In Vivo evaluation of a PAMAM-cystamine-(Gd-DO3A) conjugate as a biodegradable macromolecular MRI contrast agent

Macromolecular Gd(III) chelates are superior magnetic resonance imaging (MRI) contrast agents for blood pool and tumor imaging. However, their clinical development is limited by the safety concerns related to the slow excretion and long-term gadolinium tissue accumulation. A generation 6 PAMAM Gd(III) chelate conjugate with a cleavable disulfide spacer, PAMAM-G6-cystamine-(Gd-DO3A), was prepared as a biodegradable macromolecular MRI contrast agent with rapid excretion from the body. T(1) and T(2) relaxivities of the contrast agent were 11.6 and 13.3 mM(-1)sec(-1) at 3T, respectively. Blood pool and tumor contrast enhancement of the agent were evaluated in female nude mice bearing MDA-MB-231 human breast carcinoma xenografts with a nondegradable conjugate PAMAM-G6-(Gd-DO3A) as a control. PAMAM-G6-cystamine-(Gd-DO3A) resulted in significant contrast enhancement in the blood for about 5 mins, and Gd-DO3A was released from the conjugate and rapidly excreted via renal filtration after the disulfide spacer was cleaved. The nondegradable control had much longer blood circulation and excreted more slowly from the body. PAMAM-G6-cystamine-(Gd-DO3A) also resulted in more prominent tumor contrast enhancement than the control. However, PAMAM-G6-cystamine-(Gd-DO3A) demonstrated high toxicity due to the intrinsic toxicity of PAMAM dendrimers. In conclusion, although PAMAM-G6-cystamine-(Gd-DO3A) showed some advantages compared with the nondegradable control, PAMAM dendrimers are not suitable carriers for biodegradable macromolecular MRI contrast agents, due to their high toxicity.     

PEG-g-poly(GdDTPA-co-L-cystine): effect of PEG chain length on in vivo contrast enhancement in MRI

Biodegradable macromolecular Gd(III) complexes, Gd-DTPA cystine copolymers (GDCP), were grafted with PEG of different sizes to modify the physicochemical properties and in vivo MRI contrast enhancement of the agents and to study the effect of PEG chain length on these properties. Three new PEG-grafted biodegradable macromolecular gadolinium(III) complexes were synthesized and characterized as blood pool MRI contrast agents. One of three different lengths of MPEG-NH(2) (MW = 550, 1000, and 2000) was grafted to the backbone of GDCP to yield PEG(n)()-g-poly(GdDTPA-co-l-cystine), PEG(n)()-GDCP. The PEG chain length did not dramatically alter the T(1) relaxivity, r(1), of the modified agents. The MRI enhancement profile of PEG(n)()-GDCP with different PEG sizes was significantly different in mice with respect to both signal intensity and clearance profiles. PEG(2000)-GDCP showed more prominent enhancement in the blood pool for a longer period of time than either PEG(1000)-GDCP or PEG(550)-GDCP. In the kidney, PEG(2000)-GDCP had less enhancement at 2 min than PEG(1000)-GDCP, but both PEG(550)-GDCP and PEG(1000)-GDCP showed a more pronounced signal decay thereafter. The three agents behaved similarly in the liver, as compared to that in the heart. All three agents showed little enhancement in the muscle. Chemical grafting with PEG of different chain lengths is an effective approach to modify the physiochemistry and in vivo contrast enhancement dynamics of the biodegradable macromolecular contrast agents. The novel agents are promising for further clinical development for cardiovascular and cancer MR imaging.     

Poly(l-glutamic acid) Gd(III)-DOTA conjugate with a degradable spacer for magnetic resonance imaging

The clinical application of macromolecular Gd(III) complexes as MRI contrast agents is impeded by their slow excretion and potential toxicity due to the release of Gd(III) ions caused by the metabolism of the agents. A polymer Gd(III) chelate conjugate with a cleavable spacer has been designed to solve this problem. Poly(l-glutamic acid)-cystamine-[Gd(III)-DOTA] was prepared by the conjugation of DOTA to PGA (MW = 50,000) via cystamine, a cleavable disulfide spacer, followed by the complexation with GdCl(3). A Gd(III) DOTA chelate derivative was readily released from the polymer conjugate in the incubation with cysteine, an endogenous plasma thiol. The conjugate produced significant MRI blood pool contrast enhancement in nude mice bearing OVCAR-3 human ovarian carcinoma xenographs. Less significant contrast enhancement was observed for a small molecular contrast agent, Gd(DTPA-BMA). The pharmacokinetic MRI study showed that the Gd(III) chelate from the conjugate accumulated in the urinary bladder in a similar kinetic pattern to Gd(DTPA-BMA), suggesting that the chelate was released by the endogenous thiols and excreted through renal filtration. The preliminary results suggest that this novel design has a great potential to solve the safety problem of macromolecular MRI contrast agents.     

PEG-g-poly(GdDTPA-co-L-cystine): a biodegradable macromolecular blood pool contrast agent for MR imaging

Biodegradable PEGylated Gd-DTPA l-cystine copolymers, PEG-g-poly(GdDTPA-co-l-cystine), were prepared and tested as a blood pool contrast agent in mice. The biodegradable macromolecular agent was designed to be broken down into smaller Gd complexes by endogenous thiols via the disulfide-thiol exchange reaction to facilitate the clearance of Gd complexes after the contrast-enhanced MRI examination. Gd-DTPA l-cystine copolymers were synthesized by condensation polymerization of l-cystine and DTPA-dianhydride in water followed by chelating with Gd(OAc)(3). MPEG-NH(2) (MW = 2000) was then conjugated to the polymeric backbone in different ratios. The macromolecular contrast agent was readily degraded with the incubation of l-cysteine. It also demonstrated superior contrast enhancement in the heart and blood vessels as compared to a low molecular weight control agent, Gd-(DTPA-BMA). At 1 h postcontrast, the PEGylated macromolecular agent still showed prominent enhancement, while little contrast enhancement was detectable in the blood pool by the control agent. PEG-g-poly(GdDTPA-co-l-cystine) shows promise as an MR blood pool imaging agent.     

Synthesis and evaluation of globular Gd-DOTA-monoamide conjugates with precisely controlled nanosizes for magnetic resonance angiography

The purpose of this study was to design and prepare macromolecular contrast agents (CAs) with a precisely defined globular structure for MR angiography and tumor angiogenesis imaging. Generations 1 through 3 (Gd-DOTA-monoamide)-poly-L-lysine octasilsesquioxane dendrimers were prepared as nanoglobular MRI CAs. The nanoglobular Gd(III) chelates had a well-defined compact globular structure and high loading of Gd-DOTA-monoamide at their surface. The size of the G1, G2, and G3 nanoglobular MRI CAs was approximately 2.0, 2.4, and 3.2 nm, respectively. The T1 relaxivity of G1, G2, and G3 nanoglobular MRI CAs was approximately 6.4, 7.2, and 10.0 mM(-1) sec(-1) at 3T, respectively. The nanoglobular MRI CAs showed size-dependent contrast enhancement within the mouse vasculature, which gradually decayed to baseline after a 60 min session. The G3 nanoglobular CA resulted in more significant and prolonged vascular enhancement than the smaller nanoglobular agents at 0.03 mmol Gd/kg. The G3 agent also provided significant and prolonged contrast enhancement in the heart and vasculature at a dose as low as 0.01 mmol Gd/kg, 1/10th of the regular clinical dose. Significant enhancement was observed in tumor for all CAs. The nanoglobular CAs cleared via renal filtration and accumulated in the urinary bladder as shown in the dynamic MR images. The nanoglobular Gd(III) chelates are effective intravascular MRI CAs at substantially reduced doses. The nanoglobular MRI CAs are promising for further preclinical development for MR angiography and MR imaging of tumor angiogenesis.     

Synthesis and characterization of new porphyrazine-Gd(III) conjugates as multimodal MR contrast agents

Magnetic resonance imaging (MRI) has long been used clinically and experimentally as a diagnostic tool to obtain three-dimensional, high-resolution images of deep tissues. These images are enhanced by the administration of contrast agents such as paramagnetic Gd(III) complexes. Herein, we describe the preparation of a series of multimodal imaging agents in which paramagnetic Gd(III) complexes are conjugated to a fluorescent tetrapyrrole, namely, a porphyrazine (pz). Zinc metalated pzs conjugated to one, four, or eight paramagnetic Gd(III) complexes are reported. Among these conjugates, Zn-Pz-8Gd(III) exhibits an ionic relaxivity four times that of the monomeric Gd(III) agent, presumably because of increased molecular weight and a molecular relaxivity that is approximately thirty times larger, while retaining the intense electronic absorption and emission of the unmodified pz. Unlike current clinical MR agents, Zn-Pz-1Gd(III) is taken up by cells. This probe demonstrates intracellular fluorescence by confocal microscopy and provides significant contrast enhancement in MR images, as well as marked phototoxicity in assays of cellular viability. These results suggest that pz agents possess a new potential for use in cancer imaging by both MRI and near-infrared (NIR) fluorescence, while acting as a platform for photodynamic therapy.     

New paramagnetic supramolecular adducts for MRI applications based on non-covalent interactions between Gd(III)-complexes and beta- or gamma-cyclodextrin units anchored to chitosan

Gd(III) complexes are used as magnetic resonance imaging (MRI) contrast agents because they greatly enhance the relaxation rate of water protons of tissues in which they distribute, an effect that is much more marked if the paramagnetic complex is part of a macromolecular system. Furthermore applications in molecular imaging, require that as many units of contrast agent as possible be directed to the site of interest. To this end we synthesised a polymer made of chitosan functionalized with beta- and gamma-cyclodextrins (CDs) that is able to form high-affinity adducts with suitably functionalized Gd(III) complexes. beta- and gamma-CDs were first treated with maleic anhydride to afford 6-monosubstituted derivatives that reacted regioselectively with the amino groups of chitosan. Reaction times and yields were markedly improved by carrying out these reactions under high-intensity ultrasound or microwave irradiation. Compared to the CD monomers, beta- and gamma-CD-chitosan adducts show large increases both in terms of their binding affinity towards Gd(III) complexes and in relaxivity values and they appear promising carriers for the in vivo vehiculation of Gd(III) complexes.     

Innovative magnetic resonance imaging diagnostic agents based on paramagnetic Gd(III) complexes

Gd(III) complexes are under intense scrutiny as contrast agents for magnetic resonance imaging (MRI). They act by enhancing tissutal proton relaxation rates. Much has already been done in order to get an in-depth understanding of the relationships between structure, dynamics, and contrastographic ability of these paramagnetic complexes. Their potential in the assessment of flow, perfusion, and capillary permeability has already been established. The next challenges are in the field of molecular imaging applications, which would allow the attainment of early diagnosis based on the recognition of specific reporters of the onset of the pathological state. To this end, Gd(III) complexes have to be endowed with improved targeting capabilities by conjugating suitable recognition synthons on their surfaces. Small peptides are candidates of choice for the attainment of this goal. Moreover, the intrinsic low sensitivity of the NMR techniques implies the need to deliver large amounts of contrast agents to the target in order to get its visualization in the resulting images. Highly efficient delivery systems have been identified, which bring a great promise for the development of innovative diagnostic agents based on Gd(III) complexes.     

Novel bimodal bifunctional ligands for radioimmunotherapy and targeted MRI

The structurally novel bifunctional ligands C-NETA and C-NE3TA, each possessing both acyclic and macrocyclic moieties, were prepared and evaluated as potential chelates for radioimmunotherapy (RIT) and targeted magnetic resonance imaging (MRI). Heptadentate C-NE3TA was fortuitously discovered during the preparation of C-NETA. An optimized synthetic method to C-NETA and C-NE3TA including purification of the polar and tailing reaction intermediates, tert-butyl C-NETA (2) and tert-butyl C-NE3TA (3) using semiprep HPLC was developed. The new Gd(III) complexes of C-NETA and C-NE3TA were prepared as contrast enhancement agents for use in targeted MRI. The T 1 relaxivity data indicate that Gd(C-NETA) and Gd(C-NE3TA) possess higher relaxivity than Gd(C-DOTA), a bifunctional version of a commercially available MRI contrast agent; Gd(DOTA). C-NETA and C-NE3TA were radiolabeled with (177)Lu, (90)Y, (203)Pb, (205/6)Bi, and (153)Gd; and in vitro stability of the radiolabeled corresponding complexes was assessed in human serum. The in vitro studies indicate that the evaluated radiolabeled complexes were stable in serum for 11 days with the exception being the (203)Pb complexes of C-NETA and C-NE3TA, which dissociated in serum. C-NETA and C-NE3TA radiolabeled (177)Lu, (90)Y, or (153)Gd complexes were further evaluated for in vivo stability in athymic mice and possess excellent or acceptable in vivo biodistribution profile. (205/6)Bi- C-NE3TA exhibited extremely rapid blood clearance and low radioactivity level at the normal organs, while (205/6)Bi- C-NETA displayed low radioactivity level in the blood and all of the organs except for the kidney where relatively high renal uptake of radioactivity is observed. C-NETA and C-NE3TA were further modified for conjugation to the monoclonal antibody Trastuzumab.     

Dendritic MRI contrast agents: an efficient prelabeling approach based on CuAAC

The Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) allows the efficient and complete functionalization of dendrimers with preformed Gd chelates (prelabeling) to give monodisperse macromolecular contrast agents (CAs) for magnetic resonance imaging (MRI). This monodispersity contrasts with the typical distribution of materials obtained by classical routes and facilitates the characterization and quality control demanded for clinical applications. The potential of a new family of PEG-dendritic CA based on a gallic acid-triethylene glycol (GATG) core functionalized with up to 27 Gd complexes has been explored in vitro and in vivo, showing contrast enhancements similar to those of Gadomer-17, which reveals them to be a promising platform for the development of CA for MRI.     

Relaxometric evaluation of novel manganese(II) complexes for application as contrast agents in magnetic resonance imaging

Three novel Mn(II) complexes bearing benzyloxymethyl functionalities are reported and their ability to enhance water (1H and 17O) relaxation times is investigated in detail. Two of them contain one coordinated water molecule and display relaxivity values only slightly smaller than those shown by the most clinically used contrast agents (e.g. [Gd(DTPA)(H2O)]2-). Moreover, in these Mn(II) chelates the exchange rate of the coordinated water is ca. one order of magnitude higher if compared to the exchange rates previously reported for Gd(III) complexes with octadentate ligands. The occurrence of such fast exchange rates of the coordinated water is exploited in the formation of macromolecular adducts with human serum albumin to attain systems displaying relaxivity values in the upper range of those so far reported for analogous Gd(III) systems. These results strongly support the view that Mn(II) complexes, in spite of the lower effective magnetic moment, can be considered as viable alternatives to the currently used Gd(III) complexes as contrast agents for MRI applications.     

Method for enhancing cell penetration of Gd3+-based MRI contrast agents by conjugation with hydrophobic fluorescent dyes

Gadolinium ion (Gd(3+)) complexes are commonly used as magnetic resonance imaging (MRI) contrast agents to enhance signals in T(1)-weighted MR images. Recently, several methods to achieve cell-permeation of Gd(3+) complexes have been reported, but more general and efficient methodology is needed. In this report, we describe a novel method to achieve cell permeation of Gd(3+) complexes by using hydrophobic fluorescent dyes as a cell-permeability-enhancing unit. We synthesized Gd(3+) complexes conjugated with boron dipyrromethene (BDP-Gd) and Cy7 dye (Cy7-Gd), and showed that these conjugates can be introduced efficiently into cells. To examine the relationship between cell permeability and dye structure, we further synthesized a series of Cy7-Gd derivatives. On the basis of MR imaging, flow cytometry, and ICP-MS analysis of cells loaded with Cy7-Gd derivatives, highly hydrophobic and nonanionic dyes were effective for enhancing cell permeation of Gd(3+) complexes. Furthermore, the behavior of these Cy7-Gd derivatives was examined in mice. Thus, conjugation of hydrophobic fluorescent dyes appears to be an effective approach to improve the cell permeability of Gd(3+) complexes, and should be applicable for further development of Gd(3+)-based MRI contrast agents.     

Use of a genetically engineered protein for the design of a multivalent MRI contrast agent

The majority of clinically used contrast agents (CAs) for magnetic resonance imaging have low relaxivities and thus require high concentrations for signal enhancement. Research has turned to multivalent, macromolecular CAs to increase CA efficiency. However, previously developed macromolecular CAs do not provide high relaxivities, have limited biocompatibility, and/or do not have a structure that is readily modifiable to tailor to particular applications. We report a new family of multivalent, biomacromolecular, genetically engineered protein polymer-based CAs; the protein backbone contains evenly spaced lysines that are derivatized with gadolinium (Gd(III)) chelators. The protein's length and repeating amino acid sequence are genetically specified. We reproducibly obtained conjugates with an average of 8-9 Gd(III) chelators per protein. These multivalent CAs reproducibly provide a high relaxivity of 7.3 mM (-1) s (-1) per Gd(III) and 62.6 mM (-1) s (-1) per molecule. Furthermore, they can be incorporated into biomaterial hydrogels via chemical cross-linking of the remaining free lysines, and provide a dramatic contrast enhancement. Thus, these protein polymer CAs could be a useful tool for following the evolution of tissue engineering scaffolds.     

Synthesis and characterization of poly(L-glutamic acid) gadolinium chelate: a new biodegradable MRI contrast agent

Most currently evaluated macromolecular contrast agents for magnetic resonance imaging (MRI) are not biodegradable. The goal of this study is to synthesize and characterize poly(l-glutamic acid) (PG) gadolinium chelates as biodegradable blood-pool MRI contrast agents. Two PG chelates of gadolinium diethylenetriaminepentaacetic acid (Gd-DTPA) were synthesized through the use of difunctional and monofunctional DTPA precursors. The conjugates were characterized with regard to molecular weight and molecular weight distribution, gadolinium content, relaxivity, and degradability. Distributions of the polymeric MRI contrast agents in various organs were determined by intravenous injection of (111)In-labeled polymers into mice bearing murine breast tumors. MRI scans were performed at 1.5 T in mice after bolus injection of the polymeric chelates. PG-Hex-DTPA-Gd, obtained from aminohexyl-substituted PG and DTPA-dianhydride, was partially cross-linked and was undegradable in the presence of cathepsin B. On the other hand, PG-Bz-DTPA-Gd synthesized directly from PG and monofunctional p-aminobenzyl-DTPA(acetic acid-tert-butyl ester) was a linear polymer and was degradable. The relaxivities of the polymers at 1.5 T were 3-8 times as great as that of Gd-DTPA. Both polymers had high blood concentrations and were primarily accumulated in the kidney. However, PG-Bz-DTPA-Gd was gradually cleared from the body and had significantly less retention in the blood, the spleen, and the kidney. MRI with PG-Bz-DTPA-Gd in mice showed enhanced vascular contrast at up to 2 h after the contrast agent injection. The ability of PG-Bz-DTPA-Gd to be degraded and cleared from the body makes it a favorable macromolecular MRI contrast agent.     

Strategies for the covalent conjugation of a bifunctional chelating agent to albumin: synthesis and characterization of potential MRI contrast agents

With the purpose to develop macromolecular magnetic resonance imaging contrast agents, we herein report three different synthetic approaches to the covalent attachment of bifunctional chelating agents to human serum albumin followed by coordination to contrast enhancing gadolinium(III). Applied methods cover active ester-mediated conjugation, linkage through glutaryl spacer, as well as the connection by the employment of glutaraldehyde. The content of gadolinium(III) was evaluated by inductively-coupled-plasma mass-spectrometry (ICP-MS) measurements and indicated reproducible amounts of conjugated contrast enhancing material. Small angle X-ray scattering (SAXS) experiments provided the size and altered shape of the gadolinium loaded proteins in comparison to unmodified albumin. Finally, the magnetic resonance properties of the protein conjugates were evaluated. The results indicated suitability of the gadolinium(III) loaded protein conjugates for use as macromolecular contrast agents in magnetic resonance imaging (MRI).     

Cell-permeable MR contrast agents with increased intracellular retention

Magnetic resonance imaging (MRI) is a technique used in both clinical and experimental settings to produce high-resolution images of opaque organisms without ionizing radiation. Currently, MR imaging is augmented by contrast agents, and the vast majority these small molecule Gd(III) chelates are confined to the extracellular regions. As a result, contrast agents are confined to vascular regions reducing their ability to provide information about cell physiology or molecular pathology. We have shown that polypeptides of arginine have the capacity to transport Gd(III) contrast agents across cell membranes. However, this transport is not unidirectional, and once inside the cell, the arginine-modified contrast agents efflux rapidly, decreasing the intracellular Gd(III) concentration and corresponding MR image intensity. By exploiting the inherent disulfide reducing environment of cells, thiol compounds, Gd(III)-DOTA-SS-Arg 8 and Gd(III)-DTPA-SS-Arg 8, are cleaved from their cell-penetrating peptide transduction domains upon cell internalization. This reaction prolongs the cell-associated lifetime of the chelated Gd(III) by cleaving it from the cell transduction domain.     

Activated clearance of a biotinylated macromolecular MRI contrast agent from the blood pool using an avidin chase

The enhancement characteristics of a contrast agent are dependent on its pharmacokinetics within the body. In the case of macromolecular contrast agents, prolonged enhancement of the blood pool is seen after the first dose, limiting opportunities for repeated injection in the same session. If the enhancement within the blood pool could be intentionally switched off, the macromolecular contrast agents could be used both to define blood volume and vessel permeability, properties that could be useful in studying angiogenesis. In the current study, the avidin-biotin system was coupled to a dendrimer-based macromolecular MRI contrast agent to switch enhancement from the blood pool to the liver. Because avidin causes rapid trapping of the contrast agent in the liver, the blood pool cleared within 2 min of the injection of avidin. This system can be applied to all dendrimer-based macromolecular MRI contrast agents to investigate blood volume and vascular permeability. Moreover, it permits the repeated injection of the contrast agent and the "avidin switch" during a single MR experiment.     

Liposomes and Micelles to Target the Blood Pool for Imaging Purposes

Abstract

The blood pool is among body compartments of a special interest for imaging using magnetic resonance (MR) and computed tomography (CT), since with the help of selective blood-pool contrast agents blood perfusion and various cardiac parameters as well as a status of the blood flow and vascular system in any organ can be evaluated. Blood pool-specific imaging agents can also provide minimally invasive angiography, image guidance of minimally invasive procedures, oncologic imaging of angiogenesis, ascertaining organ blood volume, and identifying hemorrhage. Particulate contrast agents (such as liposomes and micelles) whose distribution is limited to the blood pool, should have a size larger than fenestrated capillaries (> 10 nm), contain the reporter (paramagnetic or radiopaque) moiety structurally incorporated within the particulate, and be able to stay in the blood long enough to obtain clinically useful images. We describe here a new generation of long-circulating Gd-loaded liposomes and iodine-loaded micelles to provide an efficient blood pool MR and CT imaging, respectively. In this study, we developed the optimized protocol to prepare a liposomal MR contrast agent with high relaxivity and narrow size distribution. Liposomes were loaded with Gadolinium (Gd) via so called polychelating amphiphilic polymer (PAP) that represents a low-molecular-weight DTPA-polylysine linked via its N-terminus to a lipid anchor, NGPE-PE. Gd-containing liposomes were additionally modified with PEG to provide the longevity in vivo. We demonstrated also that upon the intravenous administration in rabbits and dogs, a new preparation causes prolonged decrease in the blood Tl value, permits to obtain sharp and clear MR images of the vasculature, and may be considered as a potential contrast agent for MRI of the blood pool. In addition, to prepare micellar contrast agents for CT blood-pool imaging, we synthesized an iodine-containing amphiphilic block-copolymer consisting of methoxypoly(ethyleneglycol) and polyl?,N-(triiodobenzoyl)]-L-lysine. In aqueous solutions, it forms stable micelles with an average diameter of 80 nm and an iodine content of 35–40% wt. Iodine-containing micelles were intravenously injected into rats and rabbits at a dose of 170 mg I/kg and produced significant and sustained enhancement of the blood pool (aorta and heart), liver and spleen for a period of at least 3 hours providing clear and informative CT images.     

Targeted gadolinium-loaded dendrimer nanoparticles for tumor-specific magnetic resonance contrast enhancement

A target-specific MRI contrast agent for tumor cells expressing high affinity folate receptor was synthesized using generation five (G5) ofpolyamidoamine (PAMAM) dendrimer. Surface modified dendrimer was functionalized for targeting with folic acid (FA) and the remaining terminal primary amines of the dendrimer were conjugated with the bifunctional NCS-DOTA chelator that forms stable complexes with gadolinium (Gd III). Dendrimer-DOTA conjugates were then complexed with GdCl3 followed by ICP-OES as well as MRI measurement of their longitudinal relaxivity (T1 s(-1) mM(-1)) of water. In xenograft tumors established in immunodeficient (SCID) mice with KB human epithelial cancer cells expressing folate receptor (FAR), the 3D MRI results showed specific and statistically significant signal enhancement in tumors generated with targeted Gd(III)-DOTA-G5-FA compared with signal generated by non-targeted Gd(III)-DOTA-G5 contrast nanoparticle. The targeted dendrimer contrast nanoparticles infiltrated tumor and were retained in tumor cells up to 48 hours post-injection of targeted contrast nanoparticle. The presence of folic acid on the dendrimer resulted in specific delivery of the nanoparticle to tissues and xenograft tumor cells expressing folate receptor in vivo. We present the specificity of the dendrimer nanoparticles for targeted cancer imaging with the prolonged clearance time compared with the current clinically approved gadodiamide (Omniscan) contrast agent. Potential application of this approach may include determination of the folate receptor status of tumors and monitoring of drug therapy.     

Peptide‐based building blocks as structural elements for supramolecular Gd‐containing MRI contrast agents

Magnetic resonance imaging (MRI) is one of the most important clinic diagnostic tool used to obtain high‐quality body images. The administration of low‐molecular‐weight Gd complex–based MRI contrast agents (CAs) permits to increase the ¹H relaxation rate of nearby water molecules, thus modulating signal intensity and contrast enhancement. Even if highly accurate, MRI modality suffers from its low sensitivity. Moreover, low‐molecular‐weight CAs rapidly equilibrate between the intravascular and extravascular spaces after their administration. In order to improve their sensitivity and limit the extravasation phenomenon, several macromolecular and supramolecular multimeric gadolinium complexes (dendrimers, polymers, carbon nanostructures, micelles, and liposomes) have been designed until now. Because of their biocompatibility, low immunogenicity, low cost, and easy synthetic modification, peptides are attractive building blocks for the fabbrication of novel materials for biomedical applications. We report on the state of the art of supramolecular CAs obtained by self‐assembly of three different classes of building blocks containing a peptide sequence, a gadolinium complex, and, if necessary, a third functional portion achieving the organization process.