Synthesis of homochiral tris(2-alkyl-2-aminoethyl)amine derivatives from chiral alpha-amino aldehydes and their application in the synthesis of water soluble chelators

Preorganized Homochiral Pyrrole-Based Receptors That Display Enantioselective Anion Binding

Herein, a new scaffold for anion recognition based on a tripodal tris(pyrrolamide) motif is presented. The receptors were able to bind to a variety of anions with high affinity. Using density functional theory methods, the three-dimensional geometry of the receptor-anion complex was calculated. These calculations show that the receptors bind anions via a preorganized cavity of amide and pyrrole hydrogen bond donor groups. Based on these findings, homochiral tris(pyrrolamide) receptors were prepared, which produced as much as a 1.6-fold greater affinity for (S)-(+)-mandelate over (R)-(-)-mandelate, demonstrating the ability to differentiate between these enantiomeric anions. The interaction of (S)-(+)-mandelate and (R)-(-)-mandelate within the homochiral receptor was examined by solution NMR spectroscopy and density functional theory calculations. These findings indicate that the preorganized positioning of the pyrrole groups and subsequent sterics allows to differentiate between the stereoisomeric anions.

A novel multi-target strategy to attenuate the progression of Parkinson's disease by diamine hybrid AGE/ALE inhibitor

Instead of a conventional 'one-drug-one-target approach', this article presents a novel multi-target approach with a concept of trapping simultaneously as many detrimental factors as possible involved in the progression of Parkinson's disease. These factors include reactive carbonyl species, reactive oxygen species, Fe^3+/Cu²⁺ and ortho-quinones (o-quinone), in particular. Different from the known multi-target strategies for Parkinson's disease, it is a sort of 'vacuum cleaning' strategy. The new agent consists of reactive carbonyl species scavenging moiety and reactive oxygen species scavenging and metal chelating moiety linked by a spacer. Provided that the capacity of scavenging o-quinones is demonstrated, this type of agent can further broaden its potential therapeutic profile. In order to support this new hypothetical approach, a number of simple in vitro experiments are proposed.

Effect of a mesitylene-based ligand cap on the relaxometric properties of Gd(III) hydroxypyridonate MRI contrast agents

A series of new Gd(III) hydroxypyridonate complexes featuring a mesitylene (ME)-derived ligand cap has been prepared. Relaxometric characterization reveals that the complexes tend to form large aggregates in solution with slow tumbling rates, as estimated from NMRD analysis, and unique pH-dependent relaxivities. The solution behavior and relaxometric properties are compared with those observed for analogous TREN-capped compounds, and the potential for use of these new ME-capped complexes as pH-responsive MRI contrast agents is explored.

Gd-hydroxypyridinone (HOPO)-based high-relaxivity magnetic resonance imaging (MRI) contrast agents

Magnetic resonance imaging (MRI) is a particularly effective tool in medicine because of its high depth penetration (from 1 mm to 1 m) and ability to resolve different soft tissues. The MRI signal is generated by the relaxation of in vivo water molecule protons. MRI images can be improved by administering paramagnetic agents, which increase the relaxation rates of nearby water protons, thereby enhancing the MRI signal. The lanthanide cation Gd(3+) is generally used because of its favorable electronic properties; high toxicity, however, necessitates strongly coordinating ligands to keep Gd(3+) completely bound while in the patient. In this Account, we give a coordination chemistry overview of contrast agents (CAs) based on Gd-hydroxypyridinone (HOPO), which show improved MRI contrast and high thermodynamic stabilities. Tris-bidentate HOPO-based ligands developed in our laboratory were designed to complement the coordination preferences of Gd(3+), especially its oxophilicity. The HOPO ligands provide a hexadentate coordination environment for Gd(3+), in which all of the donor atoms are oxygen. Because Gd(3+) favors eight or nine coordination, this design provides two to three open sites for inner-sphere water molecules. These water molecules rapidly exchange with bulk solution, hence affecting the relaxation rates of bulk water molecules. The parameters affecting the efficiency of these contrast agents have been tuned to improve contrast while still maintaining a high thermodynamic stability for Gd(3+) binding. The Gd-HOPO-based contrast agents surpass current commercially available agents because of a higher number of inner-sphere water molecules, rapid exchange of inner-sphere water molecules via an associative mechanism, and a long electronic relaxation time. The contrast enhancement provided by these agents is at least twice that of commercial contrast agents, which are based on polyaminocarboxylate ligands. Advances in MRI technology have made significant contributions to the improvement of clinical diagnostics by allowing visualization of underlying pathology. However, understanding the mechanism of a disease at the molecular level requires improved imaging sensitivity. The ultimate goal is to visually distinguish between different disease targets or markers, such as enzymes, hormones, proteins, or small molecules, at biologically relevant concentrations (from micro- to nanomolar). Although MRI techniques can provide images of the organs and tissues in which these biomarkers are regulated, the high sensitivity required to visualize the biological targets within the tissues is currently lacking; contrast enhancements of 50-fold beyond current agents are required to achieve this goal. According to the theory of paramagnetic relaxation, the contrast enhancement can be further improved by slowing the tumbling rate of the MRI agent. Theoretically, this enhancement would be greater for contrast agents with an optimal rate of water exchange. The Gd-HOPO-based contrast agents have optimal water-exchange rates, whereas the commercial agents have slower non-optimal water-exchange rates; thus, the Gd-HOPO agents are ideal for attachment to macromolecules, which will slow down the tumbling rate and increase contrast. This strategy has been recently tested with the Gd-HOPO agents via covalent attachment to virus capsids, affording contrast enhancements 10-fold beyond commercial agents.

1,2-hydroxypyridonate/terephthalamide complexes of gadolinium(III): synthesis, stability, relaxivity, and water exchange properties

Four new Gd(III) complexes based on the 1,2-hydroxypyridinone chelator have been synthesized and evaluated as potential magentic resonance imaging contrast agents. Previously reported work examining Gd-TREN-1,2-HOPO (3; HOPO = hydroxypyridinone) suggests that the 1,2-HOPO unit binds strongly and selectively to Gd(III), encouraging further study of the stability and relaxivity properties of this class of compounds. Among the new complexes presented in this paper are the homopodal Gd-Ser-TREN-1,2-HOPO (Gd-5) and three heteropodal bis-1,2-HOPO-TAM complexes (Gd-6, Gd-7, and Gd-8; TAM = terephthalamide). Conditional stability constants were determined, and all pGd values are in the range of 18.5-19.7, comparable to other analogous HOPO complexes and currently used commercial contrast agents. Relaxivities for all complexes are about twice those of commercial agents, ranging from 7.8 to 10.5 mM(-1) s(-1) (20 MHz; 25 degrees C), and suggest two innersphere water molecules in fast exchange. Luminescent measurements were used to verify the number of coordinated waters for Gd-5, and VT (17)O NMR experiments were employed for the highly soluble Gd-TREN-bis-1,2-HOPO-TAM-N3 (Gd-8) complex to measure a fast water exchange rate, (298)k(ex) = 1/tau(M), of 5.1 (+/-0.4) x 10(8) s(-1) ((298)tau(M) approximately 2 ns).

High-relaxivity MRI contrast agents: where coordination chemistry meets medical imaging

The desire to improve and expand the scope of clinical magnetic resonance imaging (MRI) has prompted the search for contrast agents of higher efficiency. The development of better agents requires consideration of the fundamental coordination chemistry of the gadolinium(III) ion and the parameters that affect its efficacy as a proton relaxation agent. In optimizing each parameter, other practical issues, such as solubility and in vivo toxicity, must also be addressed, making the attainment of safe, high-relaxivity agents a challenging goal. This Minireview presents recent advances in the field, with an emphasis on gadolinium(III) hydroxypyridinone chelate complexes.

Tris(pyrone) chelates of Gd(III) as high solubility MRI-CA

Two tripodal, hexadentate pyrone-based chelators have been prepared. These ligands form stable, soluble complexes with gadolinium(III). The complexes show aqueous stability comparable to that of [Gd(DTPA)]2- at pH 7.4. Evaluation by relaxometry shows that these complexes have two inner-sphere water molecules and a very fast water exchange rate. The solution behavior of these complexes suggests that they may be attractive as high relaxivity, next-generation magnetic resonance imaging contrast agents.

The effect of ligand scaffold size on the stability of tripodal hydroxypyridonate gadolinium complexes

The variation of the size of the capping scaffold which connects the hydroxypyridonate (HOPO) binding units in a series of tripodal chelators for gadolinium (Gd) complexes has been investigated. A new analogue of TREN-1-Me-3,2-HOPO (1) (TREN = tri(ethylamine)amine) was synthesized: TREN-Gly-1-Me-3,2-HOPO (2) features a glycine spacer between the TREN cap and HOPO binding unit. TRPN-1-Me-3,2-HOPO (3) has a propylene-bridged cap, as compared to the ethylene bridges within the TREN cap of the parent complex. Thermodynamic equilibrium constants for the acid-base properties of 2 and the Gd(3+) complexation strength of 2 and 3 were measured and are compared with that of the parent ligand. The most basic ligand is 2 while 3 is the most acidic. Both 2 and 3 form Gd(3+) complexes of similar stability (pGd = 16.7 and 15.6, respectively) and are less stable than the parent complex Gd-1 (pGd = 19.2). Two of the three complexes are more stable than the bis(methylamide)diethylenetriamine pentaacetate complex Gd(DTPA-BMA) (pGd = 15.7) while the other is of comparable stability. Enlargement of the ligand scaffold decreases the stability of the Gd(3+) complexes and indicates that the TREN scaffold is superior to the TRPN and TREN-Gly scaffolds. The proton relaxivity of Gd-2 is 6.6 mM(-)(1) s(-)(1) (20 MHz, 25 degrees C, pH 7.3), somewhat lower than the parent Gd-1 but higher than that of the MRI contrast agents in clinical practice. The pH-independent relaxivity of Gd-2 is uncharacteristic of this family of complexes and is discussed.

6-carboxamido-5,4-hydroxypyrimidinones: a new class of heterocyclic ligands and their evaluation as gadolinium chelating agents

A previously unexplored class of heterocyclic bidentate chelating groups, 6-carboxamido-5,4-hydroxypyrimidinones (6-substituted-HOPYs), have been synthesized by two routes that provide a flexible entry into this ligand system. These are related to, but distinct from, the hydroxypyridonates and have been characterized in this study as a gadolinium chelating agent for magnetic resonance imaging (MRI) applications. The complex Gd[TrenHOPY] demonstrates high stability and high selectivity relative to other ions of biological interest, such as Zn(II) and Ca(II). These stability constants are comparable to those demonstrated by the previously studied 3,2-pyridinone analogues, however, the 5,4-pyrimidinones are at least an order of magnitude more soluble in water. The proton relaxation properties of Gd[TrenHOPY] in water were measured as a function of magnetic field, pH, and temperature. These results support the description of Gd[TrenHOPY] as a complex with two coordinated water molecules in fast exchange with bulk water. In addition, the influence of exogenous anions and blood serum proteins has been investigated. The favorable contrast agent properties emerging from these studies are discussed.