Actinide chelation: biodistribution and in vivo complex stability of the targeted metal ions

Because of the continuing use of nuclear fuel sources and heightened threats of nuclear weapon use, the amount of produced and released radionuclides is increasing daily, as is the risk of larger human exposure to fission product actinides. A rodent model was used to follow the in vivo distribution of representative actinides, administered as free metal ions or complexed with chelating agents including diethylenetriamine pentaacetic acid (DTPA) and the hydroxypyridinonate ligands 3,4,3-LI(1,2-HOPO) and 5-LIO(Me-3,2-HOPO). Different metabolic pathways for the different metal ions were evidenced, resulting in intricate ligand- and metal-dependent decorporation mechanisms. While the three studied chelators are known for their unrivaled actinide decorporation efficiency, the corresponding metal complexes may undergo in vivo decomposition and release metal ions in various biological pools. This study sets the basis to further explore the metabolism and in vivo coordination properties of internalized actinides for the future development of viable therapeutic chelating agents.     

Metal ions, Alzheimer's disease and chelation therapy

In the last few years, various studies have been providing evidence that metal ions are critically involved in the pathogenesis of major neurological diseases (Alzheimer, Parkinson). Metal ion chelators have been suggested as potential therapies for diseases involving metal ion imbalance. Neurodegeneration is an excellent target for exploiting the metal chelator approach to therapeutics. In contrast to the direct chelation approach in metal ion overload disorders, in neurodegeneration the goal seems to be a better and subtle modulation of metal ion homeostasis, aimed at restoring ionic balance. Thus, moderate chelators able to coordinate deleterious metals without disturbing metal homeostasis are needed. To date, several chelating agents have been investigated for their potential to treat neurodegeneration, and a series of 8-hydroxyquinoline analogues showed the greatest potential for the treatment of neurodegenerative diseases.     

Bifunctional chelators in the design and application of radiopharmaceuticals for oncological diseases

Radiopharmaceuticals constitute diagnostic and therapeutic tools for both clinical and preclinical applications. They are a blend of a tracer moiety that mediates a site specific accumulation and an effector: a radioisotope whose decay enables either molecular imaging or exhibits cytotoxic effects. Radioactive halogens and lanthanides are the most commonly used isotopes for radiopharmaceuticals. Due to their ready availability and the facile labeling metallic radionuclides offer ideal characteristics for applications in nuclear medicine. A stable link between the radionuclide and the carrier molecule is the primary prerequisite for in vivo applications. The radionuclide is selected according to its physical and chemical properties i.e. half-life, the type of decay, the energy emitted and its availability. Bifunctional chelating agents are used to stably link the radiometal to the carrier moiety of the radiopharmaceutical. The design of the bifunctional chelator has to consider the impact of the radiometal chelate on the biological properties of the target-specific pharmaceutical. Here, with an emphasis on oncology, we review applications of radiopharmaceuticals that contain bifunctional chelators, while highlighting successes and identifying the key challenges that need to be addressed for the successful translation of target binding molecules into tracers for molecular imaging and endoradiotherapy.     

The antibody-linked chelating polymers for nuclear therapy and diagnostics

This review deals with the problem of protein modification with chelating polymers. The main purpose of this approach is the preparation of monoclonal antibodies labeled with heavy metal isotopes (alpha-, beta-, and delta-emitting metals and metals used for NMR-tomography). Traditional binding of metals with proteins via chelating agents directly coupled to protein molecule does not allow binding a high number of metal atoms per single protein molecule and can also alter protein specific properties. At the same time, metal-to-protein binding via intermediate chelating polymer makes possible the binding of several dozen metal atoms per single protein without affecting its specific properties. Moreover, the variations in polymer properties and molecular weight allow controlled modified antibody biodistribution and clearance rate. Modified antibodies can be used successfully for nuclear and NMR diagnostics and for nuclear therapy. The following problems are discussed: the chemistry of the coupling of chelating groups to polymer backbone; the binding of chelating polymers to proteins, including monoclonal antibodies; the ability of chelating polymer-to-protein conjugates to bind heavy metals; the influence of the modification on protein conformation and specific properties; the behavior of metal-containing conjugates in vivo; the practical use of conjugates obtained for radioimmunoimaging, radioimmunotherapy, NMR-tomography, and in vitro immunoassays. Future prospects of the approach are also discussed.     

Dendrimer-based nanosized MRI contrast agents

Paramagnetic metals can induce T1 shortening by interaction with free water molecules. Two metal ions, Gadolinium and Manganese, are currently available for human use. Gadolinium-based MRI contrast agents (CAs) can operate using a approximately 100-fold lower concentration of Gadolinium ions in comparison to the necessary concentration of Iodine atoms employed in CT imaging in the tissues. Therefore, numerous macromolecular MRI CAs prepared employing relatively simple chemistry are readily available that can provide sufficient enhancement for multiple applications. Herein, we describe the synthesis, characteristics, and potential applications of dendrimer-based macromolecular MRI CAs in our recently reported libraries. This entire series of dendrimer-based macromolecular MRI CAs have a spherical shape and possess similar surface charges. Changes in molecular size altered the route of excretion. Smaller sized contrast agents, of less than 60 kD molecular weight, were excreted through the kidney resulting in these agents being potentially suitable as functional renal contrast agents. Less hydrophilic and larger sized contrast agents were found better suited for use as blood pool contrast agents. Hydrophobic variants of CAs formed with polypropylenimine diaminobutane dendrimer cores quickly accumulated in the liver and can function as liver contrast agents. Larger hydrophilic agents are also useful for lymphatic imaging. Finally, contrast agents conjugated with either monoclonal antibodies or with avidin are able to function as tumor-specific contrast agents and might also be employed as therapeutic drugs for either gadolinium neutron capture therapy or in conjunction with radioimmunotherapy.     

Bifunctional coupling agents for radiolabeling of biomolecules and target-specific delivery of metallic radionuclides

Receptor-based radiopharmaceuticals are of great current interest in molecular imaging and radiotherapy of cancers, and provide a unique tool for target-specific delivery of radionuclides to the diseased tissues. In general, a target-specific radiopharmaceutical can be divided into four parts: targeting biomolecule (BM), pharmacokinetic modifying (PKM) linker, bifunctional coupling or chelating agent (BFC), and radionuclide. The targeting biomolecule serves as a "carrier" for specific delivery of the radionuclide. PKM linkers are used to modify radiotracer excretion kinetics. BFC is needed for radiolabeling of biomolecules with a metallic radionuclide. Different radiometals have significant difference in their coordination chemistry, and require BFCs with different donor atoms and chelator frameworks. Since the radiometal chelate can have a significant impact on physical and biological properties of the target-specific radiopharmaceutical, its excretion kinetics can be altered by modifying the coordination environment with various chelators or coligand, if needed. This review will focus on the design of BFCs and their coordination chemistry with technetium, copper, gallium, indium, yttrium and lanthanide radiometals.     

Analysis of the degradation compounds of chemical warfare agents using liquid chromatography/mass spectrometry

The analysis of the degradation products of chemical warfare (CW) agents has been a challenge to analysts. The low volatility of these compounds makes them unsuitable for direct gas chromatography analysis without prior derivatization. Lack of a chromophore causes difficulties with classic detection methods after liquid chromatography separation. With the recent development of various interfaces that allow for the introduction of a liquid solvent stream into the mass spectrometer, the task of directly analyzing these compounds has become easier. For this report, we examined three different liquid chromatography/mass spectrometry (LC/MS) interfaces for their suitability for the analysis of CW degradation compounds. The interface types examined were particle beam electron impact ionization (PBI), electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI). Several alkylphosphonates and thiodiglycol analogs that are produced from the degradation of organophosphorus nerve agents and sulfur mustard, respectively, were analyzed using each of the three techniques. Electron impact ionization following gas chromatography or particle beam introduction typically generates very reproducible, library-searchable mass spectra. Most of the CW breakdown compounds examined using the PBI interface did not produce a molecular ion. Despite the lack of a molecular ion, the mass spectra of the various compounds contained enough different structural information from fragment ions for the positive identification of each. The mass spectra generated using ESI are generally limited to protonated molecular ions with little or no fragmentation. For positive identification and confirmation, tandem mass spectrometry techniques quite often must be used. Many of the compounds in this study were characterized by prominent sodiated adducts along with the protonated molecular ion. Methylphosphonic acid produced protonated dimers, trimers, etc. Although the various adduct ions can be used for additional confirmation of the molecular weight of a compound, the adducts also can result in suppression of ionization of the compound and thus reduce sensitivity. Another 'soft' ionization technique that results in abundant protonated molecular ions is APCI. The mass spectra of the breakdown compounds produced using APCI were characterized generally by either a prominent protonated molecular ion or a dehydrated form of it. In addition, a number of structurally significant fragment ions were observed and their relative abundances could be adjusted by altering the APCI conditions. The data presented here indicate that each of the three techniques can be used successfully for direct liquid introduction and analysis of the non-volatile compounds produced from the degradation of CW agents. The mass spectra produced using each technique are quite different and could be utilized as additional confirmation of compound identity.     

Synthesis, in vitro receptor binding, and in vivo evaluation of fluorescein and carbocyanine peptide-based optical contrast agents

Site-specific delivery of drugs and contrast agents to tumors protects normal tissues from the cytotoxic effects of drugs and enhances the contrast between normal and pathologic tissues. One approach to achieve selectivity is to target overexpressed receptors on the membranes of tumor cells and to visualize the tumors by a noninvasive optical imaging method. Accordingly, we conjugated fluorescein and carbocyanine dyes to somatostatin and bombesin receptor-avid peptides and examined their receptor binding affinities. We also prepared potential dual imaging probes consisting of a bioactive peptide for tumor targeting, a biocompatible dye for optical imaging, and a radioactive or paramagnetic metal chelator for scintigraphic or magnetic resonance imaging of tumors. Using these approaches, the resulting carbocyanine derivatives of somatostatin and bombesin analogues retained high binding for their respective receptors. Further evaluation of representative molecules in rats bearing somatostatin- and bombesin-positive tumors showed selective uptake of the agents by the tumor cells. Unlike carbocyanine derivatives, the receptor binding of fluorescein-somatostatin peptide conjugates was highly sensitive to the type of linker and the site of fluorescein attachment on the nonreceptor binding region of the peptide. In general, the presence of flexible linkers disrupted binding affinity, possibly due to the interaction of the linker's thiourea group with the peptide's cyclic disulfide bond. While the receptor binding affinity of the dual probes was not dependent on the type of chelating group examined, it was affected by the relative positions of fluorescein and chelator on the lysine linker. For somatostatin compounds, best results were obtained when the chelator was on the alpha-amino lysine linker and fluorescein was on the epsilon-amino group. In contrast, conjugation of the chelator to epsilon- and fluorescein to the alpha-amino lysine linker of bombesin peptides resulted in high receptor binding. These findings indicate that despite their small size, conjugation of dyes to truncated somatostatin and bombesin peptide analogues results in promising diagnostic agents that retain high receptor binding activity in vitro. The results further show that these contrast agents can selectively and specifically localize in receptor-positive tumors in rat models.     

Multifunctional ligands in medicinal inorganic chemistry--current trends and future directions

This review will highlight recent advances in ligand design for innovative applications in medicinal inorganic chemistry. Ligands that effectively bind metal ions and also include specific features to enhance targeting, reporting, and overall efficacy are driving innovation in areas of disease diagnosis and therapy. Increasing the potency of therapeutic compounds, while limiting side-effects, is a common goal in medicinal chemistry. In an effort to achieve this goal, compounds are being developed that either target a disease site, or are activated by a disease specific biological process. Metal complexes containing targeting functions and/or bioactive ligands, as well as agents that are activated by specific enzymes, or changes in pH and pO2, provide new avenues for drug development. Radiodiagnostic compounds, magnetic resonance imaging agents, and optical probes containing transition metals offer versatility unavailable to organic imaging agents. In certain cases, dual modality agents have been developed, and will be highlighted. Finally, we will discuss targeted metal binding compounds for treatment of metal overload disorders, and the recent application to neurodegenerative disease.     

PAMAM dendrimer based macromolecules as improved contrast agents

Dendrimers are an attractive platform for macromolecular imaging due to the presence of multiple terminal groups on the exterior of the molecule. Through application of appropriate metal ion chelates, large numbers of metal ions for imaging (paramagnetic or radioopaque) and therapy (radioactive particle emitters) may be conjugated to the dendrimer in combination with a targeting vector, through classic organic synthetic techniques. Thus, a large amount of these metal ions potentially may be site specifically delivered directly into the body with the dendrimer as the vehicle with the targeting vector directing the modified dendrimer. The development of targeted macromolecular agents with acceptable blood retention times and selective organ uptake then has the potential for various biological applications. A review of comparative studies of dendrimers with various externally appended imaging and targeting agents is presented herein.     

Hydroxypyri(mi)dine-based chelators as antidotes of toxicity due to aluminum and actinides

This review is focused on recent developments on hydroxypyri(mi)dines, as aluminum and actinide chelating agents to combat the toxicity due to accumulations of these metal ions in human body resulting from excessive metal exposure. After a brief update revision of the most common processes of aluminum (Al) exposure, as well as the associated toxicities and pathologies, we will focus on the current available Al chelators and future perspective as potential antidotes of Al toxicity. Due to the similarity between Al and Fe, a major emphasis is given to the hydroxypyridinone and hydroxypyrimidinone chelators, since they are analogues of the current iron chelators in clinical use (DFP and DFO). This review includes issues such as molecular design strategies and corresponding effects on the associated physico-chemical properties, lipo-hydrophilic balance, toxicity, in vivo bioassays and current clinical applications. The hydroxypyri(mi)dine chelators are also suitable for other hard metal ions, such as the radiotoxic actinides, and so a brief review is included on the applications of these chelators in actinides scavenging.     

The interaction of cadmium and certain other metal ions with proteins and nucleic acids

The toxic effects of cadmium and other selected divalent cations are presumed to be related to specific chemical and physical characteristics of the ion. The chemistry of cadmium and metal ions in general is reviewed from the viewpoint of such relevant properties as ion polarizability, electronic structure, and the hard-soft characteristics. The softness of metal ions is seen as a useful single parameter to correlate with the affinity for nucleic acids and proteins and with toxic effects. The effects of cadmium on nucleic acids and proteins are examined for a number of specific cases to illustrate the variety of interactions that are well recognized and to demonstrate the utility of soft metal ions as reagents and probes for examining the relationship of structure and function in these macromolecules.     

Synthesis of novel bifunctional chelators and their use in preparing monoclonal antibody conjugates for tumor targeting

Bifunctional derivatives of the chelating agents ethylenediaminetetraacetic acid and diethylenetriaminepentaacetic acid, in which a p-isothiocyanatobenzyl moiety is attached at the methylene carbon atom of one carboxymethyl arm, was synthesized by reductive alkylation of the relevant polyamine with (p-nitrophenyl)pyruvic acid followed by carboxymethylation, reduction of the nitro group, and reaction with thiophosgene. The resulting isothiocyanate derivatives reacted with monoclonal antibody B72.3 to give antibody-chelator conjugates containing 3 mol of chelator per mole of immunoglobulin, without significant loss of immunological activity. Such conjugates, labeled with the radioisotopic metal indium-111, selectively bound a human colorectal carcinoma implanted in nude mice when given intravenously. Uptake into normal tissues was comparable to or lower than that reported for analogous conjugates with known bifunctional chelators. It is concluded that substitution with a protein reactive group at this position in polyaminopolycarboxylate chelators does not alter the chelating properties of these molecules to a sufficient extent to adversely affect biodistribution and thus provides a general method for the synthesis of such chelators.     

Nano-sized MRI contrast agents with dendrimer cores

Gadolinium-based MRI contrast agents (CAs) can be effective at a approximately 100-fold lower concentration of Gadolinium ions in comparison to the concentration of Iodine atoms required for CT imaging. Therefore, a number of dendrimer based macromolecular MRI CAs of various sizes and properties prepared employing relatively simple chemistry are readily available that can provide sufficient contrast enhancement for various applications. Molecules up to 20 nm in diameter behave differently in the body depending on their size. Even if these molecules possess similar chemical properties, small changes in size can greatly impact their pharmacokinetics. Changes in molecular size up to 15 nm in diameter altered permeability across the vascular wall, excretion route, and recognition by the reticuloendothelial system. Smaller sized polyamidoamine (PAMAM) dendrimer-based contrast agents, i.e., less than 3 nm in diameter, easily "leak" across the vascular wall resulting in rapid perfusion throughout the body. Contrast agents 3-6 nm in diameter were quickly excreted through the kidney indicating these agents to be potentially suitable as functional renal contrast agents. Contrast agents 7-12 nm in diameter were retained in circulation and were better suited for use as blood pool contrast agents. Hydrophobic variants of CAs formed with polypropylenimine diaminobutane (DAB) dendrimer cores quickly accumulated in the liver and potentially have use as liver contrast agents. Larger hydrophilic agents have suitable characteristics for lymphatic imaging. Finally, contrast agents conjugated with either monoclonal antibodies or with avidin are able to function as tumor-specific contrast agents and might also be employed as either gadolinium neutron capture therapy or in conjunction with radioimmunotherapy.     

Molecular factors affecting the complex formation between deferiprone (L1) and Cu(II). Possible implications on efficacy and toxicity.

Deferiprone (1,2-dimethyl-3-hydroxypyrid-4-one, L1, CAS 30652-11-0) is a new chelating drug used worldwide for the treatment of iron overloading conditions. Spectrophotometric and potentiometric measurements were carried out to investigate the interaction of L1 with Cu(II) ions under different conditions. The complexation of Cu(II) ions with L1 in aqueous solution leads predominantly to the formation of the Cu(L1)2 species at a pH range of 4-9. The experimental results indicate that L1 has high affinity for Cu(II) with stability constants log beta 11 = 10.3 +/- 0.9 and log beta 12 = 19.2 +/- 0.6. The effect of Cu(II) ions on the affinity of L1 for Fe(III) ions by competition reactions in vitro indicate displacement of Fe(III) in a concentration dependent manner by Cu(II). Similarly, the presence of different buffers at various pH values resulted in the formation of different stoichiometry L1 complexes with Cu(II) and of mixed complexes with buffer anions. The strong interaction of L1 with Cu(II) may have implications on the therapeutic and toxicological properties of this chelating drug. In particular, L1 may be used in the treatment of copper overloading conditions, such as Wilson's disease or other conditions where copper toxicity is implicated.     

Silicon switch: Carbon–silicon Bioisosteric replacement as a strategy to modulate the selectivity,physicochemical, and drug-like properties in anticancer pharmacophores

Bioisosterism is one of the leading strategies in medicinal chemistry for the design and modification of drugs, consisting in replacing an atom or a substituent with a different atom or a group with similar chemical properties and an inherent biocompatibility. The objective of such an exercise is to produce a diversity of molecules with similar behavior while enhancing the desire biological and pharmacological properties, without inducing significant changes to the chemical framework. In drug discovery and development, the optimization of the absorption, distribution, metabolism, elimination, and toxicity (ADMETox) profile is of paramount importance. Silicon appears to be the right choice as a carbon isostere because they possess very similar intrinsic properties. However, the replacement of a carbon by a silicon atom in pharmaceuticals has proven to result in improved efficacy and selectivity, while enhancing physicochemical properties and bioavailability. The current review discusses how silicon has been strategically introduced to modulate drug-like properties of anticancer agents, from a molecular design strategy, biological activity, computational modeling, and structure–activity relationships perspectives.     

Stimulative effects of chelating agents, 2,2'-bipyridine and 1,10-phenanthroline, on lipid peroxidation in rat liver microsomes.

Well known lipid peroxidation inhibitors, 1,10-phenanthroline and 2,2'-bipyridine, stimulated microsomal NADPH- and ascorbic acid-dependent lipid peroxidation when low concentrations of these chelating agents were added to incubation mixture. The stimulatory effects of the chelating agents on lipid peroxidation were enhanced when ferrous ion was added together with the chelating agents to the mixture at a molar ratio of 1:1. Ethylenediaminetetraacetic acid (EDTA) had no stimulatory effect on lipid peroxidation. Ferrous ion-EDTA complex increased lipid peroxidation by only 20-30%, which was lower than that obtained by addition of the same concentration of ferrous ion alone. On the other hand, manganese and calcium ions, which are also inhibitors of lipid peroxidation, had no ability to stimulate lipid peroxidation even in the presence of extra ferrous ions. Changes in the lipid peroxidation by chelating agents affected the apparent activity of ethylmorphine N-demethylation.     

Chromium genotoxicity as influenced by complexation and rate effects

Conclusions as to the mutagenicity and carcinogenicity of metal salts can be ambiguous and misleading, especially for metal ions having a high charge/radius ratio, hence a strong tendency to hydrolyze. Using the rec-assay, we determined whether the mutagenicity of chromium salts was reduced by complexation, as in the case of Cr(VI), or induced in the case of Cr(III). We find that several chelants, in proportion to concentration, reduce or eliminate the mutagenicity of Cr2O32-. These include EDTA, salicylate (SA), and Tiron (disodium 1,2-dihydroxylbenzene-3,5-disulfonate). Cr(III) was rendered slightly mutagenic by salicylate and citrate. None of the chelating agents or their combinations were mutagenic.     

Radical-scavenging and iron-chelating properties of carvedilol, an antihypertensive drug with antioxidative activity

Carvedilol, an antihypertensive agent, has been in clinical use for several years. In addition to its function as a beta-blocker, carvedilol has been shown to act as an antioxidant. However, there is some controversy as to how carvedilol achieves its antioxidative ability: by radical scavenging or ion chelation? We therefore used a method of radical generation independent of metal ions to investigate the antioxidative properties of carvedilol. We showed that carvedilol decreased low-density lipoprotein (LDL) oxidation induced by a peroxyl radical-generating system [2,2'-azobis(2-amidinopropane)hydrochloride]. Formation of thiobarbituric acid-reactive substances, lipid hydroperoxides, and newly generated epitopes on oxidised LDL was used to monitor LDL oxidation. We further showed that carvedilol was consumed during reaction with peroxyl radicals. However, carvedilol showed no reaction with nitrogen-centered radicals (1,1-diphenyl-2-picrylhydrazyl and 2,2'-azino-di-[3-ethylbenzthiazoline sulphonate]), which are often used in assays for determining antioxidative properties. On the other hand, we found that carvedilol acted as a chelator of ferric ions. Using mass spectrometry and NMR spectroscopy, we observed complex formation with free and acetylacetonate-complexed ferric ions. The binding constant with Fe(3+) was in the range of 10(5) L/mol. From our data, we concluded that carvedilol acts as both a metal chelator and a radical scavenger in vitro. However, it is selective in reacting with different radicals and is not an electron-donating radical scavenger as is alpha-tocopherol. Therefore, taking into account the low physiological concentration, the antioxidative properties reported earlier may not solely be explained by its radical-scavenging activity.