Synthesis, characterization, and relaxivity of two linear Gd(DTPA)-polymer conjugates

Lymph Node Metastases Detection Using Gd2O3@PCD as Novel Multifunctional Contrast Imaging Agent in Metabolic Magnetic Resonance Molecular Imaging

Axillary lymph node detection is crucial to staging and prognosis of the lymph node metastatic spread in breast cancer. Currently, lymphoscintigraphy and blue dye, as the conventional methods to localize sentinel lymph nodes (SLNs), are invasive and can only be performed during surgery. This study has had a novel hybrid gadolinium oxide nanoparticle coating with Cyclodextrin-based polyester as a high-relaxivity T₁ magnetic resonance molecular imaging (MRMI) contrast agent (CA). Twelve female BALB/c mice were randomly divided into three groups of four mice; each group was injected with 4T₁ cells to obtain metastasis lymph nodes and diagnosed by using the 3D T₁W (VIBE) MRI (Siemens 3T, Prisma). The synthesized Gd₂O₃@PCD nanoparticles with a suitable particle size range of 20-40 nm have had much higher longitudinal relaxivity (r ₁) for Gd₂O₃@PCD and Gd-DOTA (Dotarem) with the values of 3.98 mM^-1·s^-1 ± 0.003 and 2.71 mM^-1·s^-1 ± 0.005, respectively. Identical MR images in coronal views were subsequently obtained to create time-intensity curves of the right axillary lymph nodes and to measure the contrast ratio (CR). The peak CR and qualitative assessment of axillary lymph nodes at five-time points were evaluated. After subcutaneous injection, the contrast ratio of axillary lymph node and tumor in mice exhibited CR peak of Gd₂O₃@PCD and Dotarem with the values of 2.21 ± 0.06 and 0.40 ± 0.004 for lymph node and 2.54 ± 0.04 and 1.21 ± 0.007 for the tumor, respectively. Furthermore, the lumbar-aortic lymph node is weakly visible in the original coronal image. In conclusion, the use of Gd₂O₃@PCD nanoparticles as novel MRMI CAs enables high resolution for the detection of lymph node metastasis in mice with the potential capability for breast cancer diagnostic imaging.

Recent nanotheranostics applications for cancer therapy and diagnosis: A review

Nanotheranostics has attracted much attention due to its widespread application in molecular imaging and cancer therapy. Molecular imaging using nanoparticles has attracted special attention in the diagnosis of cancer at early stages. With the progress made in nanotheranostics, studying drug release, accumulation in the target tissue, biodistribution, and treatment effectiveness are other important factors. However, according to the studies conducted in this regard, each nanoparticle has some advantages and limitations that should be examined and then used in clinical applications. The main goal of this review is to explore the recent advancements in nanotheranostics for cancer therapy and diagnosis. Then, it is attempted to present recent studies on nanotheranostics used as a contrast agent in various imaging modalities and a platform for cancer therapy.

Glucosamine Conjugated Gadolinium (III) Oxide Nanoparticles as a Novel Targeted Contrast Agent for Cancer Diagnosis in MRI

BACKGROUND

Glucose transporter (Glut), a cellular transmembrane receptor, has a key role in the metabolism of cell glucose and is also associated with various human carcinomas.

OBJECTIVE

In this study, we evaluated a magnetic resonance (MR) imaging contrast agent for tumor detection based on paramagnetic gadolinium oxide (Gd₂O₃) coated polycyclodextrin (PCD) and modified with glucose (Gd₂O₃@PCD-Glu) for the targeting of overexpressed glucose receptors.

MATERIAL AND METHODS

In this experimental study, 3T magnetic resonance imaging (MRI) scanner was used to assess the specific interactions between Glut1-overexpressing tumor cells (MDA-MB-231) and Gd₂O₃@PCD-Glu NPs. Furthermore, the capacity of transporting Gd₂O₃@PCD-Glu NPs to tumor cells was evaluated.

RESULTS

It was found that the acquired MRI T₁ signal intensity of MDA-MB-231 cells that were treated with the Gd₂O₃@PCD-Glu NPs increased significantly. Based on the results obtained, Gd₂O₃@PCD-Glu NPs can be applied in targeting Glut1-overexpressing tumor cells in vivo, as well as an MRI-targeted tumor agent to enhance tumor diagnosis.

CONCLUSION

Results have shown that glucose-shell of magnetic nanoparticles has a key role in diagnosing cancer cells of high metabolic activity.

Core–shell particles for drug-delivery, bioimaging, sensing, and tissue engineering

Core–shell particles offer significant advantages in their use for bioimaging and biosensors.

In vitro and in vivo characteristics of doxorubicin-loaded cyclodextrine-based polyester modified gadolinium oxide nanoparticles: a versatile targeted theranostic system for tumour chemotherapy and molecular resonance imaging

β-Cyclodextrine-based polyester was coated on the surface of gadolinium oxide nanoparticles (NPs) and then functionalised with folic acid to produce an efficient pH-sensitive targeted theranostic system (Gd₂O₃@PCD-FA) for doxorubicin delivery and magnetic resonance imaging (MRI). Gd₂O₃@PCD-FA was fully characterised by FTIR, vibrating sample magnetometer, TGA, XRD, SEM and TEM analyses. The dissolution profile of DOX showed a pH sensitive release. No significant toxicity was observed for the targeted NPs (Gd₂O₃@PCD-FA) and DOX-loaded NPs inhibiting M109 cells viability more efficiently than free DOX. Moreover, the negligible hemolytic activity of the targeted NPs showed their appropriate hemocompatibility. The preferential uptake was observed for the developed Gd₂O₃@PCD-FA-DOX NPs in comparison with Dotarem using T₁- and T₂-weighted MRI in the presence of folate receptor-positive and folate receptor-negative cancer cells (M109 and 4T1, respectively). Furthermore, in vivo studies revealed that Gd₂O₃@PCD-FA-DOX not only exhibited considerably relaxivity performance as a contrast agent for MRI, but also improved in vivo anti-tumour efficacy of the system. The results suggest that Gd₂O₃@PCD-FA-DOX improves its therapeutic efficacy in the treatment of solid tumours and also reduces the adverse effects, so it could be proposed as a promising drug delivery system for chemotherapy and molecular imaging diagnosis in MRI.

Gadolinium (III) oxide nanoparticles coated with folic acid-functionalized poly(β-cyclodextrin-co-pentetic acid) as a biocompatible targeted nano-contrast agent for cancer diagnostic: in vitro and in vivo studies

OBJECTIVES

In this study, a novel targeted MRI contrast agent was developed by coating gadolinium oxide nanoparticles (Gd₂O₃ NPs) with β-cyclodextrin (CD)-based polyester and targeted by folic acid (FA).

MATERIALS AND METHODS

The developed Gd₂O₃@PCD-FA MRI contrast agent was characterized and evaluated in relaxivity, in vitro cell targeting, cell toxicity, blood compatibility and in vivo tumor MR contrast enhancement.

RESULTS

In vitro cytotoxicity and hemolysis assays revealed that Gd₂O₃@PCD-FA NPs have no significant cytotoxicity after 24 and 48 h against normal human breast cell line (MCF-10A) at concentration of up to 50 µg Gd⁺³/mL and have high blood compatibility at concentration of up to 500 µg Gd⁺³/mL. In vitro MR imaging experiments showed that Gd₂O₃@PCD-FA NPs enable targeted contrast T₁- and T₂-weighted MR imaging of M109 as overexpressing folate receptor cells. Besides, the in vivo analysis indicated that the maximum contrast-to-noise ratio (CNR) of tumor in mice increased after injection of Gd₂O₃@PCD-FA up to 5.89 ± 1.3 within 1 h under T₁-weighted imaging mode and reduced to 1.45 ± 0.44 after 12 h. While CNR increased up to maximum value of 1.98 ± 0.28 after injection of Gd₂O₃@PCD within 6 h and reduced to 1.12 ± 0.13 within 12 h.

CONCLUSION

The results indicate the potential of Gd₂O₃@PCD-FA to serve as a novel targeted nano-contrast agent in MRI.

Design and Control of Nanoconfinement to Achieve Magnetic Resonance Contrast Agents with High Relaxivity

The enhanced relaxation of hydrogen atoms of surrounding water from suitable contrast agent promotes magnetic resonance imaging as one of the most important medical diagnosis technique. The key challenge for the preparation of performant contrast agents for magnetic resonance imaging with high relaxivity is to ensure a high local concentration of contrast agent while allowing a contact between water and the contrast agent. Both requirements are answered by tailoring a semipermeable confinement for a gadolinium complex used as contrast agent. A locally high concentration is achieved by successfully encapsulating the complex in polymer nanocontainers that serves to protect and retain the complex inside a limited space. The access of water to the complex is achieved by carefully controlling the chemistry of the shell and the core of the nanocontainers. The confinement of the nanocontainers enables an increased relaxivity compared to an aqueous solution of the contrast agent. The nanocontainers are successfully applied in vivo to yield enhanced contrast in magnetic resonance imaging.

Hydroxylated Mesoporous Nanosilica Coated by Polyethylenimine Coupled with Gadolinium and Folic Acid: A Tumor-Targeted T(1) Magnetic Resonance Contrast Agent and Drug Delivery System

A pH-responsive nanoplatform, hydroxylated mesoporous nanosilica (HMNS) coated by polyethylenimine (PEI) coupled with gadolinium and folic acid (FA) (Gd-FA-Si), was designed to deliver anticancer drug targeting and to promote contrast effect for tumor cells using magnetic resonance (MR) spectrometer. Doxorubicin (DOX) was chosen as the anticancer drug and loaded into nanopores of HMNS, then its release in simulated body fluid could be controlled through adjusting the pH. This nanoplatform could significantly enhance the MR contrast effect, and the highest theoretical relaxivity per nanoplatform could even be approximately 1.28 × 10(6) mm(-1)s(-1) because of the high Gd payload (2.61 × 10(5) per nanoplatform). The entire system possessed a high targeting performance to Hela and MDA-MB-231 cells because the FA located in the system could specifically bind to the folate-receptor sites on the surface of cell. Compared with free DOX, the nanoplatform presented a higher cell inhibition effect on the basis of cell assay. Therefore, this nanoplatform could be potentially applied as a tumor-targeted T1 MR contrast agent and pH-sensitive drug carrier system.

The Present and the Future of Degradable Dendrimers and Derivatives in Theranostics

Interest in dendrimer-based nanomedicines has been growing recently, as it is possible to precisely manipulate the molecular weight, chemical composition, and surface functionality of dendrimers, tuning their properties according to the desired biomedical application. However, one important concern about dendrimer-based therapeutics remains-the nondegradability under physiological conditions of the most commonly used dendrimers. Therefore, biodegradable dendrimers represent an attractive class of nanomaterials, since they present advantages over conventional nondegradable dendrimers regarding the release of the loaded molecules and the prevention of bioaccumulation of synthetic materials and subsequent cytotoxicity. Here, we present an overview of the state-of-the-art of the design of biodegradable dendritic structures, with particular focus on the hurdles regarding the use of these as vectors of drugs and nucleic acids, as well as macromolecular contrast agents.

Molecular platform for design and synthesis of targeted dual-modality imaging probes

We report a versatile dendritic structure based platform for construction of targeted dual-modality imaging probes. The platform contains multiple copies of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) branching out from a 1,4,7-triazacyclononane-N,N′,N″-triacetic acid (NOTA) core. The specific coordination chemistries of the NOTA and DOTA moieties offer specific loading of ^68/67Ga³⁺ and Gd³⁺, respectively, into a common molecular scaffold. The platform also contains three amino groups which can potentiate targeted dual-modality imaging of PET/MRI or SPECT/MRI (PET: positron emission tomography; SPECT: single photon emission computed tomography; MRI: magnetic resonance imaging) when further functionalized by targeting vectors of interest. To validate this design concept, a bimetallic complex was synthesized with six peripheral Gd-DOTA units and one Ga-NOTA core at the center, whose ion T₁ relaxivity per gadolinium atom was measured to be 15.99 mM^–1 s^–1 at 20 MHz. Further, the bimetallic agent demonstrated its anticipated in vivo stability, tissue distribution, and pharmacokinetic profile when labeled with ⁶⁷Ga. When conjugated with a model targeting peptide sequence, the trivalent construct was able to visualize tumors in a mouse xenograft model by both PET and MRI via a single dose injection.

Biodegradable polydisulfide dendrimer nanoclusters as MRI contrast agents

Gadolinium-conjugated dendrimer nanoclusters (DNCs) are a promising platform for the early detection of disease; however, their clinical utility is potentially limited due to safety concerns related to nephrogenic systemic fibrosis (NSF). In this paper, biodegradable DNCs were prepared with polydisulfide linkages between the individual dendrimers to facilitate excretion. Further, DNCs were labeled with premetalated Gd chelates to eliminate the risk of free Gd becoming entrapped in dendrimer cavities. The biodegradable polydisulfide DNCs possessed a circulation half-life of >1.6 h in mice and produced significant contrast enhancement in the abdominal aorta and kidneys for as long as 4 h. The DNCs were reduced in circulation as a result of thiol-disulfide exchange, and the degradation products were rapidly excreted via renal filtration. These agents demonstrated effective and prolonged in vivo contrast enhancement and yet minimized Gd tissue retention. Biodegradable polydisulfide DNCs represent a promising biodegradable macromolecular MRI contrast agent for magnetic resonance angiography and can potentially be further developed into target-specific MRI contrast agents.

Macromolecular Imaging Agents Containing Lanthanides: Can Conceptual Promise Lead to Clinical Potential?

Macromolecular magnetic resonance imaging (MRI) contrast agents are increasingly being used to improve the resolution of this noninvasive diagnostic technique. All clinically-approved T₁ contrast agents are small molecule chelates of gadolinium [Gd(III)] that affect bound water proton relaxivity. Both the small size and monomeric nature of these agents ultimately limits the image resolution enhancement that can be achieved for both contrast enhancement and pharmacokinetic/biodistribution reasons. The multimeric nature of macromolecules, such as polymers, dendrimers, and noncovalent complexes of small molecule agents with proteins, have been shown to significantly increase the image contrast and resolution due to their large size and ability to incorporate multiple Gd(III) chlelation sites. Also, macromolecular agents are advantageous as they have the ability to be designed to be nontoxic, hydrophilic, easily purified, aggregation-resistant, and have controllable three-dimensional macromolecular structure housing the multiple lanthanide chelation sites. For these reasons, large molecule diagnostics have the ability to significantly increase the relaxivity of water protons within the targeted tissues and thus the image resolution for many diagnostic applications. The FDA approval of a contrast agent that consists of a reversible, non-covalent coupling of a small Gd(III) chelate with serum albumin for blood pool imaging (marketed under the trade names of Vasovist and Ablivar) proved to be one of the first diagnostic agent to capitalize on these benefits from macromolecular association in humans. However, much research and development is necessary to optimize the safety of these unique agents for in vivo use and potential clinical development. To this end, recent work in the field of polymer, dendrimer, and noncovalent complex-based imaging agents are reviewed herein and the future outlook of this field is discussed.

Evaluation of poly(amidoamine) dendrimers as potential carriers of iminodiacetic derivatives using solubility studies and 2D-NOESY NMR spectroscopy

The interactions between dendrimers and different types of drugs are nowadays one of the most actively investigated areas of the pharmaceutical sciences. The interactions between dendrimers and drugs can be divided into: internal encapsulation, external electrostatic interaction, and covalent conjugation. In the present study, we investigated the potential of poly(amidoamine) (PAMAM) dendrimers for solubility of four iminodiacetic acid derivatives. We reported that PAMAM dendrimers contribute to significant solubility enhancement of iminodiacetic acid analogues. The nature of the dendrimer–drug complexes was investigated by ¹H NMR and 2D-NOESY spectroscopy. The ¹H NMR analysis proved that the water-soluble supramolecular structure of the complex was formed on the basis of ionic interactions between terminal amine groups of dendrimers and carboxyl groups of drug molecules, as well as internal encapsulation. The 2D-NOESY analysis revealed interactions between the primary amine groups of PAMAM dendrimers and the analogues of iminodiacetic acid. The results of solubility studies together with ¹H NMR and 2D-NOESY experiments suggest that the interactions between PAMAM dendrimers of generation 1–4 and derivatives of iminodiacetic acid are based on electrostatic interactions and internal encapsulation.

Block copolymer-based gadolinium nanoparticles as MRI contrast agents with high T1 relaxivity

AIMS

To synthesize block copolymer-based gadolinium nanoparticles (Gd-NPs) and evaluate their potential as a new MRI contrast agent.

MATERIALS & METHODS

The Gd-NPs were developed through coordination of gadolinium chelates into a pentablock copolymer synthesized by two sequential atom transfer radical polymerizations, and evaluated by measuring their T1 relaxivity in vitro and imaging their T1 contrast in tissue using MRI.

RESULTS

Solution and MRI studies demonstrated that these Gd-NPs exhibit remarkably high T1 relaxivity on both per particle and per gadolinium ion basis, and can generate a much better MRI contrast compared with gadolinium chelates alone.

CONCLUSION

These novel Gd-NPs may have important applications in magnetic resonance-based medical imaging.

New magnetic-resonance-imaging-visible poly(ε-caprolactone)-based polyester for biomedical applications

A great deal of effort has been made since the 1990s to enlarge the field of magnetic resonance imaging. Better tissue contrast, more biocompatible contrast agents and the absence of any radiation for the patient are some of the many advantages of using magnetic resonance imaging (MRI) rather than X-ray technology. But implantable medical devices cannot be visualized by conventional MRI and a tool therefore needs to be developed to rectify this. The synthesis of a new MRI-visible degradable polymer is described by grafting an MR contrast agent (DTPA-Gd) to a non-water-soluble, biocompatible and degradable poly(ε-caprolactone) (PCL). The substitution degree, calculated by (1)H nuclear magnetic resonance and inductively coupled plasma-mass spectrometry, is close to 0.5% and proves to be sufficient to provide a strong and clear T1 contrast enhancement. This new MRI-visible polymer was coated onto a commercial mesh for tissue reinforcement using an airbrush system and enabled in vitro MR visualization of the mesh for at least 1 year. A stability study of the DTPA-Gd-PCL chelate in phosphate-buffered saline showed that a very low amount of gadolinium was released into the medium over 52 weeks, guaranteeing the safety of the device. This study shows that this new MRI-visible polymer has great potential for the MR visualization of implantable medical devices and therefore the post-operative management of patients.

Strategies for the preparation of bifunctional gadolinium(III) chelators

The development of gadolinium chelators that can be easily and readily linked to various substrates is of primary importance for the development high relaxation efficiency and/or targeted magnetic resonance imaging (MRI) contrast agents. Over the last 25 years a large number of bifunctional chelators have been prepared. For the most part, these compounds are based on ligands that are already used in clinically approved contrast agents. More recently, new bifunctional chelators have been reported based on complexes that show a more potent relaxation effect, faster complexation kinetics and in some cases simpler synthetic procedures. This review provides an overview of the synthetic strategies used for the preparation of bifunctional chelators for MRI applications.

A small MRI contrast agent library of gadolinium(III)-encapsulated supramolecular nanoparticles for improved relaxivity and sensitivity

We introduce a new category of nanoparticle-based T(1) MRI contrast agents (CAs) by encapsulating paramagnetic chelated gadolinium(III), i.e., Gd(3+)·DOTA, through supramolecular assembly of molecular building blocks that carry complementary molecular recognition motifs, including adamantane (Ad) and β-cyclodextrin (CD). A small library of Gd(3+)·DOTA-encapsulated supramolecular nanoparticles (Gd(3+)·DOTA⊂SNPs) was produced by systematically altering the molecular building block mixing ratios. A broad spectrum of relaxation rates was correlated to the resulting Gd(3+)·DOTA⊂SNP library. Consequently, an optimal synthetic formulation of Gd(3+)·DOTA⊂SNPs with an r(1) of 17.3 s(-1) mM(-1) (ca. 4-fold higher than clinical Gd(3+) chelated complexes at high field strengths) was identified. T(1)-weighted imaging of Gd(3+)·DOTA⊂SNPs exhibits an enhanced sensitivity with a contrast-to-noise ratio (C/N ratio) ca. 3.6 times greater than that observed for free Gd(3+)·DTPA. A Gd(3+)·DOTA⊂SNPs solution was injected into foot pads of mice, and MRI was employed to monitor dynamic lymphatic drainage of the Gd(3+)·DOTA⊂SNPs-based CA. We observe an increase in signal intensity of the brachial lymph node in T(1)-weighted imaging after injecting Gd(3+)·DOTA⊂SNPs but not after injecting Gd(3+)·DTPA. The MRI results are supported by ICP-MS analysis ex vivo. These results show that Gd(3+)·DOTA⊂SNPs not only exhibits enhanced relaxivity and high sensitivity but also can serve as a potential tool for diagnosis of cancer metastasis.

Polydisulfide Based Biodegradable Macromolecular Magnetic Resonance Imaging Contrast Agents

Macromolecular Gd(III) complexes are advantageous over small molecular Gd(III) complexes in contrast enhanced magnetic resonance imaging (MRI) because of their prolonged blood circulation and preferential tumor accumulation. However, macromolecular contrast agents have not been approved for clinical applications because of the safety concerns related to their slow body excretion. Polydisulfide Gd(III) complexes have been designed and developed as biodegradable macromolecular MRI contrast agents to alleviate the concerns by facilitating the clearance of Gd(III) complexes from the body. These agents initially behave as macromolecular agents and result in superior contrast enhancement in the vasculature and tumor tissues. They can then be readily degraded in vivo into small molecular chelates that can rapidly excrete from the body via renal filtration after the MRI examinations. Various polydisulfide Gd(III) complexes have been prepared as biodegradable macromolecular MRI contrast agents. These agents have resulted in strong contrast enhancement in the vasculature and tumor tissue in animal models with minimal long-term tissue accumulation comparable to small molecular contrast agents. Polydisulfide Gd(III) complexes are promising for further clinical development as safe and effective biodegradable macromolecular MRI contrast agents for cardiovascular and cancer imaging. The review summarizes the chemistry and properties of polydisulfide Gd(III) complexes.

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.

Gd(III)-EPTPAC16, a new self-assembling potential liver MRI contrast agent: in vitro characterization and in vivo animal imaging studies

The recently reported amphiphilic chelate, [Gd(EPTPAC16)(H2O)]2-, forms supramolecular aggregates in aqueous solution by self-assembly of the monomers with a relaxometrically determined critical micellar concentration (CMC) of 0.34 mM. The effect of sonication on the aggregate size was characterized by dynamic light scattering and relaxometry, indicating the presence of premicellar aggregates and an overall decrease in aggregate size and polydispersity upon sonication, slightly below the CMC. [[153Sm](EPTPAC16)(H2O)]2- radiotracer was evaluated in vivo from gamma scintigraphy and biodistribution in Wistar rats. It was found to depend strongly on the sample concentration, below or above the CMC, and its sonication, in a way that correlates with the effect of the same factors on the size of the aggregates formed in solution. Below CMC, the very large aggregates of the [153Sm]3+ -labeled chelate were persistently and mainly taken up by the lungs, and also by the macrophage-rich liver and spleen. Sonication of this solution led to loss of the lung uptake. Above CMC, the metal chelate was mainly taken up by the liver, with very little uptake by the spleen and lungs. In vivo, dynamic contrast-enhanced (DCE)-MRI evaluation of the micellar [Gd(EPTPAC16)(H2O)]2- compound in Wistar rats showed a persistent hepatic positive-contrast effect in T1-weighted images, qualitatively similar to the clinically established Gd(III)-based hepatobiliary-selective agents. No enhancement effect was observed in the lungs because of the scarcity of mobile protons in this organ, despite the scintigraphic evidence of significant lung retention of the [153Sm]3+ -labeled chelate at concentrations below the CMC.

Polydisulfide Gd(III) chelates as biodegradable macromolecular magnetic resonance imaging contrast agents

Macromolecular gadolinium (Gd)(III) complexes have a prolonged blood circulation time and can preferentially accumulate in solid tumors, depending on the tumor blood vessel hyperpermeability, resulting in superior contrast enhancement in magnetic resonance (MR) cardiovascular imaging and cancer imaging as shown in animal models. Unfortunately, safety concerns related to these agents’ slow elimination from the body impede their clinical development. Polydisulfide Gd(III) complexes have been designed and developed as biodegradable macromolecular magnetic resonance imaging (MRI) contrast agents to facilitate the clearance of Gd(III) complexes from the body after MRI examinations. These novel agents can act as macromolecular contrast agents for in vivo imaging and excrete rapidly as low-molecular-weight agents. The rationale and recent development of the novel biodegradable contrast agents are reviewed here. Polydisulfide Gd(III) complexes have relatively long blood circulation time and gradually degrade into small Gd(III) complexes, which are rapidly excreted via renal filtration. These agents result in effective and prolonged in vivo contrast enhancement in the blood pool and tumor tissue in animal models, yet demonstrate minimal Gd(III) tissue retention as the clinically used low-molecular-weight agents. Structural modification of the agents can readily alter the contrast-enhancement kinetics. Polydisulfide Gd(III) complexes are promising for further clinical development as safe, effective, biodegradable macromolecular MRI contrast agents for cardiovascular and cancer imaging, and for evaluation of therapeutic response.

Rotational Dynamics Account for pH-Dependent Relaxivities of PAMAM Dendrimeric, Gd-Based Potential MRI Contrast Agents

The EPTPA5) chelate, which ensures fast water exchange in GdIII complexes, has been coupled to three different generations (5, 7, and 9) of polyamidoamine (PAMAM) dendrimers through benzylthiourea linkages (H5EPTPA = ethylenepropylenetriamine-N,N,N',N'',N''-pentaacetic acid). The proton relaxivities measured at pH 7.4 for the dendrimer complexes G5-(GdEPTPA)111, G7-(GdEPTPA)253 and G9-(GdEPTPA)1157 decrease with increasing temperature, indicating that, for the first time for dendrimers, slow water exchange does not limit relaxivity. At a given field and temperature, the relaxivity increases from G5 to G7, and then slightly decreases for G9 (r1 = 20.5, 28.3 and 27.9 mM(-1) s(-1), respectively, at 37 degrees C, 30 MHz). The relaxivities show a strong and reversible pH dependency for all three dendrimer complexes. This originates from the pH-dependent rotational dynamics of the dendrimer skeleton, which was evidenced by a combined variable-temperature and multiple-field 17O NMR and 1H relaxivity study performed at pH 6.0 and 9.9 on G5-(GdEPTPA)111. The longitudinal 17O and 1H relaxation rates of the dendrimeric complex are strongly pH-dependent, whereas they are not for the [Gd(EPTPA)(H2O)]2- monomer chelate. The longitudinal 17O and 1H relaxation rates have been analysed by the Lipari-Szabo spectral density functions and correlation times have been calculated for the global motion of the entire macromolecule (tau(gO)) and the local motion of the GdIII chelates on the surface (tau(lO)), correlated by means of an order parameter S2. The dendrimer complex G5-(GdEPTPA)111 has a considerably higher tau(gO) under acidic than under basic conditions (tau(298)gO = 4040 ps and 2950 ps, respectively), while local motions are less influenced by pH (tau(298)lO = 150 and 125 ps). The order parameter, characterizing the rigidity of the macromolecule, is also higher at pH 6.0 than at pH 9.9 (S2 = 0.43 vs 0.36, respectively). The pH dependence of the global correlation time can be related to the protonation of the tertiary amine groups in the PAMAM skeleton, which leads to an expanded and more rigid dendrimeric structure at lower pH. The increase of tau(gO) with decreasing pH is responsible for the pH dependent proton relaxivities. The water exchange rate on G5-(GdEPTPA)111(k(298)ex = 150 x 10(6) s(-1)) shows no significant pH dependency and is similar to the one measured for the monomer [Gd(EPTPA)(H2O)]2-. The proton relaxivity of G5-(GdEPTPA)111 is mainly limited by the important flexibility of the dendrimer structure, and to a small extent, by a faster than optimal water exchange rate.

Comparison between Gd-DTPA and several bisamide derivatives as potential MRI contrast agents

Four neutral gadolinium complexes of diethylenetriaminepentaacetic acid (DTPA)-bisamide derivatives have been synthesized and characterized. Their potential application as tissue-specific and low-osmolarity MRI contrast agents has been evaluated by in vitro and in vivo experiments. Their measured relaxivities in D(2)O, bovine serum albumin and human serum transferrin solutions showed favorable relaxation ability. In vivo studies have proven that Gd(DTPA-BDMA), Gd(DTPA-BIN), and Gd(cyclic-DTPA-1,2-pn) could be promising liver-specific MRI contrast agents and Gd(DTPA-BDMA), and Gd(cyclic-DTPA-1,2-pn) have favorable renal excretion capability. Among them, Gd(cyclic-DTPA-1,2-pn) is a more powerful hepatic contrast agent and Gd(DTPA-BIN) provides the stable imaging contrast for several hours. They also show a lower toxicity.

Extracellular biodegradable macromolecular gadolinium(III) complexes for MRI

The clinical application of macromolecular gadolinium (Gd) complexes as MRI contrast agents is limited by the slow excretion of Gd(III) complexes and consequent long-term tissue accumulation of toxic Gd ions. To alleviate the problem of slow excretion, biodegradable polydisulfide-based macromolecular Gd(III) complexes were designed and prepared based on the disulfide-thiol exchange to allow degradation of the macromolecules by endogenous thiols and to facilitate excretion of Gd(III) complexes after the MRI examination. The in vitro degradation study showed that the polydisulfide agent was readily degraded by cysteine at plasma thiol concentrations. No cross-reaction was observed between the cysteine-34 on human serum albumin (HSA) with the agent. Concentration-dependent blood pool contrast enhancement was observed for the polydisulfide agents. The agents of both high molecular weight (35,000 Da) and low molecular weight (17,700 Da) produced significant contrast enhancement in the heart and aorta in rats at relatively high doses. Except for the bladder, the signal intensities gradually decreased over time. Significant blood pool contrast enhancement was also observed for the high molecular weight agent at a low dose (0.03 mmol-Gd/kg), but not for the agent with a lower molecular weight. The contrast enhancement in the urinary bladder increased over time for the polydisulfide agents and Gd(III)-(DTPA-BMA). Degradation products were identified by mass spectrometry in the urine samples from the rats administered with both polydisulfide agents, which confirmed that the polydisulfide agents were degraded in vivo and excreted through renal filtration. The preliminary results demonstrated the in vitro and in vivo degradability, superior blood pool contrast enhancement, and rapid clearance through renal filtration of the novel biodegradable macromolecular agent. This agent has a great potential for further preclinical and clinical development with application in contrast-enhanced blood pool and cancer MR imaging.