Novel intravascular macromolecular MRI contrast agent with generation-4 polyamidoamine dendrimer core: accelerated renal excretion with coinjection of lysine

Encapsulation of Gadolinium Oxide Nanoparticle (Gd2O3) Contrasting Agents in PAMAM Dendrimer Templates for Enhanced Magnetic Resonance Imaging in Vivo

There has been growing interest in the research of nanomaterials for biomedical applications in recent decades. Herein, a simple approach to synthesize the G4.5-Gd₂O₃-poly(ethylene glycol) (G4.5-Gd₂O₃-PEG) nanoparticles (NPs) that demonstrate potential as dual (T₁ and T₂) contrasting agents in magnetic resonance imaging (MRI) has been reported in this study. Compared to the clinically popular Gd-DTPA contrasting agents, G4.5-Gd₂O₃-PEG NPs exhibited a longer longitudinal relaxation time (T₁) and better biocompatibility when incubated with macrophage cell line RAW264.7 in vitro. Furthermore, the longitudinal relaxivity (r₁) of G4.5-Gd₂O₃-PEG NPs was 53.9 s^-1 mM^-1 at 7T, which is equivalent to 4.8 times greater than to the Gd-DTPA contrasting agents. An in vivo T₁-weighted MRI results revealed that G4.5-Gd₂O₃-PEG NPs significantly enhanced signals in the intestines, kidney, liver, bladder, and spleen. In addition, the T₂-weighted MRI results revealed darker contrast in the kidney, which proves that G4.5-Gd₂O₃-PEG NPs can be exploited as T₁ and T₂ contrasting agents. In summary, these findings suggest that the G4.5-Gd₂O₃-PEG NPs synthesized by an alternative approach can be used as dual MRI contrasting agents.

Current progress in nanotechnology applications for diagnosis and treatment of kidney diseases

Significant progress has been made in nanomedicine, primarily in the form of nanoparticles, for theranostic applications to various diseases. A variety of materials, both organic and inorganic, have been used to develop nanoparticles with promise to achieve improved efficacy in medical applications as well as reduced systemic side effects compared to current standard of care medical practices. In particular, this article highlights the recent development and application of nanoparticles for diagnosing and treating nephropathologies.

Unraveling dendrimer translocation across cell membrane mimics

Poly(amidoamine) (PAMAM) dendrimers are promising candidates in several applications within the medical field. However, it is still to date not fully understood whether they are able to passively translocate across lipid bilayers. Recently, we used fluorescence microscopy to show that PAMAM dendrimers induced changes in the permeability of lipid membranes but the dendrimers themselves could not translocate to be released into the vesicle lumen. Because of the lack of resolution, these experiments could not assess whether the dendrimers were able to translocate but remained attached to the membrane. Using quartz crystal microbalance with dissipation monitoring and neutron reflectivity, a structural investigation was performed to determine how dendrimers interact with zwitterionic and negatively charged lipid bilayers. We hereby show that dendrimers adsorb on top of lipid bilayers without significant dendrimer translocation, regardless of the lipid membrane surface charge. Thus, most likely dendrimers are actively transported through cell membranes by protein-mediated endocytosis in agreement with previous cell studies. Finally, the higher activity of PAMAM dendrimers for phosphoglycerol-containing membranes is in line with their high antimicrobial activity against Gram-negative bacteria.

Dendrimer-based macromolecular conjugate for the kidney-directed delivery of a multitargeted sunitinib analogue

The development of a macromolecular conjugate of a multitargeted tyrosine kinase inhibitor is described that can be used for renal-specific delivery into proximal tubular cells. A novel sunitinib analogue, that is, 17864, is conjugated to a NH(2) -PAMAM-G3 dendrimer via the platinum (II)-based Universal Linkage System (ULS™). The activity of 17864 is retained after coordination to the ULS linker alone or when coupled to NH(2) -PAMAM-G3. 17864-UlS-NH(2) -PAMAM-G3 is non-toxic to proximal tubular cells in vitro. After intravenous administration to mice, 17864-UlS-NH(2) -PAMAM-G3 rapidly and efficiently accumulates in the kidneys. These results are encouraging for future studies focusing on the development of novel therapeutics for the treatment of renal diseases.

Biologically optimized nanosized molecules and particles: more than just size

The expanded biological and medical applications of nanomaterials place a premium on better understanding of the chemical and physical determinants of in vivo particles. Nanotechnology allows us to design a vast array of molecules with distinct chemical and biological characteristics, each with a specific size, charge, hydrophilicity, shape, and flexibility. To date, much research has focused on the role of particle size as a determinant of biodistribution and clearance. Additionally, much of what we know about the relationship between nanoparticle traits and pharmacokinetics has involved research limited to the gross average hydrodynamic size. Yet, other features such as particle shape and flexibility affect in vivo behavior and become increasingly important for designing and synthesizing nanosized molecules. Herein, we discuss determinants of in vivo behavior of nanosized molecules used as imaging agents with a focus on dendrimer-based contrast agents. We aim to discuss often overlooked or, yet to be considered, factors that affect in vivo behavior of synthetic nanosized molecules, as well as aim to highlight important gaps in current understanding.

Macromolecular and dendrimer-based magnetic resonance contrast agents

Magnetic resonance imaging (MRI) is a powerful imaging modality that can provide an assessment of function or molecular expression in tandem with anatomic detail. Over the last 20-25 years, a number of gadolinium-based MR contrast agents have been developed to enhance signal by altering proton relaxation properties. This review explores a range of these agents from small molecule chelates, such as Gd-DTPA and Gd-DOTA, to macromolecular structures composed of albumin, polylysine, polysaccharides (dextran, inulin, starch), poly(ethylene glycol), copolymers of cystamine and cystine with GD-DTPA, and various dendritic structures based on polyamidoamine and polylysine (Gadomers). The synthesis, structure, biodistribution, and targeting of dendrimer-based MR contrast agents are also discussed.

New nanosized biocompatible MR contrast agents based on lysine-dendri-graft macromolecules

Paramagnetic nanomaterials for use as magnetic resonance imaging (MRI) contrast agents have higher relaxivity than conventional low molecular weight MRI agents. However, the biocompatibility and pharmacokinetics of such nanomaterials will strongly affect the likelihood of clinical approval. We synthesized MRI contrast agents based on biocompatible lysine-dendri-grafts: Gd-BzDTPA-lysineG2 and Gd-BzDTPA-lysineG3. The relaxivity of Gd-BzDTPA-lysineG2 and Gd-BzDTPA-lysineG3 increased with sample temperature, while the relaxivity of Gd-BzDTPA-PAMAMG4 decreased with increasing sample temperature. The increase in relaxivity with increasing temperature may be attributed to accessibility of water to the internal Gd chelates with lysine-dendri-grafts, which does not occur with PAMAM dendrimers. Gd-BzDTPA-lysineG3 had a long intravascular half-life but were largely excreted by the kidneys. Therefore, Gd-BzDTPA-lysineG3 enhanced the blood vessels for longer periods than Gd-BzDTPA-PAMAMG4, but was still excreted through the kidney. Because of their biocompatibility, desirable magneto-physical characteristics and favorable pharmacokinetics, which are derived from different interior structures rather than the physical size, lysine-dendri-graft MR contrast agents may be feasible for clinical use.

Novel nanomedicine-based MRI contrast agents for gynecological malignancies

Gynecological cancers result in significant morbidity and mortality in women despite advances in treatment and diagnosis. This is due to detection of the disease in the late stages following metastatic spread in which treatment options become limited and may not result in positive outcomes. In addition, traditional contrast agents are not very effective in detecting primary metastatic tumors and cells due to a lack of specificity and sensitivity of the diagnostic tools, which limits their effectiveness. Recently, the field of nanomedicine-based contrast agents offers a great opportunity to develop highly sophisticated devices that can overcome many traditional hurdles of contrast agents including solubility, cell-specific targeting, toxicities, and immunological responses. These nanomedicine-based contrast agents including liposomes, micelles, dendrimers, multifunctional magnetic polymeric nanohybrids, fullerenes, and nanotubes represent improvements over their traditional counterparts, which can significantly advance the field of molecular imaging.

Gadolinium(III)-based blood-pool contrast agents for magnetic resonance imaging: status and clinical potential

Blood-pool MRI contrast agents have enormous potential to aid sensitive magnetic resonance detection and yield definitive diagnostic data of cancer and diseases of the cardiovascular system. Many attempts have been initiated to design macromolecular gadolinium (Gd[III]) complexes as magnetic resonance imaging blood-pool contrast agents, as macromolecules do not readily diffuse across healthy vascular endothelium, and remain intravascular. Although extremely efficacious in detecting and characterizing pathologic tissue, clinical development of these agents has been limited by potential toxicity concerns from incomplete Gd(III) clearance. Recent innovative technologies, such as reversible protein-binding contrast agents and biodegradable macromolecular contrast agents, may be valuable alternatives that combine the effective imaging characteristics of an intravascular contrast agent and the safety of clinically approved low-molecular-weight Gd(III) chelates.

Delivery of gadolinium-labeled nanoparticles to the sentinel lymph node: comparison of the sentinel node visualization and estimations of intra-nodal gadolinium concentration by the magnetic resonance imaging

Sentinel node imaging is commonly performed prior to surgery for breast cancer and melanoma. While current methods are based on radio-lymphoscintigraphy, MR lymphangiography (MRL) offers the benefits of better spatial resolution without ionizing radiation. However, the optimal nanoparticle for imaging the sentinel nodes remains unclear. Gadolinium-labeled (Gd) contrast agents ranging in diameter from <1 to 12 nm were evaluated to determine which size provides the most rapid and most concentrated delivery of contrast agent to the lymph nodes in a mouse model of lymphatic metastases. Specifically, PAMAM-G2, -G4, -G6 and -G8, and DAB-G5 Gd-dendrimer agents, as well as Gadomer-17 and Gd-DTPA, were compared. Among these agents, the G6 Gd dendrimer depicted the lymphatics and lymph nodes with the highest peak concentrations and this occurred 24-36 min post-injection (p<0.01; all except G8). Based on ex vivo concentration phantoms, high accumulations of Gd(III) ions occurred within lymph nodes (1.7-4.4 mM Gd/270-680 ppm Gd) with high target to background ratios (>100). These concentrations are sufficient to contemplate the use of Gd-neutron capture therapy of regional lymph nodes. Thus, when injected interstitially, the PAMAM-G6 Gd dendrimer not only provides excellent opacification of sentinel lymph nodes, but also provides the potential for targeted therapy of sentinel lymph nodes.

Polyamine dendrimer-based MRI contrast agents for functional kidney imaging to diagnose acute renal failure

PURPOSE

To choose an efficacious renal functional MRI contrast agent to image early renal tubular damage. We synthesized and compared smaller polyamine dendrimer-based MRI contrast agents (<60 kD) that, unlike Gd-[DTPA], transiently accumulate in renal tubules and can be used to visualize renal structural and functional damage.

MATERIALS AND METHODS

Six dendrimer-based MRI contrast agents smaller than 60 kD were studied by high resolution dynamic micro-MRI and compared to Gd-[DTPA]-dimeglumine and Gadomer-17. The best agent, DAB-G2, was further tested in a mouse ischemia/reperfusion model to validate its efficacy.

RESULTS

Despite unequal renal clearance rates, all polyamine dendrimer agents visualized the renal functional anatomy of the mice better than Gd-[DTPA]-dimeglumine and Gadomer-17. DAB-G2 was excreted most rapidly, yet was able to visualize mild renal tubular injury very early after injury.

CONCLUSION

DAB-G2 was found to be the best candidate for functional kidney imaging and enabled early diagnosis of acute renal injury.

Newly developed techniques to study and diagnose acute renal failure

Progress in treating human acute renal failure (ARF) is dependent on developing techniques that allow for the rapid diagnosis, quantification of injury, further understanding of the pathophysiology, and the effects of therapy. Therefore, four techniques that will facilitate this progress are described and illustrated by four different investigative teams. Techniques to measure rapid changes in GFR are available for rapid diagnosis and quantification of ARF in humans. State-of-the-art magnetic resonance imaging (MRI) presently allows for enhanced resolution of regional renal blood flow and functional evaluations in patients. Furthermore, new probes and techniques for MRI that allow for identification and quantitation of inflammation, applicable to human ARF, are being developed and tested in animal models. Finally, two-photon microscopy will allow for four-dimensional cellular and subcellular studies in animal models of ARF providing rapid insights into pathophysiology and the therapeutic effects of a variety of promising agents. Further development and utilization of these techniques, especially in concert with genetic, proteomic, and molecular approaches, will allow for needed insights into the pathophysiology and therapy in human ARF.

Dendrimer-based Macromolecular MRI Contrast Agents: Characteristics and Application

Numerous macromolecular MRI contrast agents prepared employing relatively simple chemistry may be readily available that can provide sufficient enhancement for multiple applications. These agents operate using a ~100-fold lower concentration of gadolinium ions in comparison to the necessary concentration of iodine employed in CT imaging. Herein, we describe some of the general potential directions of macromolecular MRI contrast agents using our recently reported families of dendrimer-based agents as examples. Changes in molecular size altered the route of excretion. Smaller-sized contrast agents less than 60 kDa molecular weight were excreted through the kidney resulting in these agents being potentially suitable as functional renal contrast agents. Hydrophilic and larger-sized contrast agents were found better suited for use as blood pool contrast agents. Hydrophobic variants formed with polypropylenimine diaminobutane dendrimer cores created liver contrast agents. Larger hydrophilic agents are useful for lymphatic imaging. Finally, contrast agents conjugated with either monoclonal antibodies or with avidin are able to function as tumor-specific contrast agents, which also might be employed as therapeutic drugs for either gadolinium neutron capture therapy or in conjunction with radioimmunotherapy.

Renal tubular damage detected by dynamic micro-MRI with a dendrimer-based magnetic resonance contrast agent

BACKGROUND

A noninvasive technique to evaluate the structure and function of the kidney would be useful to investigate renal diseases, especially acute renal failure. We have developed a novel technique to visualize functional micro-magnetic resonance (MR) images of the mouse kidney with a dendrimer-based macromolecular renal MR contrast agent.

METHOD

Mice were injected with cisplatin or vehicle, then examined three days later by contrast-enhanced, dynamic high-resolution micro-MRI with 160 microm spatial resolution using a 1.5 T clinical MRI unit, a surface coil, and the renal contrast agent G4D-(1B4M-Gd)64.

RESULTS

The cortex and outer stripe of the outer medulla of the mouse kidney were clearly visualized in the normal mice. In animals treated with cisplatin, the gradation of tubular damage as assessed by contrast enhanced dynamic MRI correlated with renal function.

CONCLUSION

Contrast-enhanced, dynamic high-resolution micro-MRI with a novel dendrimer-based macromolecular renal MR contrast agent can be a powerful tool for in vivo observation of renal structural and functional damage.

Pharmacokinetics and enhancement patterns of macromolecular MR contrast agents with various sizes of polyamidoamine dendrimer cores

Four macromolecular contrast agents are synthesized to visualize small vessels by MRI using generation-3 (G3D), -4 (G4D), -5 (G5D), and -6 (G6D) polyamidoamine dendrimers conjugated to chelated gadolinium (Gd). The pharmacokinetics, enhancement patterns, and the ability of these constructs to visualize fine vessels is evaluated by dynamic MRI in relationship to their size. Gd-G6D and -G5D exhibit a prolonged high vascular (ventricular) signal intensity (SI) with high ventricle-to-organ SI ratios. The initial high vascular SI with Gd-G4D decreases to a value as low as that obtained with Gd-G3D and Gd-dimeglumine-diethylenetriaminepentaacetic acid (Gd-DTPA). Gd-G5D, -G4D, and -G3D show high renal SIs, and Gd-DTPA prominently enhances the skin. Gd-G6D and -G5D present fine vasculature significantly more clearly than Gd-G3D and -DTPA (P < 0.005). As the molecular size increases, the excretion of the 153Gd-conjugates is retarded. In conclusion, Gd-G6D and -G5D are retained in the blood and present fine vessels with high quality and detail, and should be adequate for visualizing small tumor vasculature.

Micro-MR angiography of normal and intratumoral vessels in mice using dedicated intravascular MR contrast agents with high generation of polyamidoamine dendrimer core: reference to pharmacokinetic properties of dendrimer-based MR contrast agents

Pharmacokinetic characteristics of intravascular macromolecular magnetic resonance imaging (MRI) contrast agents with polyamidoamine dendrimer cores smaller than generation-7 were previously studied in the literature. To evaluate the effects of greater hepatic uptake on the pharmacokinetics of the larger generation dendrimers, the MRI contrast agents GxD-(1B4M-Gd)(2(x+2)) were synthesized with generation-7, -8, and -9 polyamidoamine dendrimers and 2-(p-isothiocyanatobenzyl)-6-methyl-diethylenetriaminepentaacetic acid (1B4M). Their pharmacokinetic characteristics in mice were compared with that of G6D-(1B4M-Gd)(256). In biodistribution and dynamic micro-MRI studies, significantly less renal accumulation of G7D-(1B4M-Gd)(512), G8D-(1B4M-Gd)(1024), and G9D-(1B4M-Gd)(2048) was shown compared to G6D-(1B4M-Gd)(256) (P < 0.01). There was a significantly greater accumulation of G8D-(1B4M-Gd)(1024) and G9D-(1B4M-Gd)(2048) in the liver compared to G6D-(1B4M-Gd)(256) and G7D-(1B4M-Gd)(512) (P < 0.01). The highest blood retention of all dendrimer-based MRI contrast agents was exhibited by G7D-(1B4M-Gd)(512) (P < 0.01). The normal and intratumoral fine vessels of approximately 100 microm diameter were visualized in normal or tumor-bearing mice by high resolution three-dimensional-micro-MR angiographs with G7D-(1B4M-Gd)(512) and G8D-(1B4M-Gd)(1024) with good vessel-to-soft tissue contrast. In summary, increased accumulation in the liver with concomitant decreased uptake in the kidney was caused by increased molecular sizes of the dendrimer-based MRI contrast agents.