Comparing Strategies in the Design of Responsive Contrast Agents for Magnetic Resonance Imaging: A Case Study with Copper and Zinc.

Magnetic resonance imaging (MRI) has emerged over the years as one of the preferred modalities for medical diagnostic and biomedical research. It has the advantage over other imaging modalities such as positron emission tomography and X-ray of affording high resolution three-dimensional images of the body without using harmful radiation. The use of contrast agents has further expanded this technique by increasing the contrast between regions where they accumulate and background tissues. As MRI most often measures the relaxation rate of water throughout the body, contrast agents function by modulating the intensity of the water signal either via improved relaxation or via saturation transfer to selected exchangeable proton. Among the growing class of MRI contrast agents, a subset of them called "smart" contrast agents function as responsive probes. Their ability to increase or decrease their signal intensity is modulated by the presence of an analyte. These probes offer the unique ability to image the distribution of an analyte in vivo, thereby opening new possibilities for diagnostics and for elucidating the role of specific analytes in various pathologies or biological processes. A number of different strategies can be exploited to design responsive MRI contrast agents. The majority of contrast agents are based on GdIII complexes. These complexes can be rendered responsive in either of two ways: either by modulating the number of inner-sphere water molecules, q, or via modulating the rotational correlation time, τR, of the contrast agent upon substrate binding. The longitudinal relaxivity increases with the number of inner-sphere water molecules. GdIII complexes can be rendered responsive if they contain a recognition moiety that can bind to both the open coordination site of GdIII and to the analyte. When the recognition moiety leaves the lanthanide ion to bind to the analyte, q increases and therefore so does the relaxivity. The dependence of relaxivity on rotational correlation time is more complex and more pronounced at lower magnetic fields. In general, slower tumbling macromolecules have longer rotational correlation times and higher relaxivities. Analyte-triggered formation of macromolecules thus also increases relaxivity. Such macromolecules can either be analyte-templated supramolecular assemblies, or analyte-enhanced protein-contrast agent complexes. Chemical Exchange Saturation Transfer (CEST) agents are a newer class of contrast agents that offer the possibility of multifrequency and thus ratiometric imaging, which in turn enables quantitative mapping of the concentration of an analyte in vivo under conditions where the concentration of the contrast agent is not known. Such agents can be rendered responsive if the analyte changes the number of exchangeable proton(s), its exchange rate, or its chemical shift. All of these approaches have been successfully employed for detecting and imaging both copper and zinc, including in vivo. Magnetic Iron Oxide Nanoparticles (MIONs) are powerful MRI transverse relaxation agents. They can also be rendered responsive to an analyte if the latter can control the aggregation of the nanoparticles. For metal ions, this can be achieved via chemical functionalities that only react to form conjugates in the presence of the metal ion analyte.

Gadolinium Complex for the Catch and Release of Phosphate from Water.

The ability of complexes of hard and labile metal ions with one or more open coordination sites to capture phosphates with high affinity and selectivity directly in water at neutral pH and release them under acidic conditions is evaluated with Gadolinium- 2,2',2''-(((nitrilotris(ethane-2,1-diyl))tris(azanediyl))tris(carbonyl))tris(4-oxo-4H-pyran-3-olate) (Gd-TREN-MAM). This model lanthanide complex has two open coordination sites that, at neutral pH, are filled with water molecules. In water at neutral pH, Gd-TREN-MAM binds phosphate with high affinity (Ka = 1.3 × 104) via the formation of a ternary complex in which one phosphate replaces both inner-sphere water molecules. The formation of this complex is highly pH-dependent; the phosphate is completely released from Gd-TREN-MAM below pH 2. Because the GdIII ion remains complexed by its ligand, even under strong acidic conditions, Gd-TREN-MAM can be used at least 10 times in a pH-based recycling scheme that enables the catch and release of one phosphate per cycle. Gd-TREN-MAM is highly selective for phosphate over other anions of environmental concerns, including HCO3-, HCO2-, CH3CO2-, SO42-, NO3-, NO2-, BrO3-, AsO4-, F-, Cl-, and Br- and, to a lesser extent, ClO3-. The development of such receptors that bind phosphate reversibly in a pH-dependent manner opens the possibility to design catch-and-release systems for the purification of surface waters.

Synthesis and chemical reactivity of a 6-Me-3,2-hydroxypyridinone dithiazolide with primary amines: a route to new hexadentate chelators for hard metal(III) ions.

A hydroxypyridinone building block, bifunctionalized with thiazoline, has been prepared from orthogonally protected 2-(3-(benzyloxy)-4-(ethoxycarbonyl)-6-methyl-2-oxopyridin-1(2H)-yl) acetic acid. The reactivity of the dithiazolide has been explored with two primary amines, leading to the synthesis and characterization of four new hexadentate ligands. Their complexes with selected hard trivalent ions pertinent to potential molecular imaging applications have been surveyed.

Synthesis and reactivity of (benzoxazol-2-ylmethyl)phosphonic acid.

An efficient three step synthesis of (benzoxazol-2-ylmethyl)phosphonic acid (6-H(2)) is described along with IR, mass spectrometry (MS), and (1)H, (13)C, and (31)P NMR spectroscopic characterization data, and a single crystal X-ray diffraction structure determination. 6-H(2) is unstable in acidic aqueous solutions (pH < 4) undergoing ring-opening to give [(2-hydroxyphenylcarbamoyl)methyl] phosphonic acid (7-H(2)) that is characterized by IR, MS, and NMR methods. The protonation constants (pK(a)) for 7-H(2) have been measured, and crystal structure determinations for (NH(4))(7-H) and K(7-H)·DMF are described. Reactions of NaOH and KOH with 6-H(2) in MeOH/H(2)O solutions led to isolation and crystal structure determinations of the salts [Na(6-H)·H(2)O](2), K(6-H), Na(3)(6)(6-H)·H(2)O, and [K(2)(6)](2)·3H(2)O. The complexation reactions of 7-H(2) with La(III), Nd(III), and Gd(III), as a function of pH, were also examined by titrametric methods, and a model for the 1:1 anion binding with Ln(III) cations is proposed.

Synthesis and coordination properties of trifluoromethyl decorated derivatives of 2,6-bis[(diphenylphosphinoyl)methyl]pyridine N-oxide ligands with lanthanide ions.

Phosphinoyl Grignard-based substitutions on 2,6-bis(chloromethyl)pyridine followed by N-oxidation of the intermediate 2,6-bis(phosphinoyl)methylpyridine compounds with mCPBA give the target trifunctional ligands 2,6-bis[bis(2-trifluoromethylphenyl)phosphinoylmethyl]pyridine 1-oxide (2a) and 2,6-bis[bis(3,5-bis(trifluoromethyl)phenyl)phosphinoylmethyl]pyridine 1-oxide (2b) in high yields. The ligands have been spectroscopically characterized, the molecular structures confirmed by single crystal X-ray diffraction methods, and the coordination chemistry surveyed with lanthanide nitrates. Single crystal X-ray diffraction analyses are described for the coordination complexes Nd(2a)(NO(3))(3), Nd(2a)(NO(3))(3) x (CH(3)CN)(0.5), Eu(2a)(NO(3))(3), and Nd(2b)(NO(3))(3) x (H(2)O)(1.25); in each case the ligand binds in a tridentate mode to the Ln(III) cation. These structures are compared with the structures found for lanthanide coordination complexes of the parent NOPOPO ligand, [Ph(2)P(O)CH(2)](2)C(5)H(3)NO.

Electrodeposited Fe and Fe-Au nanowires as MRI contrast agents.

Electrodeposited Fe and Fe-Au nanowires were studied for potential as MRI contrast agents, especially for T2-weighted imaging. Transverse relaxivities up to r2 = 77.1 mM Fe-1 s-1 (at 1.5 T) were achieved when Fe-Au nanowires were coated with thiol and carboxylic acid functionalized poly(ethyleneglycol). T2-Weighted images (9 T) verified successful contrast.

Evaluation of a 3-hydroxypyridin-2-one (2,3-HOPO) Based Macrocyclic Chelator for (89)Zr(4+) and Its Use for ImmunoPET Imaging of HER2 Positive Model of Ovarian Carcinoma in Mice.

A novel octadentate 3-hydroxypyridin-2-one (2,3-HOPO) based di-macrocyclic ligand was evaluated for chelation of (89)Zr; subsequently, it was used as a bi-functional chelator for preparation of (89)Zr-labeled antibodies. Quantitative chelation of (89)Zr(4+) with the octadentate ligand forming (89)ZrL complex was achieved under mild conditions within 15 minutes. The (89)Zr-complex was stable in vitro in presence of DTPA, but a slow degradation was observed in serum. In vivo, the hydrophilic (89)Zr-complex showed prevalently renal excretion; and an elevated bone uptake of radioactivity suggested a partial release of (89)Zr(4+) from the complex. The 2,3-HOPO based ligand was conjugated to the monoclonal antibodies, HER2-specific trastuzumab and an isotypic anti-gD antibody, using a p-phenylene bis-isothiocyanate linker to yield products with an average loading of less than 2 chelates per antibody. Conjugated antibodies were labeled with (89)Zr under mild conditions providing the PET tracers in 60-69% yield. Despite the limited stability in mouse serum; the PET tracers performed very well in vivo. The PET imaging in mouse model of HER2 positive ovarian carcinoma showed tumor uptake of (89)Zr-trastuzumab (29.2 ± 12.9 %ID/g) indistinguishable (p = 0.488) from the uptake of positive control (89)Zr-DFO-trastuzumab (26.1 ± 3.3 %ID/g). In conclusion, the newly developed 3-hydroxypyridin-2-one based di-macrocyclic chelator provides a viable alternative to DFO-based heterobifunctional ligands for preparation of (89)Zr-labeled monoclonal antibodies for immunoPET studies.

Di-macrocyclic terephthalamide ligands as chelators for the PET radionuclide zirconium-89.

The development of bifunctional chelators (BFCs) which can stably chelate zirconium-89 ((89)Zr) while being conjugated to targeting molecules is an area of active research. Herein we report the first octadentate terephthalamide ligands, which are easily radiolabeled with (89)Zr and are highly stable in vitro. They represent a novel class of chelators, which are worthy of further development as BFCs for (89)Zr.

Synthesis and lanthanide coordination chemistry of trifluoromethyl derivatives of phosphinoylmethyl pyridine N-oxides.

A synthetic route for the formation of 2-[bis(2-trifluoromethylphenyl)phosphinoylmethyl]pyridine N-oxide (1c) and 2-[bis(3,5-trifluoromethylphenyl)phosphinoylmethyl]pyridine N-oxide (1d) was developed and the new ligands characterized by spectroscopic methods and single-crystal X-ray diffraction analyses. The coordination chemistry of 1c was examined with Yb(NO3)3 and the molecular structure of one complex, [Yb(1c)(NO3)3(DMF)].DMF.0.5H2O, was determined by single-crystal X-ray diffraction methods. The ligand is found to coordinate in a bidentate fashion, and this is compared against lanthanide coordination chemistry observed for the related ligand, [Ph2P(O)CH2] C5H4NO.

Oxidation reactivity channels for 2-(pyridin-2-yl)-N,N-diphenylacetamides.

Synthetic routes to 2-(pyridin-2-yl)-N,N-diphenylacetamide and 2-(6-methylpyridin-2-yl)-N,N-diphenyl-acetamide are described along with results from the chemical oxidation of these compounds with peracetic acid, m-chloroperbenzoic acid, and OXONE. In each case, oxidations generate four products in varying amounts depending on the oxidant and reaction conditions. Each product has been characterized by spectroscopic methods and the molecular structures of several of the new compounds have been confirmed by X-ray crystallography.

Formation of a layered framework structure based upon 4-methyl-2,6-bis(methylphosphonic acid) phenol.

The trifunctional ligands, [(HO)2P(O)CH2]2C6H2(R)OH, (5-H4)(R = CH3, Br) were prepared in good yield via an Arbusov reaction between P(OEt)3 and the respective 4-R-2,6-bis(chloromethyl)phenols followed by acidic aqueous hydrolysis and they were spectroscopically characterized by IR and NMR techniques. The ligand 5-H4-CH3 readily dissolves lanthanide hydroxide residues and it forms a crystalline complex from aqueous LaCl3 solutions. This complex was characterized by single crystal X-ray diffraction methods and found to adopt a complex 2-D lamellar network in the bc plane. The La(III) inner coordination sphere is seven coordinate formed by oxygen atoms from two water molecules and five phosphonate oxygen atoms from three different ligands. The phenolic oxygen atom is not involved in the ligand binding to La(III).

Synthesis and lanthanide coordination properties of new 2,6-bis(N-tert-butylacetamide)pyridine and 2,6-bis(N-tert-butylacetamide)pyridine-N-oxide ligands.

The compound 2,6-bis(N-tert-butylacetamide)pyridine (2) was obtained via a Ritter synthesis, and oxidation with oxone provided the title pyridine-N-oxide (3). The compounds were characterized by spectroscopic methods, and the molecular structure of the N-oxide was determined by single-crystal X-ray diffraction methods. The coordination chemistry with Eu(NO3)3 was examined by using 1:1 and 2:1 ligand/Eu ratios, and a single-crystal X-ray analysis for Eu(3)(NO3)3(H2O) was completed. The ligand 3 is found to chelate in a tridentate fashion on the Eu(III).

A three-dimensional framework structure constructed from 2-(2-pyridyl-N-oxide) ethylphosphonic acid and Nd(III).

A multistep synthesis for 2-(2-pyridyl-N-oxide) ethylphosphonic acid 6-H2 is described along with its spectroscopic (IR, NMR) data and a single-crystal X-ray diffraction structure analysis. Combination of the ligand with Nd(OH)3 results in the formation of a complex Nd(6-H)3. Single-crystal X-ray diffraction analysis reveals a three-dimensional crystal network generated by hydrogen-bonded chains along the crystallographic c axis. The hydrogen bonds are formed between phosphonic acid anion (6-H)(-1) protons on one chain and pyridyl N-oxide oxygen atoms in neighboring chains. The asymmetric unit contains 1/3[Nd(6-H)3] and there are two unique Nd(III) atoms, each with point symmetry. As a result, each Nd(III) ion is bound to six (6-H)(-1) ligands and the symmetry about the Nd(III) ion is octahedral with each vertex occupied by a phosphonate oxygen atom. The Nd-O bond lengths are essentially identical: Nd(1)-O(3), 2.336 (1) A; Nd(2)-O(4), 2.340 (1) A. The monoanionic ligand (6-H)(-1), therefore, serves to bridge the unique Nd(III) centers.