Simple method for quantification of gadolinium magnetic resonance imaging contrast agents using ESR spectroscopy

To develop an estimation method of gadolinium magnetic resonance imaging (MRI) contrast agents, the effect of concentration of Gd compounds on the ESR spectrum of nitroxyl radical was examined. A solution of either 4-oxo-2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPONE) or 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPOL) was mixed with a solution of Gd compound and the ESR spectrum was recorded. Increased concentration of gadolinium-diethylenetriamine pentaacetic acid chelate (Gd-DTPA), an MRI contrast agent, increased the peak-to-peak line widths of ESR spectra of the nitroxyl radicals, in accordance with a decrease of their signal heights. A linear relationship was observed between concentration of Gd-DTPA and line width of ESR signal, up to approximately 50 mmol/L Gd-DTPA, with a high correlation coefficient. Response of TEMPONE was 1.4-times higher than that of TEMPOL as evaluated from the slopes of the lines. The response was slightly different among Gd compounds; the slopes of calibration curves for acua[N,N-bis[2-[(carboxymethyl)[(methylcarbamoyl)methyl]amino]ethyl]glycinato(3-)]gadolinium hydrate (Gd-DTPA-BMA) (6.22 μT·L/mmol) and gadolinium-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid chelate (Gd-DOTA) (6.62 μT·L/mmol) were steeper than the slope for Gd-DTPA (5.45 μT·L/mmol), whereas the slope for gadolinium chloride (4.94 μT·L/mmol) was less steep than that for Gd-DTPA. This method is simple to apply. The results indicate that this method is useful for rough estimation of the concentration of Gd contrast agents if calibration is carried out with each standard compound. It was also found that the plot of the reciprocal square root of signal height against concentrations of contrast agents could be useful for the estimation if a constant volume of sample solution is taken and measured at the same position in the ESR cavity every time.     

A Gd3Al tetranuclear complex as a potential bimodal MRI/optical imaging agent

A new self-assembled gadolinium(III)-aluminum(III) complex (Gd(3)Al) was synthesized and characterized. The efficacy of this Gd(3)Al complex as a potential bimodal magnetic resonance imaging (MRI)/optical imaging agent has been evaluated. Relaxivity studies showed that the Gd(3)Al complex has higher relaxation efficiency (7.18 mM(-1) s(-1)) compared with the clinically used complex gadolinium-diethylenetriaminepentaacetic acid (Gd-DTPA, 3.9 mM(-1) s(-1)) at 400 MHz and 25 °C. In vitro T(1)-MR images on a 0.5 T magnetic field exhibited a remarkable enhancement of signal contrast for Gd(3)Al compared to Gd-DTPA. Furthermore, the Gd(3)Al complex exhibits bright-green luminescence with the emission spectrum centred at 510 nm. Live-cell fluorescence imaging reveals that the Gd(3)Al complex is permeable to cells and localizes to the cytoplasm. In view of the relaxometric and luminescent properties, this Gd(3)Al complex could serve as a potential bimodal MRI/optical imaging agent.     

Liver-targeting macromolecular MRI contrast agents

Macromolecular ligands with liver-targeting group (pyridoxamine, PM) PHEA-DTPA-PM and PAEA-DTPA-PM were prepared by the incorporation of different amount of diethylenetria-minepentaacetic acid monopyridoxamine group (DTPA-PM) into poly-cc, p-[N-(2-hydroxyethyl)-L-aspartamide] (PHEA) and poly-α, β-[N-(2-aminoethyl)-L-aspartamide] (PAEA). The macromolecular ligands thus obtained were further complexed with gadolinium chloride to give macromolecular MRI contrast agents with different Gd(Ⅲ) contents. These macromolecular ligands and their gadolinium complexes were characterized by 1H NMR, 1R, UV and elementary analysis. Relaxivity studies showed that these polyaspartamide gadolinium complexes possess higher relaxation effectiveness than that of the clinically used Gd-DTPA. Magnetic resonance imaging of the liver in rats and experimental data of biodistribution in mice indicate that these macromolecular MRI contrast agents containing pyridoxamine exhibit liver-targeting property.     

Gadolinium-based cancer therapeutic liposomes for chemotherapeutics and diagnostics

Gadolinium (Gd)-based cancer therapeutic liposomes can be used for chemotherapeutics and diagnostics. In this study, dual functional liposomes co-encapsulating doxorubicin (Dox) and Gd were prepared by Dox-transition metal complexation. Preparation conditions were optimized to obtain liposomes containing high concentrations of Dox and Gd. The optimized liposomes Gd250 co-encapsulated 3.6 mM of Dox and 1.9 mM of Gd. The magnetic resonance (MR) properties of Gd250 liposomes were determined using a 4.7 T MR system. Cellular uptake of Dox was determined using a flow cytometer and a confocal microscopy and that of Gd was measured using an inductively coupled plasma-atomic emission spectrometer. Although encapsulated Gd exhibited lower relaxivity than MRbester?, which is widely used for clinical diagnosis, because of limited diffusion across the liposome membrane, Gd250 liposomes showed much higher cellular uptake than that of MRbester?. In Gd250 liposomes, Gd was highly accumulated in B16F10 cells, which could provide improved contrast sensitivity for molecular imaging. Additionally, in Gd250 liposomes, Dox was highly internalized, which could enhance its cancer therapeutic effects. Consequently, we suggest that dual functional liposomes can be used as therapeutic and diagnostic carriers.     

两亲性双核钆(Ⅲ)配合物的合成与肝选择MRI增强

通过二乙三胺五乙酸单环酸酐和乙二胺四乙酸单环酸酐分别与L-赖氨酸苄酯的双酰化反应,制得两种含有双胺羧螯合单元的配体.它们与GdCl₃·6H₂O反应得到相应的两亲性双核钆(Ⅲ)配合物.表征了配体和配合物的结构,测试了配合物的纵向弛豫效能(R1).进行了双DTPA酰(L-赖氨酸苄酯)钆(Gd₂-3_a)的急性毒性和与Gd-DTPA的动物T₁加权成像对比实验.结果表明,这两种新配合物的R1都高于Gd-DTPA.Gd₂-3_a无明显急性毒性,且比Gd-DTPA对肝区实质细胞有更长时间和更大程度的选择性增强.     

Rational design of protein-based MRI contrast agents

We describe the rational design of a novel class of magnetic resonance imaging (MRI) contrast agents with engineered proteins (CAi.CD2, i = 1, 2,..., 9) chelated with gadolinium. The design of protein-based contrast agents involves creating high-coordination Gd(3+) binding sites in a stable host protein using amino acid residues and water molecules as metal coordinating ligands. Designed proteins show strong selectivity for Gd(3+) over physiological metal ions such as Ca(2+), Zn(2+), and Mg(2+). These agents exhibit a 20-fold increase in longitudinal and transverse relaxation rate values over the conventional small-molecule contrast agents, e.g., Gd-DTPA (diethylene triamine pentaacetic acid), used clinically. Furthermore, they exhibit much stronger contrast enhancement and much longer blood retention time than Gd-DTPA in mice. With good biocompatibility and potential functionalities, these protein contrast agents may be used as molecular imaging probes to target disease markers, thereby extending applications of MRI.     

Paramagnetic porous polymersomes

The ability of chelated Gd to serve as an effective magnetic resonance (MR) contrast agent largely depends on fast exchange rates between the Gd-bound water molecules and the surrounding bulk water. Because water diffuses slowly across lipid bilayers, liposomes with encapsulated chelated Gd have not been widely adopted as MR contrast agents. To overcome this limitation, we have synthesized chemically stabilized, porous polymersomes with encapsulated gadolinium (Gd) chelates. The polymerosmes, 125 nm in diameter, were produced from the aqueous assembly of diblock copolymers, PEO(1300)- b-PBD(2500) (PBdEO), and phospholipids, 1-palmitoyl-2-oleoyl- sn-glycero-3-phosphocholine (POPC). The PBdEO was cross-linked using a chemical initiator and the POPC was extracted with surfactant, generating a highly porous outer membrane. The encapsulated Gd chelates were attached to dendrimers to prevent their leakage through the pores. It was estimated that, on average, nearly 44 000 Gd were encapsulated within each polymersome. As a result of the slower rotational correlation time of Gd-labeled dendrimers and the porous outer membrane, the paramagnetic porous polymersomes exhibited an R1 relaxivity of 7.2 mM (-1) s (1-) per Gd and 315 637 mM (-1) s (-1) per vesicle. This corresponds to a relaxivity that is amplified by a factor of approximately 10 (5) compared with Gd-DTPA.     

Trace determination of gadolinium in biomedical samples by diode laser-based multi-step resonance ionization mass spectrometry

The application of high-resolution multi-step resonance ionization mass spectrometry (RIMS) to the trace determination of the rare earth element gadolinium is described. Utilizing three-step resonant excitation into an autoionizing level, both isobaric and isotopic selectivity of >10(7) were attained. An overall detection efficiency of approximately 10(-7) and an isotope specific detection limit of 1.5 x 10(9) atoms have been demonstrated. When targeting the major isotope (158)Gd, this corresponds to a total Gd detection limit of 1.6 pg. Additionally, linear response has been demonstrated over a dynamic range of six orders of magnitude. The method has been used to determine the Gd content in various normal and tumor tissue samples, taken from a laboratory mouse shortly after injection of gadolinium diethylenetriaminepentaacetic acid dimeglumine (Gd-DTPA), which is used as a contrast agent for magnetic resonance imaging (MRI). The RIMS results show Gd concentrations that vary by more than two orders of magnitude (0.07-11.5 microg mL(-1)) depending on the tissue type. This variability is similar to that observed in MRI scans that depict Gd-DTPA content in the mouse prior to dissection, and illustrates the potential for quantitative trace analysis in microsamples of biomedical materials.     

EPR characterization of gadolinium(III)-containing-PAMAM-dendrimers in the absence and in the presence of paramagnetic probes

Gd(III)-containing dendrimers are promising contrast agents for magnetic resonance imaging (MRI). An important issue in the effectiveness and toxicity of a Gd(III) based MRI contrast agent is knowledge of the relative locations and concentrations of Gd(III) in dendrimer drug delivery hosts. In order to provide experimental information on this issue, we have investigated the electron paramagnetic resonance (EPR) of a stable Gd(III) complex with diethylenetriaminepentaacetic acid (DTPA) in various polyammidoamine (PAMAM) dendrimers as a function of dendrimer generation (G2, G4, and G6), dendrimer core (ethylenediamine = EDA, and cystamine = cys), and dendrimer surface functionality (NH(2), 5-oxo-3-pyrrolidinecarboxylic acid methyl ester = pyr, and tris(hydroxymethyl) methylamine = tris). The dendrimer systems were investigated in the presence and absence of paramagnetic probes, that is, Cu(II) and nitroxide radicals (4-(trimethylammonium and dodecyl-dimethylammonium) 2,2,6,6-tetramethylpiperidine 1-oxyl bromide = CAT1 and CAT12, respectively). The analysis of the EPR spectra revealed anisotropic locations of Gd-DTPA inside the dendrimer. Computer analysis of the EPR spectra of the probes identified the interactions of the Gd-dendrimers with ions and organic molecules. The interaction between the probes and the dendrimer internal and external surface depends on the type of core, the composition of the external surface and the generation of the dendrimer. The negatively charged Gd-DTPA complex attracts the positively charged species and this provokes spin-spin interactions between Gd and the probes, which increases with a decrease in generation, mainly from G6 to G4, and with an increase in both the Gd-dendrimer concentration and the probe concentration. The cys core increases the internal volume and decreases the packing of the branches.     

Hybrid Dextran-gadolinium Nano-suitcases as High-relaxivity MRI Contrast Agents

Dextran-poly(glycidyl methacrylate)(Dex-PGMA) nano-suitcases were synthesized efficiently via a graft copolymerization induced self-assembly(GISA) approach. On this basis, the Dex-PGMA nano-suitcases were modified with hydrazide, and the attachment of multiple chelated Gd(Ⅲ) ions to the interior of the nano-suitcases affords nanoscale MRI contrast agents with high relaxivity values. The highly fenestrated dextran shell of the nano-suitcases assures water exchange which readily occurs between the surrounding environment and the Gd(Ⅲ) ions encapsulated within the hybrid nano-suitcases. The complexation between the hydrophilic hydrazide interior of the nano-suitcases and Gd(Ⅲ) ions results in an impressive Gd payload at 22.6 wt% in the hybrid nano-suitcases. The longitudinal relaxivity(r1) of the hybrid nano-suitcases is reported as 44.4 L/(mmol·s), which is 9-14 folds of that of commercial Gd-DTPA agents. In vivo MRI studies demonstrate that the hybrid nano-suitcases accumulated in the lymph node of the rat due to their nanoscale dimensions and displayed strong signals in vivo. The results indicated that the hybrid nano-suitcases provide a promising platform for the diagnosis of lymph node related diseases.     

Synthesis of DTPA analogues derived from piperidine and azepane: potential contrast enhancement agents for magnetic resonance imaging.

Two DTPA derivatives (PIP-DTPA and AZEP-DTPA) as potential contrast enhancement agents in MRI are synthesized. The T1 and T2 relaxivities of their corresponding Gd(III) complexes are reported. At clinically relevant field strengths, the relaxivities of the complexes are comparable to that of the contrast agent, Gd(DTPA) which is in clinical use. The serum stability of the (153)Gd-labeled complexes is assessed by measuring the release of (153)Gd from the ligands. The radiolabeled Gd chelates are found to be kinetically stable in human serum for up to at least 14 days without any measurable loss of radioactivity.     

Long-circulating gadolinium-loaded liposomes: potential use for magnetic resonance imaging of the blood pool

In our previous paper, we reported a method of liposome loading with Gadolinium (Gd) via so called polychelating amphiphilic polymer (PAP). A novel Gd-containing polymeric probe, suitable for the incorporation into the liposomal membrane, was prepared from a low-molecular-weight DTPA-polylysine by linking its N-terminus to a lipid anchor, NGPE-PE. When compared with known membranotropic MR probes, such as Gd-DTPA-SA and Gd-DTPA-PE, liposomes containing new membrane-bound polychelator possess enhanced relaxivity for water protons resulting in an increase of tissue signal intensity on MR images. In this study, we developed the optimized protocol to prepare a liposomal MR contrast agent with high relaxivity and narrow size distribution. Gd-containing liposomes were additionally modified with PEG to provide longevity in vivo. We also demonstrated that upon intravenous administration in rabbit and dog, the new preparation causes a prolonged decrease in the blood T₁ value (reflecting the proton relaxation rate in the blood) and may be considered as a potential contrast agent for MRI of the blood pool.     

夹心型锰杂多配合物作为磁共振成像造影剂的研究

以两种夹心型锰杂多配合物K10[Mn4(PW9O34)2]·22H2O和Na16[Mn4(H2O)2(P2W15O56)2]·53H2O作为研究对象, 采用元素分析和红外光谱对其结构进行了表征, 测试其在水中、牛血清白蛋白及运铁蛋白溶液中的弛豫效率, 并进行了大鼠活体成像实验. 结果表明, 这两种锰杂多配合物的弛豫效率高于或接近于目前临床常用的造影剂Gd-DTPA, 对肝脏和肾脏MRI信号具有良好的增强效果, 是比较好的潜在磁共振成像造影剂候选化合物.     

肽类树状大分子磁共振成像探针的合成与功能化

通过优化设计,合成了高产率的DTPA和DOTA配体.通过液相发散法制得第三代肽类树状大分子,其外围氨基分别用两种不同保护基团保护,且两种保护基团的个数比精确控制为18∶6,通过选择性脱去保护基团,其中一种氨基与DTPA、DOTA偶联,或与丁二酸酐反应,并与金属离子钆螯合,制得G3-18Gd-DTPA-6COOH,G3-...     

Synthesis neutral rare earth complexes of diethylenetriamine-N,N'-bis(acetyl-isoniazid)-N,N',N'-triacetic acid as potential contrast enhancement agents for magnetic resonance imaging

A novel ligand, diethylenetriamine-N,N'-bis(acetyl-isoniazid)-N,N',N'-triacetic acid (H(3)L) has been synthesized from diethylene triamine pentaacetic acid (DTPA) and isoniazid. Ligand and its five neutral rare earth (RE=La, Sm, Eu, Gd, Tb) complexes holding promise of magnetic resonance imaging (MRI) were characterized on the basis of elemental analysis, molar conductivity, (1)H-NMR spectrum, FAB-MS, TG-DTA analysis and IR spectrum. The relaxivity (R(1)) of complexes and Gd(DTPA)(2-) used as a control were determined. The relaxivity of LaL, SmL, EuL, GdL, TbL and Gd(DTPA)(2-) were 0.14, 1.66, 3.14, 6.08, 2.79 and 4.34 l.mmol(-1).s(-1), respectively. The spin-lattice relaxivity of GdL was larger than that of Gd(DTPA)(2-). The relaxivity of GdL had also been investigated in human serum albumin (HSA) solution, the relaxivity of GdL was enhanced from 6.08 l.mmol(-1).s(-1) in water solution to 9.09 l.mmol(-1).s(-1) in HSA solution. In addition, thermodynamics stability constant of GdL complex was determined, the thermodynamic stability constant of GdL complex (K(GdL)=10(20.84)) was a few larger than that of Gd(DTPA)(2-) (K(Gd-DTPA)=10(20.73)). The results showed that complex of GdL may be a prospective MRI contrast agent with low osmotic pressure due to non-ion complex, high spin-lattice relaxivity, good stability and binding affinity for the serum protein.     

Gd‐DTPA‐Dopamine‐Bisphytanyl Amphiphile: Synthesis,Characterisation and Relaxation Parameters of the Nanoassemblies and Their Potential as MRI Contrast Agents

Here, a new amphiphilic magnetic resonance imaging (MRI) contrast agent, a Gd^III‐chelated diethylenetriaminepentaacetic acid conjugated to two branched alkyl chains via a dopamine spacer, Gd‐DTPA‐dopamine‐bisphytanyl (Gd‐DTPA‐Dop‐Phy), which is readily capable of self‐assembling into liposomal nanoassemblies upon dispersion in an aqueous solution, is reported. In vitro relaxivities of the dispersions were found to be much higher than Magnevist, a commercially available contrast agent, at 0.47 T but comparable at 9.40 T. Analysis of variable temperature ¹⁷O NMR transverse relaxation measurements revealed the water exchange of the nanoassemblies to be faster than that previously reported for paramagnetic liposomes. Molecular reorientation dynamics were probed by ¹H NMRD profiles using a classical inner and outer sphere relaxation model and a Lipari–Szabo “model‐free” approach. High payloads of Gd^III ions in the liposomal nanoassemblies made solely from the Gd‐DTPA‐Dop‐Phy amphiphiles, in combination with slow molecular reorientation and fast water exchange makes this novel amphiphile a suitable candidate to be investigated as an advanced MRI contrast agent.     

Studies on Hepatocyte-Targeting Magnetic Resonance Imaging Macromolecular Contrast Media

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端氨基聚(醚-氨酯-酰胺)配体及磁共振成像造影剂研究

设计合成了一种端氨基聚(醚-氨酯)PEU-((CH2)6NH2)2(Ⅱ),以Ⅱ为原料制备端氨基聚(醚-氨酯-酰胺)PEU/DTPA共聚物造影剂配体(Ⅲ).Ⅲ的特点为(1)大分子配体的端基为—(CH2)6—NH2,柔性间隔基较长,可克服较大的空间位阻,与靶向性物质(如靶向多肽、蛋白质)反应;(2)原料Ⅱ为采用聚氨酯反应制备,亲水亲油链段长短、种类可控性好,实验条件简单;(3)端氨基可以在温和条件下与多肽等反应.大分子配体Ⅲ与Gd3+络合制备了相应的Gd3+配合物(Ⅳ),研究了Ⅳ的弛豫性能,磁共振成像(MRI)结果表明,合成的造影剂Ⅳ纵向弛豫速率(R1)与医用造影剂钆喷酸葡胺Gd-DTPA相比提高了38.40%,在裸鼠肝脏内信号增强率在0.1、6、12、24 h分别是Gd-DTPA最大信号增强的1.5、3、1.8和1.3倍,在体内的半衰期明显延长.用IR、NMR、GPC、官能团滴定、ICP等方法表征产物.     

Synthesis and Characterization of 2-Decyl-DTPA and Its Gd(Ⅲ) Chelate

The present paper covers the synthesis and the characterization of ligand 2-decyl-3, 6, 9-tris(carboxymethyl)-3,6,9-triazaundecan-1,11-dioic acid, H5L, and its Gd(Ⅲ) chelate. The protonation constants for H5L(lgKHi=10.90, 8.50, 4.55, 2.92, 2.20) and the stability constant for GdL2- (lgKGdL2-=22.80) were determined by means of potentiometric titration. They are similar to the corresponding values of DTPA and Gd-DTPA, respectively. The results obtained show that the basicity of the ligand and the stability constant of its Gd(Ⅲ) chelate are not obviously altered after the introduction of a linear chain decyl group into the terminal acetic acid residue of DTPA. The Gd(Ⅲ) chelate may be a potential contrast agent with liver-specificity for magnetic resonance imaging(MRI).     

Synthesis, thermodynamics stability constant and relaxation properties of neutral Gd(III) complex with derivative from diethylene triamine pentaacetic acid and p-hydroxybenzoyl hydrazine

A novel ligand, diethylenetriamine-N,N'-bis(acetyl-p-hydroxybenzoyl hydrazine)-N,N',N'-triacetic acid (H3L) was synthesized and characterized on the basis of elemental analysis, molar conductivity, 1H-NMR spectrum, FAB-MS, TG-DTA analysis and IR spectrum. Its complex of Gd(III) holding promise of magnetic resonance imaging (MRI) was synthesized, and relaxivity (R1) of complex and Gd(DTPA)2- used as a control was determined in water solution, respectively. The relaxivity of GdL (R1 = 6.39 l.mmol(-1).s(-1)) was larger than that of Gd(DTPA)2- (R1 = 4.34 l.mmol(-1).s(-1)). The relaxivity of GdL has also been investigated in human serum albumin (HSA) solution, the relaxivity of GdL was enhanced from 6.39 l.mmol(-1).s(-1) in water solution to 7.69 l.mmol(-1).s(-1) in HSA solution. In addition, thermodynamics stability constant of GdL was determined. The results showed that complex of GdL is a prospective MRI contrast agent, although the thermodynamic stability constant of GdL complex (K(GdL) = 10(19.56)) was a little less than that of Gd(DTPA)2- (K(Gd-DTPA) = 10(20.73)).