MRI Contrast Agents in Glycobiology.

Molecular recognition involving glycoprotein-mediated interactions is ubiquitous in both normal and pathological natural processes. Therefore, visualization of these interactions and the extent of expression of the sugars is a challenge in medical diagnosis, monitoring of therapy, and drug design. Here, we review the literature on the development and validation of probes for magnetic resonance imaging using carbohydrates either as targeting vectors or as a target. Lectins are important targeting vectors for carbohydrate end groups, whereas selectins, the asialoglycoprotein receptor, sialic acid end groups, hyaluronic acid, and glycated serum and hemoglobin are interesting carbohydrate targets.

Nanozeolite-LTL with Gd(III) deposited in the large and Eu(III) in the small cavities as a magnetic resonance optical imaging probe.

The immense structural diversity of more than 200 known zeolites is the basis for the wide variety of applications of these fascinating materials ranging from catalysis and molecular filtration to agricultural uses. Despite this versatility, the potential of zeolites in medical imaging has not yet been much exploited. In this work a novel strategy is presented to selectively deposit different ions into distinct framework locations of zeolite-LTL (Linde type L) and it is demonstrated that the carefully ion-exchanged Gd/Eu-containing nanocrystals acquire exceptional magnetic properties in combination with enhanced luminescence. This smart exploitation of the framework structure yields the highest relaxivity density (13.7 s(-1) L g(-1) at 60 MHz and 25 °C) reported so far for alumosilicates, rendering these materials promising candidates for the design of dual magnetic resonance/optical imaging probes, as demonstrated in preliminary phantom studies.

Microwave-assisted seeded growth of lanthanide-based nanoparticles for imaging and therapy.

Size control: Particles designed for imaging and therapy need to be size tunable to ensure their optimal performance. A highly reproducible procedure for the preparation of uniform, spherical, lanthanide-based nanoparticles (NPs) was developed. The size of the particles can be predefined to an accuracy of up to a few nanometers by microwave-generated temperature control and the choice of aging time (see figure).

17O NMR and density functional theory study of the dynamics of the carboxylate groups in DOTA complexes of lanthanides in aqueous solution.

The rotation of the carboxylate groups in DOTA (DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate) complexes of several lanthanide ions and Sc(3+) was investigated with density functional theory (DFT) calculations and with variable temperature (17)O NMR studies at 4.7-18.8 T. The data obtained show that the rotation is much slower than the other dynamic processes taking place in these complexes. The exchange between the bound and unbound carboxylate oxygen atoms for the largest Ln(3+) ions (La(3+)→Sm(3+)) follows a pathway via a transition state in which both oxygens of the carboxylate group are bound to the Ln(3+) ion, whereas for the smaller metal ions (Tm(3+), Lu(3+), Sc(3+)) the transition state has a fully decoordinated carboxylate group. The activation free energies show a steady increase from about 75 to 125-135 kJ·mol(-1) going from La(3+) to Lu(3+). This computed trend is consistent with the results of the (17)O NMR measurements. Fast exchange between bound and unbound carboxylate oxygen atoms was observed for the diamagnetic La-DOTA, whereas for Pr-, Sm-, Lu-, and Sc-DOTA the exchange was slow on the NMR time scale. The trends in the linewidths for the various metal ions as a function of the temperature agree with trends in the rates as predicted by the DFT calculations.

Development of a liposomal delivery system for temperature-triggered release of a tumor targeting agent, Ln(III)-DOTA-phenylboronate.

Liposomes, capable of temperature-triggered content release at the site of interest, can be of great importance for imaging and therapy of tumors. The delivery of imaging agents or therapeutics can be improved by application of liposomes with a gel-to-liquid phase-transition temperature suitable for mild hyperthermia (41-43°C), and by prolonging their circulation time by incorporation of lipids containing polyethyleneglycol moieties. Still, the rapid wash out of the delivered material from the tumor tissue is a major obstacle for both imaging and therapy. In this study, we developed an optimized temperature sensitive liposomal system to be used with mild hyperthermia: highly stable at physiological temperature and with a sharp transition of the bilayer at 41.5°C, with subsequent rapid release of entrapped compounds such as calcein or tumor cell-targeting contrast agents. Intravital microscopy on calcein/rhodamine containing liposomes was applied to demonstrate the applicability of this system in vivo. The calcein loaded liposomes were injected iv into nude mice with a human BLM melanoma tumor implanted in a dorsal skin-fold window chamber. Arrival of the liposomes at the tumor site and content release after temperature increase were monitored. The results demonstrated not only accumulation of the liposomes at the tumor site, but also a massive release of calcein after increase of the temperature to 41°C. The versatility of the thermosensitive liposomes was further demonstrated by encapsulation of a tumor cell-targeting DOTA-phenylboronate conjugate and its release at elevated temperatures. The DOTA ligand in this system is able to chelate a variety of metals suitable for both diagnostic and therapeutic applications, whereas the phenylboronate function is able to target specifically to tumor cells through a covalent binding with sialic acid moieties over-expressed on their surface upon heat-triggered release from the liposomal carrier.

Evaluation of [Ln(H2cmp)(H2O)] metal organic framework materials for potential application as magnetic resonance imaging contrast agents.

Aqueous suspensions of metal organic frameworks (MOF) containing different Ln(3+) ions, consisting of a series of layered Ln(3+) networks formulated as [Ln(H(2)cmp)(H(2)O)] (where H(5)cmp is (carboxymethyl)iminodi(methylphosphonic acid), with a relatively wide size distribution (400 nm to 1 microm) were studied by relaxometry. The water (1)H longitudinal (r(1)) and transverse (r(2)) relaxivities were obtained for aqueous suspensions of these materials with different lanthanide ions. The values of r(1) are very small and varied only slightly with the effective magnetic moment (mu(eff)) of the lanthanide ions, while r(2) values are larger and proportional to the value of mu(eff)(2). The dependence of R(2) on tau(CP) (the time interval between two consecutive refocusing pulses in the train of 180 degrees pulses applied in a CPMG pulse sequence) was evaluated. The value of R(2) initially increases with tau(CP) and then saturates at higher tau(CP) at a value that is about 3 to 5 times lower than R(2p)*. This can be explained by the static dephasing regime (SDR) theory, in which the diffusion effect is taken into account and where the condition tau(D) > Delta omega(r(p))(-1) holds (tau(D) = r(p)(2)/D, where D is the diffusion coefficient, r(p) is the radius of the particle, and Delta omega(r(p)) is the Larmor frequency shift at the particle's surface). Separation of the particles into two fractions with different particle sizes led to a significant enhancement of the r(2) relaxivity of the smaller particles with a narrow size distribution. Magnetometric measurements performed with the particles containing Dy(III), Ho(III), and Gd(III) showed a typical paramagnetic behavior from 4 to 100 K, used to determine the Curie constants.

Relaxivity of manganese ferrite nanoparticles.

Manganese ferrite nanoparticles are superparamagnetic and have very high saturation magnetization, which makes them candidates for application as MRI contrast agents. Because these nanoparticles are very effective enhancers of transverse relaxation, they are particularly suitable as negative (T2-weighted) contrast agents. The magnitude of the relaxivity of nanoparticulate Mn ferrites seems to be determined mainly by the method of preparation, their dimensions, and their saturation magnetization.

PAMAM dendrimers conjugated with an uncharged gadolinium(III) chelate with a fast water exchange: the influence of chelate charge on rotational dynamics.

A bifunctional ligand, DO3A-py(NO-C) (DO3A-py(NO-C) = 10-[(4-carboxy-1-oxidopyridin-2-yl)methyl]-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid), was attached to different generations (G0, G1, G2, and G4) of ethylenediamine-cored PAMAM dendrimers (PAMAM = polyamidoamine). The gadolinium(III) complex of this ligand possesses one molecule of water in its first coordination sphere and has a unique combination of a short water residence lifetime (tau(M) = 34 ns), a neutral overall charge, and a possibility for rigid attachment to molecules bearing primary amino groups. These favorable properties predestine the ligand for constructions of highly efficient nanosized contrast agents for magnetic resonance imaging (MRI). The coupling reaction between the carboxylic group on the pyridine-N-oxide moiety of the protected ligand molecule and primary amines in the dendrimers was achieved by an active ester method under nonaqueous conditions using the coupling agent TBTU. This reaction afforded conjugates with high loadings (80-100% of the theoretically available primary amines) and of high purity. The gadolinium(III) complexes of the conjugates were studied by variable-temperature 17O NMR and 1H NMRD measurements. The water residence lifetime (tau(M) = 55 ns) found in the largest conjugate G4-[Gd(H2O)(do3a-py(NO-C))]57, though somewhat longer compared to the "monomeric" complex, is still short enough not to limit the relaxivity. Surprisingly, compared with analogous conjugates with negatively charged chelates, the prepared uncharged compounds displayed much faster global rotational correlation times (tau(g)) and lower relaxivities. This phenomenon can be explained on the basis of Coulomb interactions. The motion of the charged chelates is restrained due to interactions with their counterions and the chelates themselves, while the uncharged chelates are not affected. Comparison of the PAMAM-based conjugates bearing uncharged and (1-)-charged chelates based on relaxometric data, 1H DOSY spectra, and SAXS measurements reveals that tau(g) reflects the rotational motion of large segments (dendrons) of the conjugates rather than that of the whole macromolecule.

Calix[4]arenes as molecular platforms for magnetic resonance imaging (MRI) contrast agents.

The novel amphipilic conjugate of a calix[4]arene with four Gd-1,4,7,10- tetra(carboxymethyl)-1,4,7,10-tetraazacyclododecane (DOTA) chelates has potential as a magnetic resonance imaging contrast agent, both in its monomeric and in its micellar form. The system, illustrated here with its nuclear magnetic relaxation profile, shows good relaxivities, thanks to its high rigidity.An amphiphilic conjugate 1 of a calix[4]arene with four Gd-DOTA chelates (DOTA=1,4,7,10-tetra(carboxymethyl)-1,4,7,10-tetraazacyclododecane) was prepared and the properties relevant to its application as a magnetic resonance imaging (MRI) contrast agent were investigated by NMR, dynamic light scattering (DLS), and cryo-electron microscopy (cryo-TEM). The compound aggregates in water; its critical micelle concentration (cmc) is 0.21 mM (or 0.84 mM for Gd) at 37 degrees C. The relaxivity of the aggregates at 37 degrees C and 20 MHz (18.3 s(-1) per mM Gd or 73.2 s(-1) per mM 1) is about twice that of the monomer. Nuclear magnetic relaxation dispersion (NMRD) profiles show the relaxivity of the monomer to be almost independent of the magnetic field strength up to 60 MHz. At higher concentrations, the NMRD profiles exhibit a maximum at about 20 MHz, which is typical for high molecular volumes. The average water residence lifetime is 1.20 mus at 298 K as determined by (17)O NMR. The rotational correlation time of the monomer (390 ns at 37 degrees C) is very close to the optimal value predicted for high-field contrast agents. Monomer as well as micelles are very rigid systems with negligible local contributions to the overall rotational dynamics. The binding to human serum albumin (HSA) is significant (K(A)=1.2x10(3) M(-1)) and the relaxivity of the HSA adduct at 20 MHz is 24.6 s(-1) per mM Gd or 98.5 s(-1) per mM 1.

Structural study of Ga(III), In(III), and Fe(III) complexes of triaza-macrocycle based ligands with N3S3 donor set.

Two new ligands, 1,4,7-tris(2-mercaptoethyl)-1,4,7-triazacyclodecane (L2) and 9-methyl-1,4,7-tris(2-mercaptoethyl)-1,4,7-triazacyclodecane (L3), have been synthesized by reaction of the free triazacycloalkanes with ethylene sulfide. Complexes of L2 with Ga(III), In(III), and Fe(III), as well as Ga(III) and In(III) complexes of L3, have been characterized by single crystal X-ray diffraction. In all cases, the metal ions are coordinated in a trigonal-antiprismatic environment with varying degrees of distortion. L2 complexes are isostructural and exhibit the chair-Delta(lambda lambda)(delta delta delta) conformation, whereas GaL3 and InL3 adopt twist-boat-Delta(lambda lambda)(delta delta delta) and twist-boat-Delta(lambda lambda)(lambda delta delta) configurational isomers and their enantiomers, respectively. In addition, we report the crystal structure of the Fe(III) complex of the known ligand 1,4,7-tris(2-mercaptoethyl)-1,4,7-triazacyclononane (TS-TACN, L1), which is isostructural to its Ga(III) and In(III) complexes. All aforementioned complexes are compared (including literature data of Ga(TS-TACN) and In(TS-TACN)), and the influence of ring size and backbone substitution on coordination geometries is discussed. Furthermore, the solution structures of Ga(III) and In(III) complexes of the new ligands have been investigated employing NMR spectroscopy and density functional theory (DFT) calculation. The latter revealed that for all compounds, the most stable structures are those where the six-membered chelate ring adopts chair conformation. NMR measurements confirmed this proposal for the L2 complexes. In contrast, the GaL3 complex is present as a mixture of chair and twist-boat isomers. This implies that for this complex, interconversion between both forms is occurring, which necessarily includes intermediate partial decoordination of the metal ion. InL3 occurs as a single species, but an unambiguous determination of its conformation was not possible.

Tuning of the size of Dy2O3 nanoparticles for optimal performance as an MRI contrast agent.

The transverse 1H relaxivities of aqueous colloidal solutions of dextran coated Dy2O3 nanoparticles of different sizes were investigated at magnetic field strengths (B) between 7 and 17.6 T. The particle size with the maximum relaxivity (r2) appears to vary between 70 nm at 7 T (r2 approximately = 190 s(-1) mM(-1)) and 60 nm at 17.6 T (r2 approximately = 675 s(-1) mM(-1)). A small difference between r2 and r2* was observed, which was ascribed to the effect of the dextran coating. The value of r2 is proportional to B2 up to 12 T after which it saturates. Independent magnetization measurements on these particles at room temperature at magnetic field strengths up to 30 T, however, show a typical paramagnetic behavior with a magnetization of the particle that is proportional to the field strength. The saturation in the curve of r2 as a function of B2 was tentatively explained by the presence of an extremely fast relaxing component of the signal at high field strengths, which is not observable on the NMR time scale. The results of this study can be exploited for the rational design of MRI contrast agents, based on lanthanide oxide particles, with high efficiencies at magnetic field strengths of more than 1.5 T.

Lanthanide(III) complexes of bis(phosphonate) monoamide analogues of DOTA: bone-seeking agents for imaging and therapy.

Lanthanide complexes of DOTA derivatives 2a (BPAMD) and 2b (BPAPD), having a monoamide pendant arm with a bis(phosphonate) moiety, were comparatively tested for application in MRI, radiotherapy, and bone pain palliation. (1)H, (31)P, and (17)O NMR spectroscopy show that they are nine-coordinated, with one water molecule in the first coordination sphere of the Ln(III) ion. The bis(phosphonate) moieties are not coordinated to the lanthanide and predominantly mono- and diprotonated at physiological pH. The parameters governing the longitudinal relaxivities of the Gd complexes are similar to those of other monoamides of DOTA reported in the literature. Upon adsorption on hydroxyapatite, the relaxivities at 20 MHz and 25 degrees C of Gd-2a and Gd-2b were 22.1 and 11 s(-1) mM(-1), respectively. An in vivo gamma-ray imaging study showed that the (177)Lu complexes of 2a and 2b have a high affinity for bones, particularly for growth plates and teeth with a prolonged retention.

Surface PEG Grafting Density Determines Magnetic Relaxation Properties of Gd-Loaded Porous Nanoparticles for MR Imaging Applications.

Surface PEGylation of nanoparticles designed for biomedical applications is a common and straightforward way to stabilize the materials for in vivo administration and to increase their circulation time. This strategy becomes less trivial when MRI active porous nanomaterials are concerned as their function relies on water/proton-exchange between the pores and bulk water. Here we present a comprehensive study on the effects of PEGylation on the relaxometric properties of nanozeolite LTL (dimensions of 20 × 40 nm) ion-exchanged with paramagnetic GdIII ions. We evidence that as long as the surface grafting density of the PEG chains does not exceed the "mushroom" regime (conjugation of up to 6.2 wt % of PEG), Gd-LTL retains a remarkable longitudinal relaxivity (38 s-1 mM-1 at 7 T and 25 °C) as well as the pH-dependence of the longitudinal and transverse relaxation times. At higher PEG content, the more compact PEG layer (brush regime) limits proton/water diffusion and exchange between the interior of LTL and the bulk, with detrimental consequences on relaxivity. Furthermore, PEGylation of Gd-LTL dramatically decreases the leakage of toxic GdIII ions in biological media and in the presence of competing anions, which together with minimal cytotoxicity renders these materials promising probes for MRI applications.

The reliability of parameters obtained by fitting of (1)H NMRD profiles and (17)O NMR data of potential Gd(3+)-based MRI contrast agents.

Synthetic variable temperature (1)H NMRD profiles and (17)O NMR relaxation and shift data were generated with a model based on the Solomon-Bloembergen-Morgan and Freed theories and then fitted simultaneously or individually. The effects of the fitting procedure and of experimental uncertainties on the resulting best-fit parameters were investigated. The most reliable best-fit parameters were obtained when all data were included in a simultaneous fitting procedure. Fitting of only NMRD and/or (17)O NMR data provided considerably less accurate best-fit parameters. Very large deviations from the values of the parameters used for the construction of the datasets were obtained due to the combined effects of uncertainties resulting from the fitting and from the data. For these fittings, the accuracy of the best-fit parameters appeared to be strongly dependent on the magnitude of synthetic parameters applied. For example, the accuracy of τM (C) was low around τM (S) = 10(-8) s. The parameters τV and Δ(2) are strongly correlated in fittings of only (17)O NMR data. Consequently, only the ratio of these parameters can be evaluated in this way. The observations underline the need to reduce the number of adjustable parameters by constraining as many as possible of them at values obtained by independent techniques. The inaccuracies observed in these simulations come in addition to those caused by the inadequacy of the Solomon-Bloembergen-Morgan theory, particularly at low magnetic field strengths.

The highest water exchange rate ever measured for a Gd(III) chelate.

The complex [Gd(L)(H2O)]3- (H(6)L =N,N'-bis(6-carboxy-2-pyridylmethyl)ethylenediamine-N,N'-methylenephosphonic acid) displays the highest water exchange rate ever measured for a Gd(III) chelate (k(298)(ex)= 8.8 x 10(8) s(-1)), which is attributed to the flexibility of the metal coordination environment.

Complexation of Ln(III) and Ca(II) Cations with 3,4-Dicarboxyinulin and Model Compounds: Methyl 3,4-Dicarboxy-alpha-D-fructofuranoside and 3,4-Dicarboxynystose, As Studied by Multinuclear Magnetic Resonance Spectroscopy and Potentiometry.

Complexes of Ln(III) and Ca(II) cations with 3,4-dicarboxyinulin (DCI) and model compounds, methyl 3,4-dicarboxy-alpha-D-fructofuranoside (DCF) and 3,4-dicarboxynystose (DCN) were studied using multinuclear magnetic resonance spectroscopy and potentiometric methods. Complexes of the model compounds with Ln(III) ions provided a feasible way in which to study complexation phenomena of the dicarboxyinulin/Ca(II) system using NMR techniques. Information on complex geometry was derived from the effect of Ln(III) ions on chemical shifts and longitudinal relaxation rates. Metal-ligand stoichiometries of 1:2 and 1:1, in which the ligand coordination was tridentate as well as tetradentate, were found. Potentiometric measurements carried out with Ca(II) yielded information on the stoichiometry as well as the cooperativity of metal ion binding by the ligands.

Hydrothermal upgrading of biomass to biofuel; studies on some monosaccharide model compounds.

During the hydrothermal upgrading of biomass, hydrolysis to glucose is an important step. To elucidate some of the reaction pathways that follow this initial hydrolysis, the hydrothermal treatment (340 degrees C, 27.5 MPa, 25-204 s) of dilute (50 mM) solutions of D-glucose and some other monosaccharides were studied. As a result of the increase of Kw under subcritical conditions, both acid and base catalysed reactions occur. The acid catalysed reactions are mainly dehydrations leading initially to 5-hydroxymethylfurfural. Important base catalysed reactions result in glycolaldehyde and glyceraldehyde. Further fragmentations and dehydrations lead to a variety of low molecular weight compounds such as formic acid, acetic acid, lactic acid, acrylic acid, 2-furaldehyde and 1,2,4-benzenetriol. Important pathways leading to a decrease of the O-content of the liquid reaction products start from the intermediate glyceraldehyde, which forms pyruvaldehyde, which in its turn is converted into formic acid and acetaldehyde. The latter compound can also be formed via isomerisation of glyceraldehyde into lactic acid followed by decarbonylation.

Gadolinium oxysulfide nanoparticles as multimodal imaging agents for T2-weighted MR, X-ray tomography and photoluminescence.

We have synthesized gadolinium oxysulfide nanoparticles (NPs) doped with other lanthanides (Eu(3+), Er(3+), Yb(3+)) via a hydroxycarbonate precursor precipitation route followed by a sulfuration process under a H2S-Ar atmosphere at 750 °C in order to propose new multimodal nanoplatforms for Magnetic Resonance (MR), X-ray and photoluminescence imaging. Gd2O2S:Eu(3+) NPs strongly absorb near UV (≈ 300-400 nm) and re-emit strong red light (624 nm). They can be easily internalized by cancer cells, and imaged by epifluorescence microscopy under excitation in the NUV (365 nm). They are not cytotoxic for living cells up to 100 µg mL(-1). Consequently, they are well adapted for in vitro imaging on cell cultures. Gd2O2S:Eu(3+) NPs also show strong transverse relaxivity and strong X-ray absorption allowing their use as contrast agents for T2-weighted MRI and X-ray tomography. Our study shows that Gd2O2S:Eu(3+) NPs are considerably better than commercial Ferumoxtran-10 NPs as negative contrast agents for MRI. Upconversion emission of Gd2O2S:Er; Yb (1; 8%) NPs under infrared excitation (λ(ex) = 980 nm) shows mainly red emission (≈ 650-680 nm). Consequently, they are more specifically designed for in vivo deep fluorescence imaging, because both excitation and emission are located inside the "transparency window" of biological tissues (650-1200 nm). Magnetic relaxivity and X-ray absorption behaviors of Gd2O2S:Er; Yb NPs are almost similar to Gd2O2S:Eu(3+) NPs.

Aldol reactions mediated by a tetrahedral boronate.

The base is a key factor in aldol reactions in organic media, determining the selectivity. Here, we describe a tetrahedral phenylboronate salt as a mild non-nucleophilic base that is able to catalyse the aldol reaction and significantly decrease the formation of undesired elimination products.

The Gd3+ complex of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid mono(p-isothiocyanatoanilide) conjugated to inulin: a potential stable macromolecular contrast agent for MRI.

Reaction of DOTA-NCSA [1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid mono(p-isothiocyanatoanilide)] with O-(aminopropyl)inulin (degree of polymerization 25) provided a chelate that formed a kinetically extremely stable Gd(3+) complex. No transmetalation was observed with Zn(2+). The conjugate has a relaxivity of 21.7 s(-1) mM(-1) at 20 MHz and 37 °C, and each molecule of the inulin carries on average 35 Gd(3+) ions. The parameters governing the relaxivity of this material and of a low-molecular-weight model compound prepared by conjugation of DOTA-NCSA and propylamine were evaluated by investigation of their water (1)H longitudinal relaxation rate enhancements at different magnetic fields (NMRD) and by studying variable temperature (17)O NMR data. The high relaxivity of the inulin conjugate can be ascribed to the efficient slowing down of the molecular tumbling by this carrier. The rotational correlation time at 37 °C of this material is 1460 ps, whereas that of the model compound is 84 ps. Furthermore, both complexes do not interact significantly with human serum albumin, as shown by their NMRD profiles, and do not undergo transmetallation by zinc ions. The inulin conjugate thus has potential for application as a contrast agent for MRI, particularly as a blood pool agent.