Methoxylation of desazadesferrithiocin analogues: enhanced iron clearing efficiency

Deferoxamine to Minimize Fibrosis During Radiation Therapy

Significance: By 2030, there will be >4 million radiation-treated cancer survivors living in the United States. Irradiation triggers inflammation, fibroblast activation, and extracellular matrix deposition in addition to reactive oxygen species generation, leading to a chronic inflammatory response. Radiation-induced fibrosis (RIF) is a progressive pathology resulting in skin pigmentation, reduced elasticity, ulceration and dermal thickening, cosmetic deformity, pain, and the need for reconstructive surgery. Recent Advances: Deferoxamine (DFO) is a U.S. Food and Drug Administration (FDA)-approved iron chelator for blood dyscrasia management, which has been found to be proangiogenic, to decrease free radical formation, and reduce cell death. DFO has shown great promise in the treatment and prophylaxis of RIF in preclinical studies. Critical Issues: Systemic DFO has a short half-life and is cumbersome to deliver to patients intravenously. Transdermal DFO delivery is complicated by its high atomic mass and hydrophilicity, preventing stratum corneum penetration. A transdermal drug delivery system was developed to address these challenges, in addition to a strategy for topical administration. Future Directions: DFO has great potential to translate from bench to bedside. An important step in translation of DFO for RIF prophylaxis is to ensure that DFO treatment does not affect the efficacy of radiation therapy. Furthermore, after an initial plethora of studies reporting DFO treatment by intravenous and subcutaneous routes, a significant advantage of recent studies is the success of transdermal and topical delivery. Given the strong foundation of basic scientific research supporting the use of DFO treatment on RIF, clinicians will be closely following the results of the ongoing human studies.

Lecithin-based deferoxamine nanoparticles accelerated cutaneous wound healing in diabetic rats

Nanoparticles have higher frequency of being exposed to cells or tissue, and are thus more likely to gain access into cytoplasm or nuclei to modulate molecular events due to significantly larger surface area to volume ratio. As a result, they present amplified response or even different physiochemical and biomedical properties from bigger particles. Deferoxamine accelerates wound healing in diabetic rats by increased neovascularization, reduced inflammation and improved maturation of wound. We investigated the wound healing potential of deferoxamine-nanoparticles in diabetic rats. Lecithin based nanoparticles of deferoxamine were prepared and characterized. The diabetic rats were divided into five Groups, of which Group I was treated with pluronic-gel f-127 (25%), Group II with deferoxamine 0.1% and Group III, IV and V were treated with deferoxamine-nanoparticles incorporated in pluronic-gel f-127 25% at 0.03% (0.01% deferoxamine), 0.1% (0.03% deferoxamine) and 0.3% (0.1% deferoxamine) w/v respectively. The wound closure was significantly accelerated in group V as compared to control groups. HIF-1α, VEGF, SDF-1α, TGF-β₁, and IL-10 protein levels were significantly higher in group V. The collagen deposition and neovascularization was greater in deferoxamine-nanoparticle treated rats. In contrast, TNF-α level was lowest in group V. In summary, the deferoxamine-nanoparticle formulation we developed, when applied topically on diabetic wounds results in faster wound healing as compared to simple deferoxamine formulation. This formulation may prove to be an effective therapy for treatment of diabetic wounds.

Targeting Oncogenic Nuclear Factor Kappa B Signaling with Redox-Active Agents for Cancer Treatment

SIGNIFICANCE

Nuclear factor kappa B (NF-κB) signaling is essential under physiologically relevant conditions. However, aberrant activation of this pathway plays a pertinent role in tumorigenesis and contributes to resistance. Recent Advances: The importance of the NF-κB pathway means that its targeting must be specific to avoid side effects. For many currently used therapeutics and those under development, the ability to generate reactive oxygen species (ROS) is a promising strategy.

CRITICAL ISSUES

As cancer cells exhibit greater ROS levels than their normal counterparts, they are more sensitive to additional ROS, which may be a potential therapeutic niche. It is known that ROS are involved in (i) the activation of NF-κB signaling, when in sublethal amounts; and (ii) high levels induce cytotoxicity resulting in apoptosis. Indeed, ROS-induced cytotoxicity is valuable for its capabilities in killing cancer cells, but establishing the potency of ROS for effective inhibition of NF-κB signaling is necessary. Indeed, some cancer treatments, currently used, activate NF-κB and may stimulate oncogenesis and confer resistance.

FUTURE DIRECTIONS

Thus, combinatorial approaches using ROS-generating agents alongside conventional therapeutics may prove an effective tactic to reduce NF-κB activity to kill cancer cells. One strategy is the use of thiosemicarbazones, which form redox-active metal complexes that generate high ROS levels to deliver potent antitumor activity. These agents also upregulate the metastasis suppressor, N-myc downstream regulated gene 1 (NDRG1), which functions as an NF-κB signaling inhibitor. It is proposed that targeting NF-κB signaling may proffer a new therapeutic niche to improve the efficacy of anticancer regimens.

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.

Co-delivery of deferoxamine and hydroxysafflor yellow A to accelerate diabetic wound healing via enhanced angiogenesis

Nonhealing chronic wounds on foot induced by diabetes is a complicated pathologic process. They are mainly caused by impaired neovascularization, neuropathy, and excessive inflammation. A strategy, which can accelerate the vessel network formation as well as inhibit inflammatory response at the same time, makes it possible for effective diabetic ulcers treatment. Co-delivery of multiple drugs with complementary bioactivity offers a strategy to properly treat diabetic wound. We previously demonstrated that hydroxysafflor yellow A (HSYA) could accelerate diabetic wound healing through promoting angiogenesis and reducing inflammatory response. In order to further enhance blood vessel formation, a pro-angiogenic molecular called deferoxamine (DFO) was topically co-administrated with HSYA. The in vitro results showed that the combination of DFO and HSYA exerted synergistic effect on enhancing angiogenesis by upregulation of hypoxia inducible factor-1 alpha (HIF-1α) expression. The interpenetrating polymer networks hydrogels, characterized by good breathability and water absorption, were designed for co-loading of DFO and HSYA aiming to recruit angiogenesis relative cells and upgrade wound healing in vivo. Both DFO and HSYA in hydrogel have achieved sustained release. The in vivo studies indicated that HSYA/DFO hydrogel could accelerate diabetic wound healing. With a high expression of Hif-1α which is similar to that of normal tissue. The noninvasive US/PA imaging revealed that the wound could be recovered completely with abundant blood perfusion in dermis after given HSYA/DFO hydrogel for 28 days. In conclusion, combination of pro-angiogenic small molecule DFO and HSYA in hydrogel provides a promising strategy to productively promote diabetic wound healing as well as better the repair quality.

Targeting cancer by binding iron: Dissecting cellular signaling pathways

Newer and more potent therapies are urgently needed to effectively treat advanced cancers that have developed resistance and metastasized. One such strategy is to target cancer cell iron metabolism, which is altered compared to normal cells and may facilitate their rapid proliferation. This is supported by studies reporting the anti-neoplastic activities of the clinically available iron chelators, desferrioxamine and deferasirox. More recently, ligands of the di-2-pyridylketone thiosemicarbazone (DpT) class have demonstrated potent and selective anti-proliferative activity across multiple cancer-types in vivo, fueling studies aimed at dissecting their molecular mechanisms of action. In the past five years alone, significant advances have been made in understanding how chelators not only modulate cellular iron metabolism, but also multiple signaling pathways implicated in tumor progression and metastasis. Herein, we discuss recent research on the targeting of iron in cancer cells, with a focus on the novel and potent DpT ligands. Several key studies have revealed that iron chelation can target the AKT, ERK, JNK, p38, STAT3, TGF-β, Wnt and autophagic pathways to subsequently inhibit cellular proliferation, the epithelial-mesenchymal transition (EMT) and metastasis. These developments emphasize that these novel therapies could be utilized clinically to effectively target cancer.

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.

Transdermal deferoxamine prevents pressure-induced diabetic ulcers

There is a high mortality in patients with diabetes and severe pressure ulcers. For example, chronic pressure sores of the heels often lead to limb loss in diabetic patients. A major factor underlying this is reduced neovascularization caused by impaired activity of the transcription factor hypoxia inducible factor-1 alpha (HIF-1α). In diabetes, HIF-1α function is compromised by a high glucose-induced and reactive oxygen species-mediated modification of its coactivator p300, leading to impaired HIF-1α transactivation. We examined whether local enhancement of HIF-1α activity would improve diabetic wound healing and minimize the severity of diabetic ulcers. To improve HIF-1α activity we designed a transdermal drug delivery system (TDDS) containing the FDA-approved small molecule deferoxamine (DFO), an iron chelator that increases HIF-1α transactivation in diabetes by preventing iron-catalyzed reactive oxygen stress. Applying this TDDS to a pressure-induced ulcer model in diabetic mice, we found that transdermal delivery of DFO significantly improved wound healing. Unexpectedly, prophylactic application of this transdermal delivery system also prevented diabetic ulcer formation. DFO-treated wounds demonstrated increased collagen density, improved neovascularization, and reduction of free radical formation, leading to decreased cell death. These findings suggest that transdermal delivery of DFO provides a targeted means to both prevent ulcer formation and accelerate diabetic wound healing with the potential for rapid clinical translation.

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.

Wound healing: an update

Wounds, both chronic and acute, continue to be a tremendous socioeconomic burden. As such, technologies drawn from many disciplines within science and engineering are constantly being incorporated into innovative wound healing therapies. While many of these therapies are experimental, they have resulted in new insights into the pathophysiology of wound healing, and in turn the development of more specialized treatments for both normal and abnormal wound healing states. Herein, we review some of the emerging technologies that are currently being developed to aid and improve wound healing after cutaneous injury.

Dose titration of deferasirox iron chelation therapy by magnetic resonance imaging for chronic iron storage disease in three adult red bald-headed uakari (Cacajao calvus rubicundus)

Iron overload is common in lemurs and some New World nonhuman primates raised in captivity, but there is no such documentation in the red bald-headed uakari (Cacajao calvus rubicundus). This study describes postmortem documentation of severe iron storage disease in one red bald-headed uakari and the use of iron chelation with oral deferasirox in the three surviving members of the colony. Magnetic resonance imaging was used to quantify pretreatment iron burden and to follow the response to therapy in two females, 22 and 28 yr of age, and one male 33 yr of age. Baseline liver iron concentrations ranged from 16 to 23 mg/g dry weight. In humans, a liver iron concentration greater than 15 mg/g is considered severe and associated with endocrine and cardiac toxicity. The uakaris were otherwise asymptomatic, generally healthy, nonpregnant, and on a stable, low-iron diet. Quantitative magnetic resonance imaging indicated that dosage escalations up to 100 mg/kg were needed to produce meaningful reductions in iron stores. After 5 yr of therapy, two animals continue at a dosage of 100 mg/kg per day, and the third was transitioned to twice-weekly maintenance dosing because of successful de-ironing. The animals tolerated iron chelation therapy well, having stable hematologic, renal, and hepatic function profiles before, during, and after treatment. Deferasirox monotherapy may represent a therapeutic option in primates with iron storage disease when dietary measures are ineffective and phlebotomy is logistically challenging.

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.

Halogenated 2'-benzoylpyridine thiosemicarbazone (XBpT) chelators with potent and selective anti-neoplastic activity: relationship to intracellular redox activity

Iron chelators of the 2'-benzoylpyridine thiosemicarbazone (BpT) class show substantial potential as anticancer agents. To explore structure-activity relationships, new BpT analogues were designed that incorporated halogen substituents on the noncoordinating phenyl group (XBpTs). These XBpT ligands exhibited potent antiproliferative activity with some analogues exceeding that of the parent BpT compound. Importantly, there was an appreciable therapeutic index in vitro, as mortal cells were significantly less affected by these chelators relative to neoplastic cells. The addition of a halogen led to a halogen-specific increase in the redox potential of XBpT-Fe complexes. Probing for chelator-induced intracellular reactive oxygen species (ROS) with the fluorescent probe, 2',7'-dichlorofluorescein, revealed a 1.5-4.7-fold increase in fluorescence upon incorporation of Cl, Br, or I to the parent analogues. Furthermore, an important structure-activity relationship was deduced where the addition of halogens led to a positive correlation between intracellular ROS generation and antiproliferative activity in the more hydrophilic BpT parent compounds.

Thiosemicarbazones: the new wave in cancer treatment

The anticancer effects of thiosemicarbazones were once solely attributed to the inhibition of ribonucleotide reductase, an enzyme involved in the rate-limiting step of DNA synthesis. However, the mechanism behind this inhibition was initially not described. The ability of thiosemicarbazones to chelate metal ions has now been recognized as a major factor in their antiproliferative effects. This mini-review discusses current advances of an emerging 'new wave' of thiosemicarbazones as potent anticancer agents, describing recent insights into their mechanism of action. The redox activity of Fe-thiosemicarbazone complexes is critical in their anticancer activity, resulting in oxidative damage and the inhibition of ribonucleotide reductase. In vivo analysis indicates that some thiosemicarbazones show potential as chemotherapeutic agents.

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.

Design, Synthesis, and Testing of Polyamine Vectored Iron Chelators

Iron chelators have been shown to control the growth of cancer cells in culture by sequestering exogenous iron in the media. Thus, the ligands prevent cellular access to the metal. However, because transferrin provides iron to tumor cells in animals, chelators have not been effective antitumor agents. Polyamine chelator conjugates in which the polyamine vectored ligands into cells were far more active than the free chelators themselves. However, the free ligands were not released from the vector once in the cell. The current study focuses on the synthesis and preliminary evaluation of a polyamine chelator conjugate capable of releasing the free ligand intracellularly via a nonspecific esterase.

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.

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.

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.

The Evolution of Iron Chelators for the Treatment of Iron Overload Disease and Cancer

The evolution of iron chelators from a range of primordial siderophores and aromatic heterocyclic ligands has lead to the formation of a new generation of potent and efficient iron chelators. For example, various siderophore analogs and synthetic ligands, including ICL670A [4-[3,5-bis-(hydroxyphenyl)-1,2,4-triazol-1-yl]-benzoic acid], 4'-hydroxydesazadesferrithiocin, and Triapine, have been developed from predecessors and illustrate potent iron-mobilizing or antineoplastic activities. This review focuses on the evolution of iron chelators from initial lead compounds through to the development of novel chelating agents, many of which show great potential to be clinically applied in the treatment of iron overload disease and cancer.

Synthesis and structural modeling of the amphiphilic siderophore rhizobactin-1021 and its analogs

We describe two convenient syntheses of rhizobactin-1021 (Rz), a citrate-based siderophore amphiphile produced by the nitrogen-fixing root symbiont Rhizobium meliloti-1021, and several analogs. Our approach features a singly amidated, tert-butyl-protected citrate intermediate that easily affords a variety of Rz analogs in the late stages of the synthesis. Structural modeling and the monolayer behavior of Rz and its metal complexes are consistent with a structural reorganization upon Rz-mediated iron chelation.