Bifunctional Gd(III) and Tb(III) chelates based on a pyridine–bis(iminodiacetate) platform,suitable optical probes and contrast agents for magnetic resonance imaging

To study the physicochemical properties of lanthanide complexes derived from a bifunctional chelating agent based on a PMN‐tetraacetic acid moiety {PMN‐tetraacetic acid (1): [2,6‐pyridinediylbis(methylene nitrilo)‐tetraacetic acid]}, 4‐carboxylic acid substituted pyridine derivative (2) was synthesized. This ligand forms heptadentate (N₃O₄) Ln(III) complexes (Ln = Gd, Eu, Tb), with two water molecules completing the inner coordination sphere of the metal. The parameters that govern the relaxivity of the Gd(III) complex and the luminescence of Eu(III) and Tb(III) complexes were obtained by ¹⁷O and ¹H NMR studies and time‐resolved fluorescence experiments, respectively. The gadolinium and terbium complexes show interesting properties either for MRI or FOR optical imaging; that is, for the Gd complex, a high proton relaxivity (r₁ = 6.4 s^?1 mm ^?1 at 20 MHz) with short water residence time (τ_M = 38.5 ns); for the Tb complex, a luminescence lifetime of 1.22 ms at room temperature and a luminescence quantum yield of 10%. The kinetic stability of these complexes toward blood protein, cation or bioactive oxyanion was also examined. The Gd(2)(H₂O)₂ complex does not interact with human serum albumin, but undergoes a transmetalation reaction with Zn(II) in a phosphate buffer solution (pH 7.4), rather similar to that of Gd–DTPA–BMA(H₂O). On the other hand, as observed for Eu and Tb complexes, these chelates do not form ternary complexes with bidentate anions such as l ‐lactate, citrate or carbonate. Finally, a phosphatidylserine‐specific hexapeptide (TLVSSL) was grafted on Gd or Tb chelates, and the Gd–peptide conjugate was used in vitro for targeting apoptotic cells. Copyright © 2014 John Wiley & Sons, Ltd.     

Pharmacokinetic and in vivo evaluation of a self‐assembled gadolinium(III)‐iron(II) contrast agent with high relaxivity

A high‐molecular weight tetrametallic supramolecular complex [(Ln‐DTPA‐phen)₃Fe]^? (Ln = Gd, Eu, La) has been obtained upon self‐assembly around one iron(II) ion of three 1,10‐phenantroline‐based molecules substituted in 5′‐position with the polyaminocarboxylate diethylenetriamine‐N,N,N′,N′,N′‐pentaacetate, DTPA‐phen^4?. The ICP‐MS measurements indicated that the lanthanide:iron ratio is 3:1. Photoluminescence spectra of [Eu‐DTPA‐phen]^? and of [(Eu‐DTPA‐phen)₃Fe]^? are nearly identical, implying that the first coordination sphere of the lanthanide(III) ion has not been changed upon coordination of phenantroline unit to iron(II) ion. NMRD measurements revealed that at 20 MHz and 310 K the relaxivity of the [(Gd‐DTPA‐phen)₃Fe]^? is equal to 9.5 ± 0.3 s^?1 mM^?1 of Gd (28.5 s^?1 per millimole per liter of complex) which is significantly higher than that for Gd‐DTPA (3.9 s^?1 mM^?1). The pharmacokinetic parameters of [(Gd‐DTPA‐phen)₃Fe]^? in rats indicate that the elimination of [(Gd‐DTPA‐phen)₃Fe]^? is significantly slower than that of Gd‐DTPA and is correlated with a reduced volume of distribution. The low volume of distribution and the longer elimination time (T_e1/2) suggest that the agent is confined to the blood compartment, so it could have an important potential as a blood pool contrast agent. The biodistribution profile of [(Gd‐DTPA‐phen)₃Fe]^? 2 h after injection indicates significantly higher concentrations of [(Gd‐DTPA‐phen)₃Fe]^? as compared with Gd‐DTPA in kidney, liver, lungs, heart and spleen. The images obtained on rats by MR angiography show the enhancement of the abdominal blood vessels. The signal intensity reaches a maximum of 55% at 7 min post‐contrast and remains around 25% after 90 min. MRI‐histomorphological correlation studies of [Gd‐DTPA‐phen]^? and [(Gd‐DTPA‐phen)₃Fe]^? showed that both agents displayed potent contrast enhancement in organs including the liver. The necrosis avidity tests indicated that, in contrast to the [Gd‐DTPA‐phen]^? precursor complex, the supramolecular complex [(Gd‐DTPA‐phen)₃Fe]^? exhibits necrosis avidity. Copyright © 2006 John Wiley & Sons, Ltd.     

The effects of intramolecular H‐bond formation on the stability constant and water exchange rate of the Gd(III)‐diethylenetriamine‐N′‐(3‐amino‐1,1‐propylenephosphonic)‐N,N,N″,N″‐tetraacetate complex

The binding interaction of metal chelates to biological macromolecules, though driven by properly devoted recognition synthons, may cause dramatic changes in some property associated with the coordination cage such as the thermodynamic stability or the exchange rate of the metal coordinated water. Such changes are due to electrostatic and H‐bonding interactions involving atoms of the coordination cage and atoms of the biological molecule at the binding site. To mimic this type of H‐bonding interactions, lanthanide(III) complexes with a DTPA–monophosphonate ligand bearing a propylamino moiety (H₆NP–DTPA) were synthesized. Their thermodynamic stabilities and the exchange lifetime of the coordinated water molecule (for the Gd‐complex) were compared with those of the analog complexes with DTPA and the parent DTPA–monophosphonate derivative (H₆P–DTPA). It was found that the intramolecular H‐bond between the ε‐amino group and the phosphonate moiety in NP–DTPA complexes causes displacements of electric charges in their coordination cage that are markedly pH dependent. In turn, this affects the characteristic properties of the coordination cage. In particular it results in a marked elongation of the exchange lifetime of the coordinated water molecule. Copyright © 2007 John Wiley & Sons, Ltd.     

Assessment of tumor angiogenesis: dynamic contrast‐enhanced MRI with paramagnetic nanoparticles compared with Gd‐DTPA in a rabbit Vx‐2 tumor model

The purpose of this study was to evaluate the suitability of a macromolecular MRI contrast agent (paramagnetic nanoparticles, PNs) for the characterization of tumor angiogenesis. Our aim was to estimate the permeability of PNs in developing tumor vasculature and compare it with that of a low molecular weight contrast agent (Gd‐DTPA) using dynamic contrast‐enhanced MRI (DCE). Male New Zealand white rabbits (n = 5) underwent DCE MRI 12–14 days after Vx‐2 tumor fragments were implanted into the left hind limb. Each contrast agent (PNs followed by Gd‐DTPA) was evaluated using a DCE protocol and transendothelial transfer coefficient (K_i) maps were calculated using a two‐compartment model. Two regions of interest (ROIs) were located within the tumor core and hindlimb muscle and five ROIs were placed within the tumor rim. Comparisons were performed using repeated measures analysis of variance (ANOVA). The K_i values estimated using PNs were significantly lower than those obtained for Gd‐DTPA (p = 0.018). When PNs and Gd‐DTPA data were analyzed separately, significant differences were identified among tumor rim ROIs for PNs (p < 0.0001), but not for Gd‐DTPA data (p = 0.34). The mean K_i for the tumor rim was significantly greater than that of either the core or the hindlimb muscle for both contrast agents (p < 0.05 for each comparison). In summary, the extravasation of Gd‐DTPA was far greater than that of PNs, suggesting that PNs can reveal regional differences in tumor vascular permeability that are not otherwise apparent with clinical contrast agents such as Gd‐DTPA. These results suggest that PNs show potential for the noninvasive delineation of tumor angiogenesis. Copyright © 2010 John Wiley & Sons, Ltd.     

Connexin 43‐targeted T1 contrast agent for MRI diagnosis of glioma

Glioblastoma multiforme is the most aggressive form of brain tumor. Early and accurate diagnosis of glioma and its borders is an important step for its successful treatment. One of the promising targets for selective visualization of glioma and its margins is connexin 43 (Cx43), which is highly expressed in reactive astrocytes and migrating glioma cells. The purpose of this study was to synthesize a Gd‐based contrast agent conjugated with specific antibodies to Cx43 for efficient visualization of glioma C6 in vivo. We have prepared stable nontoxic conjugates of monoclonal antibody to Cx43 and polylysine–DTPA ligands complexed with Gd(III), which are characterized by higher T₁ relaxivity (6.5 mM^?1 s^?1 at 7 T) than the commercial agent Magnevist® (3.4 mM^?1 s^?1). Cellular uptake of Cx43‐specific T₁ contrast agent in glioma C6 cells was more than four times higher than the nonspecific IgG‐contrast agent, as detected by flow cytometry and confocal analysis. MRI experiments showed that the obtained agents could markedly enhance visualization of glioma C6 in vivo after their intravenous administration. Significant accumulation of Cx43‐targeted contrast agents in glioma and the peritumoral zone led not only to enhanced contrast but also to improved detection of the tumor periphery. Fluorescence imaging confirmed notable accumulation of Cx43‐specific conjugates in the peritumoral zone compared with nonspecific IgG conjugates at 24 h after intravenous injection. All these features of Cx43‐targeted contrast agents might be useful for more precise diagnosis of glioma and its borders by MRI. Copyright © 2015 John Wiley & Sons, Ltd.     

Spectroscopic,radiochemical, and theoretical studies of the Ga3+‐N‐2‐hydroxyethyl piperazine‐N′‐2‐ethanesulfonic acid (HEPES buffer) system: evidence for the formation of Ga3+‐ HEPES complexes in 68 Ga labeling reactions

Recent reports have claimed a superior performance of HEPES buffer in comparison to alternative buffer systems for ^67/68 Ga labeling in aqueous media. In this paper we report spectroscopic (¹H and ⁷¹ Ga NMR), radiochemical, mass spectrometry and theoretical modeling studies on the Ga³⁺/HEPES system (HEPES = N‐2‐hydroxyethylpiperazine‐N′‐2‐ethanesulfonic acid) performed with the aim of elucidating a potential contribution of HEPES in the ^68/67 Ga radiolabeling process. Our results demonstrate that HEPES acts as a weakly but competitive chelator of Ga³⁺ and that this interaction depends on the relative Ga³⁺: HEPES concentration. A by‐product formed in the labeling mixture has been identified as a [⁶⁸ Ga]Ga(HEPES) complex via chromatographic comparison with the nonradioactive analog. The formation of this complex was verified to compete with [⁶⁸ Ga]Ga(NOTA) complexation at low NOTA concentration. Putative chelation of Ga³⁺ by the hydroxyl and adjacent ring nitrogen of HEPES is proposed on the basis of ¹H NMR shifts induced by Ga³⁺ and theoretical modeling studies. Copyright © 2013 John Wiley & Sons, Ltd.     

Synthesis and characterization of new low‐molecular‐weight lysine‐conjugated Gd‐DTPA contrast agents

Various blood pool contrast agents (CAs), characterized by intravascular distribution, have been developed to assist contrast enhanced magnetic resonance angiography (MRA). Among these CAs, the DTPA derivatives conjugated to synthetic polypeptides, such as polylysine, represent attractive candidates for blood pool imaging. However, due to the presence of charged residues located on their backbone, these agents are retained in the kidneys and this compromises their long blood half‐life. In order to overcome this major drawback of the polylysine compounds, two new low‐molecular‐weight CAs were synthesized in the present work by conjugating four or six 1‐p‐isothiocyanatobenzyl‐DTPA moieties to tri‐ or penta‐Lys peptides [(Gd‐DTPA)₄Lys₃ and (Gd‐DTPA)₆Lys₅], respectively. All the –NH₂ groups of Lys were thus blocked by covalent conjugation to DTPA. The stability and relaxometric properties of these compounds, as well as their pharmacokinetic and biodistribution characteristics, were then evaluated. The half‐life in blood of these new polylysine derivatives, as determined in rats, is twofold longer than that of Gd‐DTPA. The compounds could thus be optimal blood pool markers for MRA, which typically uses fast acquisition times. The absence of positive molecular charge did not limit their retention in kidneys 2 h after administration. On the other hand, (Gd‐DTPA)₄Lys₃ is retained in kidneys to a lesser extent than (Gd‐DTPA)₆Lys₅. Their moderate retention in blood and their higher stability and relaxivity in comparison with Gd‐DTPA highlight these polylysine derivatives as optimal compared with previously developed polylysine compounds. Copyright © 2010 John Wiley & Sons, Ltd.     

Synthesis and characterization of a porphyrazine–Gd(III) MRI contrast agent and in vivo imaging of a breast cancer xenograft model

Porphyrazines (Pz), or tetraazaporphyrins, are being studied for their potential use in detection and treatment of cancer. Here, an amphiphilic Cu–Pz–Gd(III) conjugate has been prepared via azide–alkyne Huisgen cycloaddition or ‘click’ chemistry between an azide functionalized Pz and alkyne functionalized DOTA–Gd(III) analog for use as an MRI contrast agent. This agent, Cu–Pz–Gd(III), is synthesized in good yield and exhibits solution‐phase ionic relaxivity (r₁ = 11.5 mm ^?1 s^?1) that is approximately four times higher than that of a clinically used monomeric Gd(III) contrast agent, DOTA–Gd(III). Breast tumor cells (MDA‐MB‐231) associate with Cu–Pz–Gd(III) in vitro, where significant contrast enhancement (9.336 ± 0.335 contrast‐to‐noise ratio) is observed in phantom cell pellet MR images. This novel contrast agent was administered in vivo to an orthotopic breast tumor model in athymic nude mice and MR images were collected. The average T₁ of tumor regions in mice treated with 50 mg kg^?1 Cu–Pz–Gd(III) decreased relative to saline‐treated controls. Furthermore, the decrease in T₁ was persistent relative to mice treated with the monomeric Gd(III) contrast agent. An ex vivo biodistribution study confirmed that Cu–Pz–Gd(III) accumulates in the tumors and is rapidly cleared, primarily through the kidneys. Differential accumulation and T₁ enhancement by Cu–Pz–Gd(III) in the tumor's core relative to the periphery offer preliminary evidence that this agent would find application in the imaging of necrotic tissue. Copyright © 2014 John Wiley & Sons, Ltd.     

A novel one‐step synthesis of Gd3+‐incorporated mesoporous SiO2 nanoparticles for use as an efficient MRI contrast agent

Molecular imaging has generated a demand for more sensitive contrast agents for magnetic resonance (MR) imaging. We synthesized, by a novel one‐step method, Gd³⁺ incorporated mesoporous SiO₂ nanoparticles, Gd₂O₃@SiO₂, for use as an efficient contrast agent. The prepared nanoparticles were also coated with poly(lactic‐co‐glycolic acid) (PLGA). The size, morphology, composition and Brunauer–Emmett–Teller specific surface area of the nanoparticles were evaluated. The Gd₂O₃@SiO₂ nanoparticles possess intragranular network morphology with a uniform size distribution and an average size of approximately 20–40 nm. The PLGA‐coated nanoparticles were spherical or near‐spherical in shape with a diameter of approximately 120 nm, a smooth surface, and neither aggregation nor adhesion tendencies. No free Gd ions were detected to dissociate from Gd₂O₃@SiO₂ even up to the limit (<0.03 mg/l) of the ICP equipment (IRIS Advantage). Our theoretical computation based on density functional theory (using DMol3, Materials Studio) revealed that the Gd₂O₃ molecules are fully absorbed on the interface of mesoporous SiO₂ with a stable state of lower energy. Both Gd₂O₃@SiO₂ and PLGA‐coated Gd₂O₃@SiO₂ samples have a larger T₁ relaxivitiy than commercial gadolinium diethylene triaminepentaacetate (Gd‐DTPA). In vitro and in vivo MR images using the Gd₂O₃@SiO₂ nanoparticles were observed with a 1.5 T clinical MR scanner and compared with the images using Gd‐DTPA. The Gd₂O₃@SiO₂ nanoparticles display a better magnetic property than commercial Gd‐DTPA. In vivo MR imaging demonstrated that the nanoparticles were mainly distributed in the liver. Strong enhancement was also detected in nasopharyngeal carcinoma CNE‐2 xenografted tumors. The Gd₂O₃@SiO₂ nanoparticles are not only potential candidates for highly efficient contrast agents for MR imaging, but also might be developed into potent targeted probes for in vivo molecular imaging of cancer. Copyright © 2010 John Wiley & Sons, Ltd.     

Lectin Conjugates as Biospecific Contrast Agents for MRI. Coupling of <Emphasis Type="Italic">Lycopersicon esculentum</Emphasis> Agglutinin to Linear Water-Soluble DTPA-Loaded Oligomers

Magnetic resonance imaging (MRI) requires synthesis of contrast media bearing targeting groups and numerous gadolinium chelating groups generating high relaxivity. This paper explores the results of linking the gadolinium chelates to the targeting group, a protein molecule, via various types of linkers. Polycondensates of diethylenetriaminepentaacetic acid (DTPA) with either diols or diamines were synthesised and coupled to the targeting group, a lectin (Lycopersicon esculentum agglutinin, tomato lectin) which binds with high affinity to specific oligosaccharide configurations in the endothelial glycocalyx. The polycondensates bear up to four carboxylic groups per constitutive unit. Gd-chelate bonds are created through dative interactions with the unshared pair of electrons on each oxygen and nitrogen atom on DTPA. This is mandatory for complexation of Gd(III) and avoidance of the severe toxicity of free gadolinium ions. The polymer–DTPA compounds were characterised by ¹H NMR and mass spectrometry. The final lectin–DTPA–polycondensate conjugates were purified by fast protein liquid chromatography (FPLC). The capacity for specific binding was assessed, and the MRI properties were examined in order to evaluate the use of these oligomers as components of selective perfusional contrast agents.     

Quantitative imaging of the tissue contrast agent [Gd(DTPA)]2− in articular cartilage by laser ablation inductively coupled plasma mass spectrometry

Laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) is an emerging analytical technique in the generation of quantitative images of MR contrast agent distribution in thin tissue sections of articular cartilage. An analytical protocol is described that includes sample preparation by cryo‐cutting of tissue sections, mass spectrometric measurements by LA‐ICP‐MS and quantification of gadolinium images by one‐point calibration, standard addition method (employing matrix‐matched laboratory standards) and isotope dilution analysis using highly enriched stable Gd‐155 isotope (abundance 92 vs 14.8% in the [Gd(DTPA)]^2? contrast agent). The tissue contrast agent concentrations of [Gd(DTPA)]^2? in cartilage measured in this work are in agreement with findings obtained by magnetic resonance imaging and other analytical methodologies. The LA‐ICP‐MS imaging data also confirm the observation that the spatial distribution of [Gd(DTPA)]^2? in the near‐equilibrium state is highly inhomogeneous across cartilage thickness with the highest concentration measured in superficial cartilage and a strong decrease toward the subchondral bone. In the present work, it is shown for the first time that LA‐ICP‐MS can be applied to validate the results from quantitative gadolinium‐enhanced MRI technique of articular cartilage. Copyright © 2012 John Wiley & Sons, Ltd.     

MRI of cells and mice at 1 and 7 Tesla with Gd‐targeting agents: when the low field is better!

Tumor cells were targeted with Gd‐loaded/LDL (low density lipoproteins) adducts consisting of ca 300 Gd(III) amphiphilic complexes incorporated in the lipophilic LDL particles. The long reorientational time of the Gd(III) complex in the supramolecular adduct yielded a relaxivity peak at ca 1 T, whereas its relaxivity at 7 T was 5 times less. The field‐dependent relaxivity markedly affected the signal enhancement attainable at the two magnetic fields. As tumor cells showed up‐regulation of LDL transporters, B16 melanoma cells were labeled with the Gd‐loaded/LDL adduct. Each cell contained ca 2 × 10⁹ Gd atoms. Upon dispersion of 5000 labeled cells in 1 μl of agar, signal intensity (SI) enhancements of about 30 and 7% were observed at 1 and 7 T, respectively. The results obtained on cellular systems were confirmed in vivo upon the administration of Gd‐loaded/LDL particles to C57 mice bearing a transplanted melanoma (B16) tumor. From the herein reported results, one may conclude that, for slowly moving Gd complexes, it is possible to obtain in vivo sensitivity enhancements at 1 T several times higher than that attained at high fields. Copyright © 2011 John Wiley & Sons, Ltd.     

Relaxivity enhancement of aquated Tris(β‐diketonate)gadolinium(III) chelates by confinement within ultrashort single‐walled carbon nanotubes

Ultrashort single‐walled carbon nanotubes loaded with gadolinium ions (gadonanotubes) have been previously shown to exhibit extremely high T₁‐weighted relaxivities (>100 mm ^?1 s^?1). To further examine the effect of nanoconfinement on the relaxivity of gadolinium‐based contrast agents for magnetic resonance imaging, a series of ultrashort single‐walled carbon nanotube (US‐tube) materials internally loaded with gadolinium chelates have been prepared and studied. US‐tubes were loaded with Gd(acac)₃ · 2H₂O, Gd(hfac)₃ · 2H₂O, and Gd(thd)₃ (acac = acetylacetone, hfac = hexafluoroacetylacetone, thd = tetramethylheptanedione). The longitudinal relaxivities of the prepared materials determined at 25°C in a 1.5 T field were 103 mm ^?1 s^?1 for Gd(acac)₃ · 2H₂O@US‐tubes, 105 mm ^?1 s^?1 for Gd(hfac)₃ · 2H₂O@US‐tubes and 26 mm ^?1 s^?1 for Gd(thd)₃@US‐tubes. Compared with the relaxivities obtained for the unloaded chelates (<10 mm ^?1 s^?1) as well as accounting for the T₁ reduction observed for the empty US‐tubes, the boost in relaxivity for chelate‐loaded US‐tubes is attributed to confinement within the nanotube and depends on the number of coordinated water molecules. Copyright © 2014 John Wiley & Sons, Ltd.     

Cellular labeling with Gd(III) chelates: only high thermodynamic stabilities prevent the cells acting as ‘sponges’ of Gd3+ ions

MR‐labeling of cells may be carried out by adding a Gd‐based contrast agent to the incubation media. The amount of gadolinium internalized in HTC and C6 cells upon incubation with Gd–DTPA–BMA is circa one order of magnitude higher than those found with Gd–DTPA, Gd–DOTA and Gd–HPDO3A, respectively. The comparison of relaxometric and mass spectrometry determinations allows us to establish that only a minor fraction of intact Gd–DTPA–BMA is internalized into the cells. Moreover the binding/uptake behavior shown by Gd–DTPA–BMA resembles that found when GdCl₃ is added to the incubation medium. We suggest that the lower stability of Gd–DTPA–BMA is responsible for a shift in the dissociation equilibrium that results in the net transfer of Gd³⁺ ions on the cell membrane followed by a slower internalization process. The transmetallation process is mediated by components of the incubation media, among which a dominant role is represented by phosphate anions. The uptake of Gd³⁺ ions is clearly reflected in the drastic decrease of cell viability observed for cells labeled with Gd–DTPA–BMA. Copyright © 2006 John Wiley & Sons, Ltd.     

The presence of halide salts influences the non‐covalent interaction of MRI contrast agents and human serum albumin

The rationale and objectives of the study were to evaluate the influence of the experimental conditions (buffer, salt, etc.) on the data characterizing the non‐covalent interaction between MRI contrast agents and human serum albumin and hence their in vivo relaxivity. The interaction of three gadolinium contrast agents (Gd‐EOB‐DTPA, Gd‐BOPTA and MP‐2269) with human serum albumin was assessed through the measurement of proton relaxation rate enhancement in various experimental conditions. The data show the negative effect of halide salts on the paramagnetic relaxation enhancement of the three contrast agents. The presence of halide salts can thus have a negative effect on the efficacy of MRI contrast agents interacting with HSA. In addition, careful attention must be paid to comparisons of the binding parameters of various contrast agents reported in different studies since the composition of the medium can greatly influence the non‐covalent interaction. Copyright © 2007 John Wiley & Sons, Ltd.     

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 Gd³⁺ complex. No transmetalation was observed with Zn²⁺. The conjugate has a relaxivity of 21.7 s^?1 m m ^?1 at 20 MHz and 37 °C, and each molecule of the inulin carries on average 35 Gd³⁺ 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 ¹H longitudinal relaxation rate enhancements at different magnetic fields (NMRD) and by studying variable temperature ¹⁷O 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. Copyright © 2011 John Wiley & Sons, Ltd.     

A dual CT‐MR dendrimer contrast agent as a surrogate marker for convection‐enhanced delivery of intracerebral macromolecular therapeutic agents

The feasibility of using Gd dendrimer‐based macromolecules (Gd‐G8 dendrimer) as a dual CT and MR contrast agent for monitoring convection‐enhanced delivery of therapy in the brain is evaluated both in vitro and in vivo with optimal dosing established. In vitro CT attenuation values of the Gd‐based agents (～6.0 HU mM ^?1) were ～1.6 times greater than iodine‐based agents and the attenuation of the Gd‐DTPA was comparable to Gd‐G8 dendrimer. Visible enhancement was observed on both CT and MR using Gd‐G8 dendrimer over a range of 23–78 mM ; however, a concentration of at least 47 mM in Gd was required for adequate delineation of the injection site on both CT and MR. MR offers greater sensitivity than CT in estimating the volume of distribution (V_d) and effectively quantified the agent's concentration and diffusion using T₁ mapping at much lower concentrations of Gd (<10 mM in [Gd]). Copyright © 2008 John Wiley & Sons, Ltd.     

Thermodynamic stability and kinetic inertness of a Gd–DTPA bisamide complex grafted onto gold nanoparticles

Gold nanoparticles coated by gadolinium (III) chelates (Au@DTDTPA) where DTDTPA is a dithiolated bisamide derivative of diethylenetriamine‐N,N,N′,N′′,N′′‐pentaacetic acid (DTPA), constituted contrast agents for both X‐ray computed tomography and magnetic resonance imaging. In an MRI context, highly stable Gd³⁺ complexes are needed for in vivo applications. Thus, knowledge of the thermodynamic stability and kinetic inertness of these chelates, when grafted onto gold nanoparticles, is crucial since bisamide DTPA chelates are usually less suited for Gd³⁺ coordination than DTPA. Therefore, these parameters were evaluated by means of potentiometric titrations and relaxivity measurements. The results showed that, when the chelates were grafted onto the nanoparticle, not only their thermodynamic stability but also their kinetic inertness were improved. These positive effects were correlated to the chelate packing at the nanoparticle surface that stabilized the corresponding Gd³⁺ complexes and greatly enhanced their kinetic inertness. Copyright © 2014 John Wiley & Sons, Ltd.     

Radiolabelled GLP‐1 analogues for in vivo targeting of insulinomas

Internalizing agonists are usually selected for peptide receptor targeting. There is increasing evidence that non‐internalizing receptor antagonists can be used for this purpose. We investigated whether the glucagon‐like peptide‐1 receptor (GLP‐1R) antagonist exendin(9–39) can be used for in vivo targeting of GLP‐1R expressing tumours and compared the in vitro and in vivo characteristics with the GLP‐1R agonists exendin‐3 and exendin‐4. The binding and internalization kinetics of labelled [Lys⁴⁰(DTPA)]exendin‐3, [Lys⁴⁰(DTPA)]exendin‐4 and [Lys⁴⁰(DTPA)]exendin(9–39) were determined in vitro using INS‐1 cells. The in vivo targeting properties of [Lys⁴⁰(¹¹¹In‐DTPA)]exendin‐3, [Lys⁴⁰(¹¹¹In‐DTPA)]exendin‐4 and [Lys⁴⁰(¹¹¹In‐DTPA)]exendin(9–39) were examined in BALB/c nude mice with subcutaneous INS‐1 tumours. ^natIn‐labelled [Lys⁴⁰(DTPA)]exendin‐3, [Lys⁴⁰(DTPA)]exendin‐4 and [Lys⁴⁰(DTPA)]exendin(9–39) exhibited similar IC₅₀ values (13.5, 14.4 and 13.4 n m , respectively) and bound to 26 × 10³, 41 × 10³ and 37 × 10³ receptors per cell, respectively. [Lys⁴⁰(¹¹¹In‐DTPA)]exendin‐3 and [Lys⁴⁰(¹¹¹In‐DTPA)]exendin‐4 showed rapid in vitro binding and internalization kinetics, whereas [Lys⁴⁰(¹¹¹In‐DTPA)]exendin(9–39) showed lower binding and minimal internalization in vitro. In mice, high specific uptake of [Lys⁴⁰(¹¹¹In‐DTPA)]exendin‐3 [25.0 ± 6.0% injected dose (ID) g^?1] in the tumour was observed at 0.5 h post‐injection (p.i.) with similar uptake up to 4 h p.i. [Lys⁴⁰(¹¹¹In‐DTPA)]exendin‐4 showed higher tumour uptake at 1 and 4 h p.i. (40.8 ± 7.0 and 41.9 ± 7.2% ID g^?1, respectively). Remarkably, [Lys⁴⁰(¹¹¹In‐DTPA)]exendin(9–39) showed only low specific uptake in the tumour at 0.5 h p.i. (3.2 ± 0.7% ID g^?1), rapidly decreasing over time. In conclusion, the GLP‐1R agonists [Lys⁴⁰(DTPA)]exendin‐3 and [Lys⁴⁰(DTPA)]exendin‐4 labelled with ¹¹¹In could be useful for in vivo GLP‐1R targeting, whereas [Lys⁴⁰(DTPA)]exendin(9–39) is not suited for in vivo targeting of the GLP‐1R. Copyright © 2012 John Wiley & Sons, Ltd.     

How to determine the number of inner‐sphere water molecules in Lanthanide(III) complexes by 17O NMR spectroscopy. A technical note

Lanthanide(III) complexes of polyaminocarboxylates are widely used in MRI as contrast agents. The paramagnetic properties of the metal ion contribute to the increase of ¹H relaxation rates, while the chelate offers a stable binding with the metal. The number of water molecules, coordinated directly to the Ln(III) ion, is very important for the relaxivity and, thus, the efficacy of these contrast agents. Here, we describe convenient methods to determine this parameter by measurement of Ln(III)‐induced shifts of the water ¹⁷O NMR resonance. Copyright © 2007 John Wiley & Sons, Ltd.