing structurally very similar chelators. There is a profound difference in toxicity between the DADMDFT and DADFT series: DADMDFTs are less toxic. Within the DADFT family of ligands, the more lipophilic ligands are generally more toxic. Lipophilicity can have a profound effect on ligand organ distribution, and ligands can thus be targeted to organs compromised in iron overload disease, for example, the heart.

(S)-4,5-dihydro-2-(2-hydroxy-4-hydroxyphenyl)-4-methyl-4-thiazolecarboxylic acid polyethers: a solution to nephrotoxicity

Previous studies revealed that within a family of ligands the more lipophilic chelators have better iron-clearing efficiency. The larger the log P(app) value of the compound, the better the iron-clearing efficiency. What is also clear from the data is that although the relative effects of log P(app) changes are essentially the same through different families, there are differences in absolute value between families. However, there also exists a second, albeit somewhat more disturbing, relationship. In all sets of ligands, the most lipophilic chelator is always the most toxic. The current study focuses on designing ligands that balance the lipophilicity/toxicity problem while iron-clearing efficiency is maintained. Earlier studies with (S)-4,5-dihydro-2-(2-hydroxy-4-methoxyphenyl)-4-methyl-4-thiazolecarboxylic acid [(S)-4'-(CH(3)O)-DADFT, 6] indicated that this methyl ether was a ligand with excellent iron-clearing efficiency in both rodents and primates; however, it was too toxic. On the basis of this finding, a less lipophilic, more water-soluble ligand than 6 was assembled, (S)-4,5-dihydro-2-[2-hydroxy-4-(3,6,9-trioxadecyloxy)phenyl]-4-methyl-4-thiazolecarboxylic acid [(S)-4'-(HO)-DADFT-PE, 11], a polyether analogue, along with its ethyl and isopropyl esters. The parent polyether and its isopropyl and ethyl esters were all shown to be highly efficient iron chelators in both rodents and primates. A comparison of 11 in rodents with the desferrithiocin analogue (S)-4,5-dihydro-2-(2,4-dihydroxyphenyl)-4-methyl-4-thiazolecarboxylic acid [(S)-4'-(HO)-DADFT, 1] revealed the polyether to be more tolerable, achieving higher concentrations in the liver and significantly lower concentrations in the kidney. The lower renal drug levels are in keeping with the profound difference in the architectural changes seen in the kidney of rodents given 1 versus those treated with 11.

Polyamine-vectored iron chelators: the role of charge

The utility of polyamines as vectors for the intracellular transport of iron chelators is further described. Consistent with earlier results with polyamine analogues, these studies underscore the importance of charge in the design of polyamine-vectored chelators. Four polyamine conjugates are synthesized, two of terephthalic acid [N(1)-(4-carboxy)benzoylspermine (7) and its methyl ester (6)] and two of (S)-2-(2,4-dihydroxyphenyl)-4,5-dihydro-4-methyl-4-thiazolecarboxylic acid [(S)-4'-(HO)-DADFT] [(S)-4,5-dihydro-2-[2-hydroxy-4-(12-amino-5,9-diazadodecyl-oxy)phenyl]-4-methyl-4-thiazolecarboxylic acid (10) and its ethyl ester (9)]. These four molecules were evaluated in murine leukemia L1210 cells for their impact on cell proliferation (48- and 96-h IC(50) values), their ability to compete with spermidine for the polyamine transport apparatus (K(i)), and their intracellular accumulation. The data revealed that when neutral molecules (cargo fragments) were fixed to the polyamine vector, the conjugates competed well with spermidine for transport and were accumulated intracellularly to millimolar levels. However, this was not the case when the cargo fragments were negatively charged. Metabolic studies of the polyamine-vectored (S)-4'-(HO)-DADFTs in rodents indicated that not only did the expected deaminopropylation step occur, but also a surprisingly high level of oxidative deamination at the terminal primary nitrogens took place. Finally, the iron-clearing efficiency of the (S)-4'-(HO)-DADFT conjugates was determined in a bile-duct-cannulated rodent model. Attaching the ligand to a polyamine vector had a profound effect on increasing the iron-clearing efficiency of this chelator relative to its parent drug.

Partition-variant desferrithiocin analogues: organ targeting and increased iron clearance

Altering the lipophilicity (log P(app)) of desferrithiocin analogues can change the organ distribution of the chelators and lead to enhanced iron clearance. For example, alkylation of (S)-2-(2,4-dihydroxyphenyl)-4,5-dihydro-4-methyl-4-thiazolecarboxylic acid [(S)-4'-(HO)-DADFT] and its analogues to more lipophilic compounds, such as (S)-4,5-dihydro-2-(2-hydroxy-4-methoxyphenyl)-4-methyl-4-thiazolecarboxylic acid [(S)-4'-(CH3O)-DADFT], provides ligands that achieved between a 3- and 8-fold increase in chelator concentrations in the heart, liver, and pancreas (the organs most at risk in iron-overload disease) of treated rodents. The 4'-O-methylated compounds are demethylated to their hydroxylated counterparts in rodents; furthermore, this O-demethylation takes place in both rodent and human liver microsomes. The relationship between chelator lipophilicity and iron-clearing efficacy in the iron-overloaded Cebus apella primate is further underscored by a comparison of the iron-clearing efficiency of (S)-2-(2,3-dihydroxyphenyl)-4,5-dihydro-4-methyl-4-thiazolecarboxylic acid [(S)-3'-(HO)-DADFT] and its 3'-(CH3O) counterpart. Finally, these DFT analogues are shown to be both inhibitors of the iron-mediated oxidation of ascorbate as well as effective radical scavengers.

Structure-activity relationships among desazadesferrithiocin analogues

Desferrithiocin, a natural product iron chelator (siderophore), offers an excellent platform from which to construct orally active iron chelators which have a good therapeutic window. A systematic structure-activity study on desferrithiocin identified the structural fragments necessary for the compound's oral iron-clearing activity. There are strict requirements regarding the distance between the ligating centers; they cannot be altered without loss of efficacy. The thiazoline ring must remain intact. Benz-fusions, which were designed to improve the ligands' tissue residence time and possibly iron-clearing efficiency, are ineffective. The maintenance of an (S)-configured C-4 carbon is optimal in the design of desferrithiocin-based iron chelators. With this information in hand, alteration of the redox potential of the aromatic ring was initiated. Introduction of a hydroxy in the 4'-position of at least three different desazadesferrithiocin analogues resulted in moderate to small changes in iron clearing efficacy yet dramatic reductions in the toxicity of the compounds were observed. Although the toxicity studies of these desferrithiocin analogues are continuing, it is clear that it is possible to alter a siderophore in such a way as to ameliorate its toxicity profile while maintaining its iron-clearing properties.

Iron chelation promoted by desazadesferrithiocin analogs: An enantioselective barrier

For patients who require lifelong blood transfusions, there is no efficient means, unless chelation therapy is employed, for elimination of excess iron. Alternatives to desferrioxamine, the currently accepted treatment for transfusional iron overload, are being investigated. The current article focuses on an enantiomeric pair of analogs of desferrithiocin, (+)-(S)- and (-)-(R)-2-(2,4-dihydroxyphenyl)-4,5-dihydro-4-methyl-4-thiazolecarboxylic acid (4'-hydroxydesazadesferrithiocin). The crystal structure corroborated the absolute configuration of the two compounds, (+) and (-) for the (S)- and (R)-enantiomers, respectively. Job's plots established the tridentate nature of both analogs and circular dichroism spectra confirmed the ligands' antipodal relationship. (+)-(S)-4'-Hydroxydesazadesferrithiocin is a more efficient deferration agent than is the (-)-(R)-enantiomer in a Cebus apella model of iron overload. Pharmacokinetic analyses and IC(50) measurements in L1210 murine leukemia cells were undertaken in an effort to account for the contrast in efficacy between the two enantiomers. Some differences exist in the plasma pharmacokinetic parameters between the two analogs. However, a more plausible explanation may be the apparent differences in transport across the cell membrane; the IC(50) value in L1210 cells of the (+)-(S)-enantiomer was at least 5-fold lower than that of the (-)-(R)-compound.

Methoxylation of desazadesferrithiocin analogues: enhanced iron clearing efficiency

The impact of altering the octanol-water partition properties (log P) of analogues of desazadesferrithiocin, (S)-4,5-dihydro-2-(2-hydroxyphenyl)-4-methyl-4-thiazolecarboxylic acid, on the ligands' iron clearing properties is described. Increasing chelator lipophilicity can both substantially augment iron clearing efficiency in Cebus apella primates as well as alter the mode of iron excretion, favoring fecal over urinary output. The complications of iron overload are often associated with the metal's interaction with hydrogen peroxide, generating hydroxyl radicals (Fenton chemistry) and, ultimately, other related deleterious species. In fact, some iron chelators actually promote this chemistry. All of the compounds synthesized and tested in the current study are shown to be both inhibitors of the iron-mediated oxidation of ascorbate, thus removing the metal from the Fenton cycle, and effective radical scavengers.

Desferrithiocin analogue based hexacoordinate iron(III) chelators

Traditional thinking has been that hexacoordinate Fe(III) ligands are more effective at preventing iron's interactions with reactive oxygen species, most particularly the Fe(II)-mediated reduction of hydrogen peroxide to the hydroxyl radical (i.e., Fenton chemistry), than are ligands of lower denticity. Thus, a hexacoordinate derivative of the well-characterized tricoordinate ligand (S)-2-(2,4-dihydroxyphenyl)-4,5-dihydro-4-thiazolecarboxylic acid [4'-(HO)-DADMDFT], (S,S)-1,11-bis[5-(4-carboxy-4,5-dihydrothiazol-2-yl)-2,4-dihydroxyphenyl]-4,8-dioxaundecane, was designed with the aid of a molecular modeling program and synthesized. Evaluations both in vitro and in vivo were carried out to determine whether there is any advantage, at the level of prevention of Fenton chemistry, radical trapping, or iron clearance, to constructing a desferrithiocin-based hexacoordinate analogue. The hexacoordinate analogue was more effective at preventing the iron-mediated oxidation of ascorbate at low ligand/metal ratios than was its tricoordinate parent and can function as an excellent radical scavenger. At equivalent iron binding doses in the bile duct cannulated rodent, oral administration of the tricoordinate ligand was 3-fold more effective than was po administration of the hexacoordinate derivative. However, sc administration of the hexacoordinate derivative resulted in an efficiency that was 3 times greater than that of the tricoordinate chelator. Unfortunately, the rodent findings were not substantiated in the primates. The hexacoordinate ligand was only about one-half as efficient as its tricoordinate parent when administered sc. Owing to these results, po dosing was not attempted. Thus, there appears to be no overall advantage to coupling two molecules of 4'-(HO)-DADMDFT to afford a hexacoordinate derivative.

HBED ligand: preclinical studies of a potential alternative to deferoxamine for treatment of chronic iron overload and acute iron poisoning

We have continued the preclinical evaluation of the efficacy and safety of the hexadentate phenolic aminocarboxylate iron chelator N, N'-bis(2-hydroxybenzyl) ethylenediamine-N, N'-diacetic acid monosodium salt (NaHBED) for the treatment of both chronic transfusional iron overload and acute iron poisoning. We examined the effect of route of administration by giving equimolar amounts of NaHBED and deferoxamine (DFO) to Cebus apella monkeys as either a subcutaneous (SC) bolus or a 20-minute intravenous (IV) infusion. By both routes, NaHBED was consistently about twice as efficient as DFO in producing iron excretion. For both chelators at a dose of 150 micromol/kg, SC was more efficient than IV administration. The biochemical and histopathologic effects of NaHBED administration were assessed. No systemic toxicity was found after either IV administration once daily for 14 days to iron-loaded dogs or after SC administration every other day for 14 days to dogs without iron overload. Evidence of local irritation was found at some SC injection sites. When the NaHBED concentration was reduced to 15% or less in a volume comparable to a clinically useful one, no local irritation was found with SC administration in rats. Because treatment of acute iron poisoning may require rapid chelator infusion, we compared the effects of IV bolus administration of the compounds to normotensive rats. Administration of DFO produced a prompt, prolonged drop in blood pressure and acceleration of heart rate; NaHBED had little effect. NaHBED may provide an alternative to DFO for the treatment of both chronic transfusional iron overload and of acute iron poisoning.

Desferrithiocin is a more potent antineoplastic agent than desferrioxamine

Desferrithiocin (DFT) is an orally effective Fe chelator, with a similar high affinity and selectivity for Fe to desferrioxamine (DFO), which has been shown clinically to possess antineoplastic activity. In this study, DFT was assessed for antineoplastic potential in hepatocellular carcinoma cell lines (HCC). This was done as there are few treatments for this aggressive neoplasm. The effects of DFT on cell proliferation, cell cycle progression, Fe uptake and toxicity were examined. To establish whether DFT was selective for cancer cells a comparison was made with normal (non-proliferating) hepatocytes and non-tumorigenic (proliferating) fibroblasts (SWISS-3T3). DFT was a potent inhibitor of HCC proliferation (IC(50) approximately 40 microM). DFO also inhibited HCC proliferation under the same conditions, but was much less active (IC(50)=110 - 210 microM). When saturated with Fe, the activity of DFT, like DFO, was greatly diminished, suggesting it may act by depriving the cells of Fe or inactivating essential Fe pool(s). Indeed DFT rapidly decreased Fe uptake from Tf-(59)Fe by hepatoma cells and also by normal hepatocytes. However, DFT (and DFO) had much less effect on cell survival in hepatocytes and fibroblasts than in hepatoma cells. DFT may, like DFO, inhibit the cell cycle in the S phase of DNA synthesis. Both chelators showed low toxicity. These results indicate that DFT has potent antineoplastic activity in HCC. Further investigation into the DFT class of Fe chelators seems warranted, particularly in view of its high activity in relation to DFO, a chelator which is already in clinical trial for neuroblastoma.

Significance of asymmetric sites in choosing siderophores as deferration agents

The syntheses of the microbial iron chelators L-fluviabactin, its unnatural enantiomer, D-fluviabactin, L-homofluviabactin, and L-agrobactin, are described. The key steps involve the selective bis-acylation of the terminal nitrogens of norspermidine, spermidine, or homospermidine with 2,3-bis(benzyloxy)benzoic acid in the presence of 1,1-carbonyldiimidazole, followed by coupling of the N-hydroxysuccinimide ester of CBZ-protected L- or D-threonine with the central nitrogen. The effectiveness of each of these ligands in supporting the growth of Paracoccus denitrificans in a low-iron environment and the ability of these compounds to promote iron uptake are evaluated. The stereochemical configuration of the oxazoline ring is shown to be the major structural factor controlling both microbial growth stimulation and iron uptake. L-Fluviabactin, L-homofluviabactin, and L-agrobactin all promoted growth and iron uptake; D-fluviabactin was only marginally active. As with the microorganism's native siderophore, L-parabactin, all three ligands in the L-configuration investigated exhibited biphasic, i.e., both high-affinity and low-affinity, kinetics. The high-affinity system (iron concentration < 1 microM) yielded K(m) values between 0.11 and 0.23 microM and V(max) values from 157 to 129 pg-atoms Fe min(-1) (mg of protein)(-1), whereas the low-affinity scheme (iron concentration > 1 microM) gave K(m) values from 0.53 to 3.5 microM and V(max) values between 96 and 413 pg-atoms Fe min(-1) (mg of protein)(-1). Both L- and D-fluviabactin are very effective at clearing iron from the bile duct-cannulated rodent; when given subcutaneously at a dose of 150 micromol/kg, both ligands had iron clearing efficiencies of >13%, which is much greater than that of desferrioxamine in this model. Thus, by altering the stereochemistry of certain microbial siderophores, it is possible to generate deferration agents that are still effective at clearing iron from animals, yet do not promote microbial growth.

Chelator-induced iron excretion in iron-overloaded marmosets

In order to test new orally active iron chelators in a predictive way, a primate model has been developed. This model makes use of the marmoset monkey (Callithrix jacchus) and its overall design is similar to a previously reported monkey model. However, this new model enables a higher compound throughput and requires lower amounts of test compound because the animals are much easier to handle and have much lower body weights. The marmosets were iron-overloaded by three intraperitoneal injections of iron (III) hydroxide polyisomaltose. For the iron-balance studies, the animals were kept in metabolic cages and were maintained on a low-iron diet in order to reduce faecal background. After compound administration, the excretion of iron in urine and faeces was followed for 2 d. A series of well-known chelators was tested for validation of the model. In particular, comparison of the iron-clearing properties of DFO, L1, CP94 and HBED in marmosets and humans demonstrated the predictive value of the model and justify our expectation that if iron chelators such as CGP65015, ICL670A and CGP75254A are active in marmosets, they will be active in humans as well.

Improving the oral bioavailability of the iron chelator HBED by breaking the symmetry of the intramolecular H-bond network

Physicochemical analysis and Monte Carlo simulations were used to identify structural features which prevent oral absorption of HBED, a potent iron chelator. In water the dominant conformations of HBED involve the hydrophobic collapse of the two aromatic rings. These conformations are favored in polar media because they expose the polar phenolic hydroxy groups to the solvent and partially shield the nonpolar aromatic rings. In a less polar solvent such as chloroform, a symmetrical H-bond network between the carboxylates and the amines dominates the conformational space. This leads to the exposure of the phenolic hydroxy groups to the solvent, which is unfavorable for solvation. The low solubility of HBED in nonpolar solvents was confirmed experimentally by determination of the partition coefficients in octanol, chloroform, and cyclohexane and may explain the poor membrane permeability of this compound. The high conformational stability which disfavors partitioning into phospholipids is mainly due to the symmetrical H-bond network. Potentiometric titrations of a monoester of HBED in MeOH/water indicate that the protonation sequence was changed compared to that of the parent compound, suggesting that the symmetrical H-bond network was disrupted. Conformational analysis in chloroform confirmed that, in contrast to HBED, no symmetric interaction between the carboxylate and the nitrogen amines is possible in the half-ester and a variety of conformations which allow partial shielding of the polar phenolic OH groups are energetically possible. This theoretical model predicting a better solubility of the half-esters in nonpolar solvents was supported by the large increase in the partition coefficients in octanol, chloroform, and cyclohexane measured experimentally. The high absorbability predicted by physicochemical and computer simulation methods was corroborated by in vivo experiments in marmoset monkeys where the monoethyl ester derivative of HBED was well-absorbed orally while the parent compound was nearly ineffective in the same model.

Evaluation of the desferrithiocin pharmacophore as a vector for hydroxamates

A series of (S)-desmethyldesferrithiocin (DMDFT, 1) hydroxamates and a bis-salicyl polyether hydroxamate are evaluated for their iron-clearing properties in rodents; some of these are further assessed in primates. These hydroxamates include (S)-desmethyldesferrithiocin, N-methylhydroxamate (2); (S)-desmethyldesferrithiocin, N-[5-(acetylhydroxyamino)pentyl]hydroxamate (3); desmethyldesferrithiocin, N-benzylhydroxamate (4); (S,S)-N(1), N(8)-bis[4,5-dihydro-2-(3-hydroxy-2-pyridinyl)-4-thiazoyl]-N(1), N(8)-dihydroxy-3,6-dioxa-1,8-octanediamine (5); and N(1), N(8)-bis(2-hydroxybenzoyl)-N(1),N(8)-dihydroxy-3,6-dioxa-1, 8-octanediamine (6). The ligands are evaluated when given both orally (po) and subcutaneously (sc) in the bile-duct-cannulated rodent model. In iron-overloaded primates, ligands 1-4 are assessed when administered po and sc. The efficiencies of the hydroxamates are shown to vary considerably; giving the compounds sc consistently resulted in greater chelating efficiency in vivo. After oral administration in the primate, compound 3, a pentacoordinate unsymmetrical dihydroxamate, produces iron excretion sufficient to warrant further preclinical evaluation both as a potential orally active iron-chelating agent and as a parenteral iron chelator. The increased iron clearance of several of these ligands when administered sc versus po also underscores the idea that parenteral administration is a reasonable alternative to a less efficient, orally active device which would require large and frequent doses.

Effects of C-4 stereochemistry and C-4' hydroxylation on the iron clearing efficiency and toxicity of desferrithiocin analogues

Additional structure-activity studies of desferrithiocin analogues are carried out. The effects of stereochemistry at C-4 on the ligands' iron clearing efficiency are reviewed and assessed using the enantiomers 4,5-dihydro-2-(2, 4-dihydroxyphenyl)thiazole-4(R)-carboxylic acid and 4,5-dihydro-2-(2, 4-dihydroxyphenyl)thiazole-4(S)-carboxylic acid. The utility of 4'-hydroxylation as a method of reducing the toxicity of desazadesferrithiocin analogues is also examined further with the synthesis and in vivo comparison of 4, 5-dihydro-2-(2-hydroxyphenyl)-4-methylthiazole-4(S)-carboxylic acid, which is the natural product 4-methylaeruginoic acid, and 4, 5-dihydro-2-(2,4-dihydroxyphenyl)-4-methylthiazole-4(S)-carboxylic acid. The stereochemistry at C-4 is shown to have a substantial effect on the iron clearing efficiency of desferrithiocin analogues, as does C-4'-hydroxylation on the toxicity profile. All of the compounds are evaluated in a bile-duct-cannulated rodent model to determine iron clearance efficiency and are carried forward to the iron-overloaded primate for iron clearing measurements. On the basis of the results of the present work, although 4,5-dihydro-2-(2, 4-dihydroxyphenyl)thiazole-4(S)-carboxylic acid is still the most promising candidate for clinical evaluation, 4,5-dihydro-2-(2, 4-dihydroxyphenyl)-4-methylthiazole-4(S)-carboxylic acid (4'-hydroxydesazadesferrithiocin) also merits further preclinical assessment.

In vitro and in situ permeability of a 'second generation' hydroxypyridinone oral iron chelator: correlation with physico-chemical properties and oral activity

PURPOSE

The in vitro and in situ transport of CGP 65015 ((+)-3-hydroxy-1-(2-hydroxyethyl)-2-hydroxyphenyl-methyl-1H-pyridin-4-on e), a novel oral iron chelator, is described. The predictive power of these data in assessing intestinal absorption in man is described.

METHODS

Caco-2 epithelial monolayer and in situ rat jejunum perfusion intestinal permeability models were utilized. In vivo iron excretion and preliminary animal pharmacokinetic experiments were described. Ionization constants and octanol/aqueous partition coefficients were measured potentiometrically. Solubilities and intrinsic dissolution rates were determined using standard procedures.

RESULTS

Caco-2 cell (Papp approximately 0.25 x 10(-6) cm x s(-1)) and rat jejunum (Pw approximately 0.4) permeabilities of CGP 65015 were determined. The log D(pH 7.4) of CGP 65015 was 0.58 and its aqueous solubility was < 0.5 mg x ml(-1) (pH 3-9). The intrinsic dissolution rate of CGP 65015 in USP simulated intestinal fluid was 0.012 mg x min(-1) x cm(-2). CGP 65015 promotes iron excretion effectively and dose dependently in animals.

CONCLUSIONS

Caco-2 and rat intestinal permeabilities predict incomplete oral absorption of CGP 65015 in man. Preliminary rat pharmacokinetics support this. Physico-chemical data are, also, in line and suggest that CGP 65015 may, in addition, be solubility/dissolution rate limited in vivo. Nevertheless, early animal pharmacological data demonstrate that CGP 65015 is a viable oral iron chelator candidate.

Domain-structured N1,N2-derivatized hydrazines as inhibitors of ribonucleoside diphosphate reductase: redox-cycling considerations

Eight analogues of 1-[5-halogenosalicylidene]-2-[2'-pyridinoyl]hydrazine and -[2'-pyridyl]hydrazine, four of 1-[pyridoxylidene]-2-[2'-pyridinoyl]hydrazine, seven of 1-[pyridoxylidene]-2-[2'-pyridyl]hydrazine, and one each of 1, 2-bis[pyridoxylidene]diaminoethane and bis[pyridoxylidenehydrazino]phthalazine were synthesized. Their solutions in DMF were assayed for activity against the metalloenzyme ribonucleoside diphosphate reductase (RdR), prepared from a subcutaneously growing murine tumor (sarcoma 180) implanted in B6D2F3 male mice. The 14C-labeled CDP reductase was assayed by the modified method of Takeda and Weber, in which [14C]cytidine was separated from deoxycytidine by thin-layer chromatography (TLC) on cellulose foil. Distribution of radioactivity was assessed with an automatic TLC linear analyzer. Of the 31 compounds tested, 13 were essentially inactive, 7 were highly active against RdR, and the remaining 20 were slightly more active than hydroxyurea (used as a reference compound). The mechanism of inhibition is discussed in terms of three alternative pathways, initiated by sequestration of iron embedded in the R1 subunit of the metalloenzyme to form a C-centered chelate radical (via redox cycling). Alternatively, the latter could either reduce the tyrosyl radical or intercept radicals generated in the reduction process.

Desazadesmethyldesferrithiocin analogues as orally effective iron chelators

Further structure-activity studies of desferrithiocin analogues are carried out. (S)-Desazadesmethyldesferrithiocin, 2-(2-hydroxyphenyl)-Delta2-thiazoline-4(S)-carboxylic acid, serves as the principal framework in the current paper. Desazadesmethyldesferrithiocin can be structurally altered with facility, and data are already available on its iron-clearing properties and toxicity parameters. Four different kinds of structural modifications of this framework are undertaken: introduction of hydroxy, carboxy, or methoxy groups on the aromatic ring; alteration of the thiazoline ring; increasing the distance between the ligand donor atoms; and benz-fusion of the aromatic rings. The structural modifications described are shown to have a tremendous impact on both the iron clearance and toxicity profiles of the desazadesmethyldesferrithiocin molecule. All of the compounds are assessed in a bile-duct-cannulated rodent model to determine iron clearance efficiency. Ligands which demonstrate an efficiency of greater than 2% are carried forward to the iron-overloaded primate for iron-clearing measurements. Ligands with efficiencies greater than 3% in the primate are then evaluated in a formal toxicity study in rodents. On the basis of the results of the present work, 2-(2, 4-dihydroxyphenyl)-Delta2-thiazoline-4(S)-carboxylic acid is a promising candidate for clinical evaluation.

HBED: The Continuing Development of a Potential Alternative to Deferoxamine for Iron-Chelating Therapy

To further examine the potential clinical usefulness of the hexadentate phenolic aminocarboxylate iron chelatorN,N′-bis(2-hydroxybenzyl)ethylenediamine-N,N′-diacetic acid (HBED) for the chronic treatment of transfusional iron overload, we performed a subchronic toxicity study of the HBED monosodium salt in rodents and have evaluated the iron excretion in primates induced by HBED. The HBED-induced iron excretion was determined for the monohydrochloride dihydrate that was first dissolved in a 0.1-mmol/L sodium phosphate buffer at pH 7.6 and administered to the primates either orally (PO) at a dose of 324 μmol/kg (149.3 mg/kg, n = 5), subcutaneously (sc) at a dose of 81 μmol/kg (37.3 mg/kg, n = 5), sc at 324 μmol/kg (n = 5), and sc at 162 μmol/kg (74.7 mg/kg) for 2 consecutive days for a total dose of 324 μmol/kg (n = 3). In addition, the monosodium salt of HBED in saline was administered to the monkeys sc at a single dose of 150 μmol/kg (64.9 mg/kg, n = 5) or at a dose of 75 μmol/kg every other day for three doses, for a total dose of 225 μmol/kg (n = 4). For comparative purposes, we have also administered deferoxamine (DFO) PO and sc in aqueous solution at a dose of 300 μmol/kg (200 mg/kg). In the iron-loadedCebus apella monkey, whereas the PO administration of DFO or HBED even at a dose of 300 to 324 μmol/kg was ineffective, the sc injection of HBED in buffer or its monosodium salt, 75 to 324 μmol/kg, produced a net iron excretion that was nearly three times that observed after similar doses of sc DFO. In patients with transfusional iron overload, sc injections of HBED may provide a much needed alternative to the use of prolonged parenteral infusions of DFO.  Note: After the publication of our previous paper (Blood, 91:1446, 1998) and the completion of the studies described here, it was discovered that the HBED obtained from Strem Chemical Co (Newburyport, MA) that was labeled and sold as a dihydrochloride dihydrate was in fact the monohydrochloride dihydrate. Therefore, the actual administered doses were 81, 162, or 324 μmol/kg; not 75, 150, or 300 μmol/kg as was previously reported. The new data have been recalculated accordingly, and the data from our earlier study, corrected where applicable, are shown in parentheses.

HBED: The Continuing Development of a Potential Alternative to Deferoxamine for Iron-Chelating Therapy

To further examine the potential clinical usefulness of the hexadentate phenolic aminocarboxylate iron chelatorN,N′-bis(2-hydroxybenzyl)ethylenediamine-N,N′-diacetic acid (HBED) for the chronic treatment of transfusional iron overload, we performed a subchronic toxicity study of the HBED monosodium salt in rodents and have evaluated the iron excretion in primates induced by HBED. The HBED-induced iron excretion was determined for the monohydrochloride dihydrate that was first dissolved in a 0.1-mmol/L sodium phosphate buffer at pH 7.6 and administered to the primates either orally (PO) at a dose of 324 μmol/kg (149.3 mg/kg, n = 5), subcutaneously (sc) at a dose of 81 μmol/kg (37.3 mg/kg, n = 5), sc at 324 μmol/kg (n = 5), and sc at 162 μmol/kg (74.7 mg/kg) for 2 consecutive days for a total dose of 324 μmol/kg (n = 3). In addition, the monosodium salt of HBED in saline was administered to the monkeys sc at a single dose of 150 μmol/kg (64.9 mg/kg, n = 5) or at a dose of 75 μmol/kg every other day for three doses, for a total dose of 225 μmol/kg (n = 4). For comparative purposes, we have also administered deferoxamine (DFO) PO and sc in aqueous solution at a dose of 300 μmol/kg (200 mg/kg). In the iron-loadedCebus apella monkey, whereas the PO administration of DFO or HBED even at a dose of 300 to 324 μmol/kg was ineffective, the sc injection of HBED in buffer or its monosodium salt, 75 to 324 μmol/kg, produced a net iron excretion that was nearly three times that observed after similar doses of sc DFO. In patients with transfusional iron overload, sc injections of HBED may provide a much needed alternative to the use of prolonged parenteral infusions of DFO.  Note: After the publication of our previous paper (Blood, 91:1446, 1998) and the completion of the studies described here, it was discovered that the HBED obtained from Strem Chemical Co (Newburyport, MA) that was labeled and sold as a dihydrochloride dihydrate was in fact the monohydrochloride dihydrate. Therefore, the actual administered doses were 81, 162, or 324 μmol/kg; not 75, 150, or 300 μmol/kg as was previously reported. The new data have been recalculated accordingly, and the data from our earlier study, corrected where applicable, are shown in parentheses.

Caco-2 cell permeability of a new (hydroxybenzyl)ethylenediamine oral iron chelator: correlation with physicochemical properties and oral activity

This study describes the transport of CGP 75254A, a novel oral iron chelator, across Caco-2 cells in an attempt to model intestinal epithelial cell permeability in man. CGP 75254A was dosed to the apical side of Caco-2 cell monolayers, together with [14C]mannitol as an internal permeability standard. The apparent permeability (Papp) was calculated from the cumulative appearance of drug in the basolateral fluid with time. The [14C]mannitol Papp indicated that the Caco-2 monolayers remained intact and that the iron chelator was not toxic to the cells. Permeabilities of CGP 75254A were compared with the Caco-2 permeabilities of compounds of known absorption in man. The results predict that absorption of CGP 75254A is likely to be virtually complete at pH values between 5.5 and 7.0. However, at pH 8.0 permeability is predicted as negligible. Cell permeability data are in full accordance with key physicochemical properties of CGP 75254A and suggest that the drug is passively absorbed. The results, which suggest likely quantitative absorption in vivo, are supported by preliminary pharmacological experiments in marmosets.

The origin of the differences in (R)- and (S)-desmethyldesferrithiocin. Iron-clearing properties

The iron clearance properties, toxicity, and pharmacokinetics of (R)- and (S)-desmethyldesferrithiocin (DMDFT) are described. The studies were performed in rodent and primate models. While both enantiomers were found to be effective iron chelators with minimal toxicity in the rodents, only (S)-DMDFT was able to induce the clearance of any iron in the primates. In addition, two out of nine of the monkeys given (R)-DMDFT died within 24 h of drug administration. The reason for the differences in iron clearance properties and the apparent toxicity of the (R)-enantiomer in the primates is likely related to the disparities in the pharmacokinetics of the two analogues. The pharmacokinetic data suggest enantioselectivity in renal clearance of the desferrithiocins and their iron complexes with (S)-DMDFT clearance 3.5 times greater than that of (R)-DMDFT, and FeIII [(S)-DMDFT]2 clearance 6.8 times greater than that of FeIII [R-DMDFT]2. In all primates studied FeIII [(R)-DMDFT]2 in the plasma exceeded 25 mg/L (50 microM) for several hours and remained above 10 mg/L (20 microM) at 8 h while levels of FeIII [(S)-DMDFT]2 never exceeded 50 microM and were at or below the limits of detection 8 h post-injection.

HBED: A Potential Alternative to Deferoxamine for Iron-Chelating Therapy

To examine the potential clinical usefulness of the hexadentate phenolic aminocarboxylate iron chelatorN,N-bis(2-hydroxybenzyl)ethylenediamine-N,N-diacetic acid (HBED) for the chronic treatment of transfusional iron overload, we compared the iron excretion induced by subcutaneous (SC) injection of HBED and deferoxamine (DFO), the reference chelator, in rodents and primates. In the non–iron-overloaded, bile-duct–cannulated rat, a single SC injection of HBED, 150 μmol/kg, resulted in a net iron excretion that was more than threefold greater than that after the same dose of DFO. In the iron-loaded Cebus apella monkey, a single SC injection of HBED, 150 μmol/kg, produced a net iron excretion that was more than twice that observed after the same dose of SC DFO. In patients with transfusional iron overload, SC injections of HBED may provide a much needed alternative to the use of prolonged parenteral infusions of DFO.

Synthesis and biological evaluation of naphthyldesferrithiocin iron chelators

The synthesis and iron-clearing properties of the naphthyldesferrithiocins 2-(2'-hydroxynaphth-1'-yl)-delta2-thiazoline-(4R)-carboxylic acid, 2-(2'-hydroxynaphth-1'-yl)-delta2-thiazoline-(4S)-carboxylic acid, 2-(3'-hydroxynaphth-2'-yl)-delta2-thiazoline-(4R)-carboxylic acid, and 2-(3'-hydroxynaphth-2'-yl)-delta2-thiazoline-(4S)-carboxylic acid are described. While the bile duct-cannulated rat model clearly demonstrates that the 3'-hydroxynaphthyl-2'-yl compounds are orally active iron-clearing agents and the corresponding 2'-hydroxynaphthyl-1'-yl compounds are not, in the primate model none of the benz-fused desazadesferrithiocin analogues are active. Oral versus subcutaneous administration of these ligands strongly suggests that metabolism is a key issue in their iron-clearing properties and that these benz-fused desferrithiocins are not good candidates for orally active iron-clearing drugs.

Oral iron-chelating therapy: the L1 experience

L1 is the most widely studied oral iron-chelating drug and at present the only one shown to be effective at causing negative iron balance in long-term clinical trials for thalassemia major and other transfusion-dependent refractory anaemias. Because of side-effects, both in experimental animals and in humans, its development as a widely available pharmaceutical agent has been delayed. However, for the large numbers of transfusion-dependent, iron-overloaded patients who do not use DFX because of poor compliance, adverse effects or unavailability of the drug, L1 may be a suitable alternative for iron chelation. However, its use should be restricted to Ethical Committee approved clinical trials. Patients who are capable of using DFX effectively should be encouraged to continue doing so until an oral iron chelator has been fully established for clinical use. It is hoped that 3-hydroxypyrid-4-one analogues of L1 as well as compounds related to pyridoxal isonicotinyl hydrazone, HBED or hydroxamic acid can be found both orally effective and safe for long-term administration. Current and future trials of L1 could address some of the following issues, beside extending present studies on the efficacy and adverse effects of L1: 1. The effect of administering a reduced dose of L1 (< 75 mg/kg per day) on the incidence of adverse effects and on long-term efficacy. 2. The efficacy and adverse effects of L1 at a low dose in patients with non-transfusional iron overload such as thalassaemia intermedia, primary haemochromatosis and congenital haemolytic anaemias. 3. The effect of combining oral L1 with intravenous or subcutaneous DFX on the incidence of adverse effects and efficacy. 4. Elucidation of the mechanisms involved in agranulocytosis and joint toxicity and finding methods to predict for individual susceptibility to these adverse effects and ways of preventing them.

Desferrithiocin is an effective iron chelator in vivo and in vitro but ferrithiocin is toxic

The efficacy and toxicity of the siderophore desferrithiocin (DFT), which has shown potential application in iron chelation therapy, were assessed in vivo and in vitro. DFT was evaluated in vivo in two ways: firstly, by measuring the effect of a single dose of DFT (10-100 mg/kg) on 59Fe excretion in iron-loaded rats labelled with 59Fe; and secondly, by examining the effect of the daily oral administration for 2 weeks of DFT (10-25 mg/kg/d) on the growing rat. DFT and its ferric complex, ferrithiocin (FT), were assessed in vitro from their effects on transferrin and iron uptake and mobilization from rat hepatocytes in culture using transferrin doubly labelled with 125I and 59Fe. Both oral and subcutaneous DFT were highly effective in promoting iron excretion in vivo, but showed evidence of toxicity after oral administration for 2 weeks at 25 mg/kg/d. In addition, DFT was much more effective than desferrioxamine or pyridoxal isonicotinyl hydrazone in reducing hepatocyte iron in vitro. However, FT was cytotoxic, causing membrane disruption and release of intracellular aspartate aminotransferase. It was concluded that DFT should not be considered for chronic iron chelation therapy without extensive further evaluation.