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

A Glimpse into Dendrimers Integration in Cancer Imaging and Theranostics

Cancer is a result of abnormal cell proliferation. This pathology is a serious health problem since it is a leading cause of death worldwide. Current anti-cancer therapies rely on surgery, radiation, and chemotherapy. However, these treatments still present major associated problems, namely the absence of specificity. Thus, it is urgent to develop novel therapeutic strategies. Nanoparticles, particularly dendrimers, have been paving their way to the front line of cancer treatment, mostly for drug and gene delivery, diagnosis, and disease monitoring. This is mainly derived from their high versatility, which results from their ability to undergo distinct surface functionalization, leading to improved performance. In recent years, the anticancer and antimetastatic capacities of dendrimers have been discovered, opening new frontiers to dendrimer-based chemotherapeutics. In the present review, we summarize the intrinsic anticancer activity of different dendrimers as well as their use as nanocarriers in cancer diagnostics and treatment.

Real-Time Longitudinal Evaluation of Tumor Blood Vessels Using a Compact Preclinical Fluorescence Imaging System

Tumor angiogenesis is enhanced in all types of tumors to supply oxygen and nutrients for their growth and metastasis. With the development of anti-angiogenic drugs, the importance of technology that closely monitors tumor angiogenesis has also been emerging. However, to date, the technology for observing blood vessels requires specialized skills with expensive equipment, thereby limiting its applicability only to the laboratory setting. Here, we used a preclinical optical imaging system for small animals and, for the first time, observed, in real time, the entire process of blood vessel development in tumor-bearing mice injected with indocyanine green. Time-lapse sequential imaging revealed blood vessel volume and blood flow dynamics on a microscopic scale. Upon analyzing fluorescence dynamics at each stage of tumor progression, vessel volume and blood flow were found to increase as the tumor developed. Conversely, these vascular parameters decreased when the mice were treated with angiogenesis inhibitors, which suggests that the effects of drugs targeting angiogenesis can be rapidly and easily screened. The results of this study may help evaluate the efficacy of angiogenesis-targeting drugs by facilitating the observation of tumor blood vessels easily in a laboratory unit without large and complex equipment.

Developments in Treatment Methodologies Using Dendrimers for Infectious Diseases

Dendrimers comprise a specific group of macromolecules, which combine structural properties of both single molecules and long expanded polymers. The three-dimensional form of dendrimers and the extensive possibilities for use of additional substrates for their construction creates a multivalent potential and a wide possibility for medical, diagnostic and environmental purposes. Depending on their composition and structure, dendrimers have been of interest in many fields of science, ranging from chemistry, biotechnology to biochemical applications. These compounds have found wide application from the production of catalysts for their use as antibacterial, antifungal and antiviral agents. Of particular interest are peptide dendrimers as a medium for transport of therapeutic substances: synthetic vaccines against parasites, bacteria and viruses, contrast agents used in MRI, antibodies and genetic material. This review focuses on the description of the current classes of dendrimers, the methodology for their synthesis and briefly drawbacks of their properties and their use as potential therapies against infectious diseases.

Gadolinium Complexes Attached to Poly Ethoxy Ethyl Glycinamide (PEE-G) Dendrons: Magnetic Resonance Imaging Contrast Agents with Increased Relaxivity

A range of poly ethoxy ethyl glycinamide (PEE-G) dendron scaffolds with gadolinium (III) complexes attached were synthesized with a focus on product purity and high Gd(III) loading. The nuclear magnetic resonance relaxivity of these products was measured and compared with commercially available low-molecular-weight magnetic resonance imaging contrast agents. Over twice the relaxivity based on Gd(III) concentration, and up to 20-fold increase in relaxivity were observed based on molecular concentration. Relaxivity properties were observed to increase with both increasing molecular weight and number of Gd(III) complexes attached, however a plateau was reached for molecular weight increase. T₁ and T₂ relaxivity properties were also investigated at two different magnetic fields. Transverse relaxivity is unaffected by magnetic field strength whereas increase in longitudinal relaxivity was not as pronounced at the higher field.

Self-assembly of four generations of RNA dendrimers for drug shielding with controllable layer-by-layer release

Chemical dendrimers have been shown to be a promising drug delivery platform due to their advantageous properties such as monodispersity, multivalency and branched structure. Taking advantage of self-assembly and its intrinsic negative charge, we used RNA as the building block for dendrimer construction to eliminate complex synthesis procedures and cationic charge-related toxicity. Oligo ribonucleotides produced by solid phase chemical synthesis allow the large-scale manufacture of homologous RNA dendrimers. Employing concepts from RNA nanotechnology enabled the controllable production of dendrimers with generations from G1, G2, G3, to G4 with layer-by-layer release capability. The conjugation of functional groups into individual RNA strands and the incorporation of functionalized RNA strands into the dendrimers at different sites have been reported. Anticancer drugs loaded into RNA dendrimers showed comparable cancer cell inhibition effect to free drugs. Encapsulation of cell binding ligands and hydrophobic drugs within the dendrimer significantly reduced the efficiency of cell binding and protein binding respectively, demonstrating the shielding effect of RNA dendrimers. The results imply a potential application of RNA dendrimer for delivery, shielding and controlled release of hydrophobic drugs in vivo.

Dendrimer-Based Drug Delivery Systems for Brain Targeting

Human neuroscience has made remarkable progress in understanding basic aspects of functional organization; it is a renowned fact that the blood-brain barrier (BBB) impedes the permeation and access of most drugs to central nervous system (CNS) and that many neurological diseases remain undertreated. Therefore, a number of nanocarriers have been designed over the past few decades to deliver drugs to the brain. Among these nanomaterials, dendrimers have procured an enormous attention from scholars because of their nanoscale uniform size, ease of multi-functionalization, and available internal cavities. As hyper-branched 3D macromolecules, dendrimers can be maneuvered to transport diverse therapeutic agents, incorporating small molecules, peptides, and genes; diminishing their cytotoxicity; and improving their efficacy. Herein, the present review will give exhaustive details of extensive researches in the field of dendrimer-based vehicles to deliver drugs through the BBB in a secure and effectual manner. It is also a souvenir in commemorating Donald A. Tomalia on his 80th birthday.

3-D Longitudinal Imaging of Tumor Angiogenesis in Mice in Vivo Using Ultrafast Doppler Tomography

Angiogenesis, the formation of new vessels, is one of the key mechanisms in tumor development and an appealing target for therapy. Non-invasive, high-resolution, high-sensitivity, quantitative 3-D imaging techniques are required to correctly depict tumor heterogeneous vasculature over time. Ultrafast Doppler was recently introduced and provides an unprecedented combination of resolution, penetration depth and sensitivity without requiring any contrast agents. The technique was further extended to three dimensions with ultrafast Doppler tomography (UFD-T). In this work, UFD-T was applied to the monitoring of tumor angiogenesis in vivo, providing structural and functional information at different stages of development. UFD-T volume renderings revealed that our murine model's vasculature stems from pre-existing vessels and sprouts to perfuse the whole volume as the tumor grows until a critical size is reached. Then, as the network becomes insufficient, the tumor core is no longer irrigated because the vasculature is concentrated mainly in the periphery. In addition to spatial distribution and growth patterns, UFD-T allowed a quantitative analysis of vessel size and length, revealing that the diameter distribution of vessels remained relatively constant throughout tumor growth. The network is dominated by small vessels at all stages of tumor development, with more than 74% of the vessels less than 200 µm in diameter. This study also found that cumulative vessel length is more closely related to tumor radius than volume, indicating that the vascularization becomes insufficient when a critical mass is reached. UFD-T was also compared with dynamic contrast-enhanced ultrasound and found to provide complementary information regarding the link between structure and perfusion. In conclusion, UFD-T is capable of in vivo quantitative assessment of the development of tumor vasculature (vessels with blood speed >1 mm/s [sensitivity limit] assessed with a resolution limit of 80 µm) in 3 dimensions. The technique has very interesting potential as a tool for treatment monitoring, response assessment and treatment planning for optimal drug efficiency.

An emerging class of amphiphilic dendrimers for pharmaceutical and biomedical applications: Janus amphiphilic dendrimers

In recent years, a new class of dendrimer, known as Janus dendrimers (JDs), has attracted much attention due to their different structures and properties to the conventional symmetric forms. The broken symmetry of JDs offers the opportunity to form complex self-assembled materials, and presents new sets of properties that are presently inconceivable for homogeneous or symmetrical dendrimers. Due to their unique features, JDs have a promising future in pharmaceutical and biomedical fields, as seen from the recent interest in their application in conjugating multiple drugs and targeting moieties, forming supramolecular hydrogels, enabling micellar delivery systems, and preparing nano-vesicles, known as dendrimersomes, for drug encapsulation. The present paper is the first review, with an emphasis on various emerging applications of JDs, in the drug delivery and biomedical field reported so far. In addition, the paper describes different synthetic methods for producing JDs that can guide the design of new biocompatible forms with pharmacological activities, and that have the potential to be nano drug delivery vehicles. Furthermore, future studies to optimize the applications of JDs in drug delivery sciences and biomedical field to realize their potential to treat various disease conditions are identified and highlighted. Overall, this review identifies the current status of JDs in terms of their synthesis and applications, as well as the future research for their translation into macromolecules for clinical applications to solve health problems. It highlights the future combined efforts needed to be taken by dendrimer chemists, formulation scientist and microbiologists to develop novel antibacterials and nanomedicines from JDs.

Tweaking dendrimers and dendritic nanoparticles for controlled nano-bio interactions: potential nanocarriers for improved cancer targeting

Nanoparticles have shown great promise in the treatment of cancer, with a demonstrated potential in targeted drug delivery. Among a myriad of nanocarriers that have been recently developed, dendrimers have attracted a great deal of scientific interests due to their unique chemical and structural properties that allow for precise engineering of their characteristics. Despite this, the clinical translation of dendrimers has been hindered due to their drawbacks, such as scale-up issues, rapid systemic elimination, inefficient tumor accumulation and limited drug loading. In order to overcome these limitations, a series of reengineered dendrimers have been recently introduced using various approaches, including: (i) modifications of structure and surfaces; (ii) integration with linear polymers and (iii) hybridization with other types of nanocarriers. Chemical modifications and surface engineering have tailored dendrimers to improve their pharmacokinetics and tissue permeation. Copolymerization of dendritic polymers with linear polymers has resulted in various amphiphilic copolymers with self-assembly capabilities and improved drug loading efficiencies. Hybridization with other nanocarriers integrates advantageous characteristics of both systems, which includes prolonged plasma circulation times and enhanced tumor targeting. This review provides a comprehensive summary of the newly emerging drug delivery systems that involve reengineering of dendrimers in an effort to precisely control their nano-bio interactions, mitigating their inherent weaknesses.

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.

Recent advances in targeted drug delivery approaches using dendritic polymers

Since they were first synthesized over 30 years ago, dendrimers have seen rapid translation into various biomedical applications. A number of reports have not only demonstrated their clinical utility, but also revealed novel design approaches and strategies based on the elucidation of underlying mechanisms governing their biological interactions. This review focuses on presenting the latest advances in dendrimer design, discussing the current mechanistic understandings, and highlighting recent developments and targeted approaches using dendrimers in drug/gene delivery.

Emerging concepts in dendrimer-based nanomedicine: from design principles to clinical applications

Dendrimers are discrete nanostructures/nanoparticles with 'onion skin-like' branched layers. Beginning with a core, these nanostructures grow in concentric layers to produce stepwise increases in size that are similar to the dimensions of many in vivo globular proteins. These branched tree-like concentric layers are referred to as 'generations'. The outer generation of each dendrimer presents a precise number of functional groups that may act as a monodispersed platform for engineering favourable nanoparticle-drug and nanoparticle-tissue interactions. These features have attracted significant attention in medicine as nanocarriers for traditional small drugs, proteins, DNA/RNA and in some instances as intrinsically active nanoscale drugs. Dendrimer-based drugs, as well as diagnostic and imaging agents, are emerging as promising candidates for many nanomedicine applications. First, we will provide a brief survey of recent nanomedicines that are either approved or in the clinical approval process. This will be followed by an introduction to a new 'nanoperiodic' concept which proposes nanoparticle structure control and the engineering of 'critical nanoscale design parameters' (CNDPs) as a strategy for optimizing pharmocokinetics, pharmocodynamics and site-specific targeting of disease. This paradigm has led to the emergence of CNDP-directed nanoperiodic property patterns relating nanoparticle behaviour to critical in vivo clinical translation issues such as cellular uptake, transport, elimination, biodistribution, accumulation and nanotoxicology. With a focus on dendrimers, these CNDP-directed nanoperiodic patterns are used as a strategy for designing and optimizing nanoparticles for a variety of drug delivery and imaging applications, including a recent dendrimer-based theranostic nanodevice for imaging and treating cancer. Several emerging preclinical dendrimer-based nanotherapy concepts related to inflammation, neuro-inflammatory disorders, oncology and infectious and ocular diseases are reviewed. Finally we will consider challenges and opportunities anticipated for future clinical translation, nanotoxicology and the commercialization of nanomedicine.

Engineering Gd-loaded nanoparticles to enhance MRI sensitivity via T(1) shortening

Magnetic resonance imaging (MRI) is a noninvasive imaging technique capable of obtaining high-resolution anatomical images of the body. Major drawbacks of MRI are the low contrast agent sensitivity and inability to distinguish healthy tissue from diseased tissue, making early detection challenging. To address this technological hurdle, paramagnetic contrast agents have been developed to increase the longitudinal relaxivity, leading to an increased signal-to-noise ratio. This review focuses on methods and principles that enabled the design and engineering of nanoparticles to deliver contrast agents with enhanced ionic relaxivities. Different engineering strategies and nanoparticle platforms will be compared in terms of their manufacturability, biocompatibility properties, and their overall potential to make an impact in clinical MR imaging.

Dendrimers as high relaxivity MR contrast agents

Dendrimers are versatile macromolecules with tremendous potential as magnetic resonance imaging (MRI) contrast agents. Dendrimer-based agents provide distinct advantages over low-molecular-weight gadolinium chelates, including enhanced r1 relaxivity due to slow rotational dynamics, tunable pharmacokinetics that can be adapted for blood pool, liver, kidney, and lymphatic imaging, the ability to be a drug carrier, and flexibility for labeling due to their inherent multivalency. Clinical applications are increasingly being developed, particularly in lymphatic imaging. Herein we present a broad overview of dendrimer-based MRI contrast agents with attention to the unique chemistry and physical properties as well as emerging clinical applications.

Magnetic resonance angiography and vascular corrosion casting as tools in biomedical research: application to transgenic mice modeling Alzheimer's disease

In vivo imaging technologies are presently receiving considerable attention in the biomedical and pharmaceutical research areas. One of the principal imaging modalities is magnetic resonance imaging (MRI). The multiparametric nature of MRI enables anatomical, functional and even molecular information to be obtained non-invasively from intact organisms at high spatial resolution. Here we describe the use of one MRI modality, namely angiography (MRA), to non-invasively study the arterial vascular architecture of APP23 transgenic mice modeling Alzheimer's disease. Because the spatial resolution of the technique is limited, the in vivo studies are complemented by a powerful analysis of the vasculature using vascular corrosion casting. Both techniques revealed age-dependent blood flow alterations and cerebrovascular abnormalities in these mice. Our experience suggests that MRA complemented by cast analysis are important tools to describe vascular alterations and test new therapy concepts in animal models of AD. Furthermore, being non-invasive, MRA can also be applied to studies in patients suffering from this disease.

Dendrimer pharmacokinetics: the effect of size, structure and surface characteristics on ADME properties

Dendrimers show increasing promise as drug-delivery vectors and can be generated with a wide range of scaffold structures, sizes and surface functionalities. To this point, the majority of studies of dendrimer-based drug-delivery systems have detailed pharmacodynamic outcomes, or have followed the pharmacokinetics of a solubilized or conjugated drug. By contrast, detailed commentary on the in vivo fate of the dendrimer carrier is less evident, even though the pharmacokinetics of the carrier will likely dictate both pharmacodynamic and toxicokinetic outcomes. In the current article, the influence of size, structure and surface functionality on the absorption, distribution, metabolism and elimination (ADME) properties of dendrimers have been examined and the implications of these findings for delivery system design are discussed.

Targeting the lymphatics using dendritic polymers (dendrimers)

Dendrimers are unique biomaterials that are constructed by the stepwise addition of layers (generations) of polymer around a central core. They can be constructed with a range of molecular weights and have a polyfunctional surface that facilitates the attachment of drugs and pharmacokinetic modifiers such PEG or targeting moieties. These properties have led to considerable interest in the development of dendrimers for a range of biomedical applications. After subcutaneous administration, larger dendrimers in particular (> 8 nm), preferentially drain from the injection site into the peripheral lymphatic capillaries and therefore have potential as lymphatic imaging agents for magnetic resonance and optical fluorescence lymphangiography and as vectors for drug-targeting to lymphatic sites of disease progression. In general, lymphatic targeting of dendrimers is enhanced by increasing size although ultimately larger constructs may be incompletely absorbed from the injection site. Increasing hydrophilicity and reducing surface charge enhances drainage from subcutaneous injection sites, but the reverse is true of uptake into lymph nodes where charge and hydrophobicity promote retention. Larger hydrophilic dendrimers are also capable of extravasation from the systemic circulation, absorption into the lymphatic system and recirculation into the blood. Lymphatic recirculation may therefore be a characteristic of PEGylated dendrimers with long systemic circulation times.

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.

Dendrimer-based MRI contrast agents: the effects of PEGylation on relaxivity and pharmacokinetics

Polyethylene glycol (PEG) surface modification can make nanomaterials highly hydrophilic, reducing their sequestration in the reticuloendothelial system. In this study, polyamidoamine (PAMAM) dendrimers bearing gadolinium (Gd) chelates were PEGylated with different PEG-chain lengths, and the effects on paramagnetic and pharmacokinetic properties were evaluated. Specifically, Gd chelate-bearing PAMAM dendrimers (generations 4 and 5; G4 and G5) were conjugated with two different PEG chains (2 kDa and 5 kDa; 2k and 5k). Long PEG chains (5k) on the smaller (G4) dendrimer resulted in reduced relaxivity compared to non-PEGylated dendrimers, whereas short PEG chains (2k) on a larger (G5) dendrimer produced relaxivities comparable to non-PEGylated G4 dendrimers. The relaxivity of all PEGylated or lysine-conjugated dendrimers increased at higher temperature, whereas that of intact G4 Gd-PAMAM dendrimer decreased. All PEGylated dendrimers had minimal liver and kidney uptake and remained in circulation for at least 1 hour. Thus, surface-PEGylated Gd-PAMAM dendrimers showed decreased plasma clearance and prolonged retention in the blood pool. Shorter PEG, higher generation conjugates led to higher relaxivity.

FROM THE CLINICAL EDITOR

In this study, polyamidoamine dendrimers bearing gadolinium (Gd) chelates were PEGylated with different PEG-chain lengths, and the effects on paramagnetic and pharmacokinetic properties were evaluated.

N-acetylgalactosamine-functionalized dendrimers as hepatic cancer cell-targeted carriers

There is an urgent need for novel polymeric carriers that can selectively deliver a large dose of chemotherapeutic agents into hepatic cancer cells to achieve high therapeutic activity with minimal systemic side effects. PAMAM dendrimers are characterized by a unique branching architecture and a large number of chemical surface groups suitable for coupling of chemotherapeutic agents. In this article, we report the coupling of N-acetylgalactosamine (NAcGal) to generation 5 (G5) of poly(amidoamine) (PAMAM-NH₂) dendrimers via peptide and thiourea linkages to prepare NAcGal-targeted carriers used for targeted delivery of chemotherapeutic agents into hepatic cancer cells. We describe the uptake of NAcGal-targeted and non-targeted G5 dendrimers into hepatic cancer cells (HepG2) as a function of G5 concentration and incubation time. We examine the contribution of the asialoglycoprotein receptor (ASGPR) to the internalization of NAcGal-targeted dendrimers into hepatic cancer cells through a competitive inhibition assay. Our results show that uptake of NAcGal-targeted G5 dendrimers into hepatic cancer cells occurs via ASGPR-mediated endocytosis. Internalization of these targeted carriers increased with the increase in G5 concentration and incubation time following Michaelis-Menten kinetics characteristic of receptor-mediated endocytosis. These results collectively indicate that G5-NAcGal conjugates function as targeted carriers for selective delivery of chemotherapeutic agents into hepatic cancer cells.

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.

A new approach in the preparation of dendrimer-based bifunctional diethylenetriaminepentaacetic acid MR contrast agent derivatives

In this paper, we report a new method to prepare and characterize a contrast agent based on a fourth-generation (G4) polyamidoamine (PAMAM) dendrimer conjugated to the gadolinium complex of the bifunctional diethylenetriamine pentaacetic acid derivative (1B4M-DTPA). The method involves preforming the metal-ligand chelate in alcohol prior to conjugation to the dendrimer. The dendrimer-based agent was purified by a Sephadex G-25 column and characterized by elemental analysis. The analysis and SE-HPLC data gave a chelate to dendrimer ratio of 30:1 suggesting conjugation at approximately every other amine terminal on the dendrimer. Molar relaxivity of the agent measured at pH 7.4 displayed a higher value than that of the analogous G4 dendrimer based agent prepared by the postmetal incorporation method (r(1) = 26.9 vs 13.9 mM(-1) s(-1) at 3 T and 22 degrees C). This is hypothesized to be due to the higher hydrophobicity of this conjugate and the lack of available charged carboxylate groups from noncomplexed free ligands that might coordinate to the metal and thus also reduce water exchange sites. Additionally, the distribution populations of compounds that result from the postmetal incorporation route are eliminated from the current product simplifying characterization as quality control issues pertaining to the production of such agents for clinical use as MR contrast agents. In vivo imaging in mice showed a reasonably fast clearance (t(1/2) = 24 min) suggesting a viable agent for use in clinical application.

Effects of nanomaterial physicochemical properties on in vivo toxicity

It is well recognized that physical and chemical properties of materials can alter dramatically at nanoscopic scale, and the growing use of nanotechnologies requires careful assessment of unexpected toxicities and biological interactions. However, most in vivo toxicity concerns focus primarily on pulmonary, oral, and dermal exposures to ultrafine particles. As nanomaterials expand as therapeutics and as diagnostic tools, parenteral administration of engineered nanomaterials should also be recognized as a critical aspect for toxicity consideration. Due to the complex nature of nanomaterials, conflicting studies have led to different views of their safety. Here, the physicochemical properties of four representative nanomaterials (dendrimers, carbon nanotubes, quantum dots, and gold nanoparticles) as it relates to their toxicity after systemic exposure is discussed.

A method for assessing the microvasculature in a murine tumor model using contrast-enhanced ultrasonography

OBJECTIVE

The purpose of this study was to develop a method for assessing tumor vascularity in a preclinical model of breast cancer using contrast-enhanced ultrasonography.

METHODS

Eight mice were injected with 67NR breast cancer cells on their hind limbs and imaged with ultrasonography 8 days later. Mice were injected with an ultrasound contrast agent (UCA), and a sequence of images of the resultant backscattered echoes was recorded before and after high-power "destruction" pulses for each of multiple parallel planes. From these, data maps of the maximum contrast enhancement (within each time course) were constructed for each pixel, which enabled reconstruction of high-resolution coregistered sections into a 3-dimensional (3D) volume reflecting tumor vascularity. Additional studies were performed to determine the duration and repeatability of image enhancement, and images were correlated with conventional 3D power Doppler measurements.

RESULTS

The lifetime of the UCA in vivo was found to be 4.3 +/- 1.09 minutes (mean +/- SD). The 3D contrast-enhanced ultrasonographic technique produced images that correlated well with power Doppler images in specific regions but also depicted additional regions of flow surrounding the power Doppler signal. The mean correlation coefficient between voxel measurements of the central slice for each animal was 0.64 +/- 0.07 (P < .01). In addition, sequential studies in each animal were reproducible.

CONCLUSIONS

A method producing high-resolution volumetric assessments of tumor vascularity in a preclinical model of breast cancer is shown that correlates with other ultrasonographic measures of blood flow, which may provide greater sensitivity to the microvasculature.

Nanovehicular intracellular delivery systems

This article provides an overview of principles and barriers relevant to intracellular drug and gene transport, accumulation and retention (collectively called as drug delivery) by means of nanovehicles (NV). The aim is to deliver a cargo to a particular intracellular site, if possible, to exert a local action. Some of the principles discussed in this article apply to noncolloidal drugs that are not permeable to the plasma membrane or to the blood-brain barrier. NV are defined as a wide range of nanosized particles leading to colloidal objects which are capable of entering cells and tissues and delivering a cargo intracelullarly. Different localization and targeting means are discussed. Limited discussion on pharmacokinetics and pharmacodynamics is also presented. NVs are contrasted to micro-delivery and current nanotechnologies which are already in commercial use. Newer developments in NV technologies are outlined and future applications are stressed. We also briefly review the existing modeling tools and approaches to quantitatively describe the behavior of targeted NV within the vascular and tumor compartments, an area of particular importance. While we list "elementary" phenomena related to different level of complexity of delivery to cancer, we also stress importance of multi-scale modeling and bottom-up systems biology approach.

Flow-compensated self-gating

OBJECTIVE

Self-gating (SG) is a method to record cardiac movement during MR imaging. It uses information from an additional short, non-spatially encoded data acquisition. This usually lengthens TE and increases the sensitivity to flow artifacts. A new flow compensation scheme optimized for self-gating sequences is introduced that has very little or no time penalty over self-gating sequences without flow compensation.

MATERIALS AND METHODS

Three variants of a self-gated 2D spoiled gradient echo or fast low angle shot (FLASH) sequence were implemented: without (noFC), with a conventional, serial (cFC), and with a new, time-efficient flow compensation (sFC). In experiments on volunteers and small animals, the sequence variants were compared with regard to the SG signal and the flow artifacts in the images.

RESULTS

Both cFC and sFC reduce flow artifacts in cardiac images. The SG signal of the sFC is more sensitive to physiological motion, so that a cardiac trigger can be extracted more precisely as in cFC. In a typical setting for small animal imaging, sFC technique reduces the echo/repetition time over cFC by about 23%/14%.

CONCLUSION

The time-efficient sFC technique provides flow-compensated images with cardiac triggering in both volunteers and small animals.

Imaging Probe Development Center: a National Institutes of Health core synthesis resource for imaging probes

The Imaging Probe Development Center (IPDC) has been set up under the auspices of the National Institutes of Health (NIH) Roadmap as part of the molecular libraries and imaging initiatives. It comprises a core synthesis facility dedicated to the preparation of imaging probes, initially for intramural NIH scientists, and later, for the extramural scientific community. The facility opened fully in late 2006, in refurbished laboratories in Rockville, Maryland, and a staff of around a dozen was recruited into place by early 2007; the director was hired in late 2005. The IPDC provides a mechanism for the production of sensitive probes for use by imaging scientists who cannot obtain such probes commercially. The probes to be made will encompass all major imaging modalities including radionuclide, magnetic resonance, and optical. The operation of the IPDC is outlined, together with the results of interim achievements while the IPDC maintained a small temporary laboratory in Bethesda. As of December 2006, a total of eleven probe compositions had been made, and several of these are described with particular mention of those probes intended for use in optical applications.

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.

Cationic poly-L-lysine dendrimers: pharmacokinetics, biodistribution, and evidence for metabolism and bioresorption after intravenous administration to rats

Cationic poly-L-lysine 3H-dendrimers with either 16 or 32 surface amine groups (BHALys [Lys]4 [3H-Lys]8 [NH2]16 and BHALys [Lys]8 [3H-Lys]16 [NH2]32, generation 3 and 4, respectively) have been synthesized and their pharmacokinetics and biodistribution investigated after intravenous administration to rats. The species in plasma with which radiolabel was associated was also investigated by size exclusion chromatography (SEC). Rapid initial removal of radiolabel from plasma was evident for both dendrimers (t(1/2) < 5 min). Approximately 1 h postdose, however, radiolabel reappeared in plasma in the form of free lysine and larger (but nondendrimer) species that coeluted with albumin by SEC. Plasma and whole blood pharmacokinetics were similar, precluding interaction with blood components as a causative factor in either the rapid removal or reappearance of radioactivity in plasma. Administration of monomeric 3H L-lysine also resulted in the appearance in plasma of a radiolabeled macromolecular species that coeluted with albumin by SEC, suggesting that biodegradation of the dendrimer to L-lysine and subsequent bioresorption may explain the pharmacokinetic profiles. Capping the Lys8 dendrimer with D-lysine to form BHALys [Lys]4 [3H-Lys]8 [D-Lys]16 [NH2]32 resulted in similar, and very rapid, initial disappearance kinetics from plasma when compared to the L-lysine capped dendrimer. Since significant extravasation of these large hydrophilic molecules seems unlikely, this most likely reflects both elimination and extensive binding to vascular surfaces. Capping with "non-natural" D-lysine also appeared to render the dendrimer essentially inert to the biodegradation process. For the L-lysine capped dendrimers, radiolabel was widely distributed throughout the major organs, with no apparent selectivity for organs of the reticuloendothelial system. In contrast, a greater proportion of the administered radiolabel was recovered in the organs of the reticuloendothelial system for the D-lysine capped system, as might be expected for a nondegrading circulating foreign colloid. To our knowledge this is the first data to demonstrate the biodegradation/bioresorption of poly-L-lysine dendrimers and has significant implications for the utility of these systems as drugs or drug delivery systems.

MR angiography of tumor-related vasculature: from the clinic to the micro-environment

Angiogenesis is a very important process for tumor growth and proliferation. Given its high temporal and spatial resolution, magnetic resonance (MR) imaging is well suited for use in the assessment of angiogenesis. MR angiography can be used clinically and experimentally for identification of tumor feeding and draining vessels, for tumor characterization, and for treatment planning. The morphologic structure of tumor vessels can be investigated in relation to tumor vessel permeability with use of specific contrast agents. To gain insight into tumor angiogenesis in vivo, the authors compared images obtained with digital photography, high-resolution MR angiography, and intravital microscopy through a dorsal skin-fold window in a rodent model. The close correlation between images obtained with these various modalities, with regard to the depiction of the developing tumor vasculature, indicates that noninvasive quantification of angiogenesis may be possible with MR imaging. Future directions in tumor imaging may include so-called four-dimensional MR angiography, in which high-resolution three-dimensional MR angiography is combined with dynamic contrast-enhanced MR imaging.

Imaging Acute Renal Failure with Polyamine Dendrimer-Based MRI Contrast Agents

Acute renal failure (ARF) induced by sepsis has a high mortality but lacks effective treatments. To develop novel therapies we must diagnose renal injury early and accurately in septic patients and identify any additional insults such as nephrotoxic drugs and ischemia. In this short review we describe our experience using MRI with dendrimer-based contrast agents in mouse models of ARF. This technique can diagnose early renal injury before serum creatinine is elevated, distinguish different ARF etiologies, track drug therapy and predict outcome. As an ARF biomarker, MRI with dendrimer-based contrast is a promising technique deserving further development.

Magnetic resonance angiography of collateral vessel growth in a rabbit femoral artery ligation model

Collateral vessel growth was visualized in a rabbit femoral artery ligation model by serial contrast-enhanced magnetic resonance angiography (MRA) at 1.5 T in comparison with X-ray angiography (XRA). XRA and MRA were performed directly after femoral artery ligation (day 0+) and after 7 and 21 days. XRA (in-plane resolution, 0.3x0.3 mm) was performed with arterial catheterization for fast injection of iodinated contrast agent just proximal to the aortic bifurcation. MRA (in-plane, 0.6x0.6 mm) was performed at 1.5 T with a five-element phased-array coil and slow injection of gadolinium-based MR contrast agent into an ear vein. Collateral vessel scores on two-dimensional XRA projections and on three-dimensional digitally subtracted rotational MRA maximum intensity projections were obtained by two observers and compared. Collateral vessel counts and minimal detectable vessel diameters for MRA and XRA were combined in a computational flow model to interpret differences in spatial detection limits between imaging modalities in terms of flow. Collateral vessel scores were significantly higher in the ligated limb at day 7 (P < 0.05) and more so at day 21 (P < 0.001), in comparison with day 0+ or in the non-ligated control limb on both XRA and MRA. Significantly more (smaller) vessels were visualized with XRA than with MRA, particularly on day 21 (P < 0.05). Inter-observer agreement was high for both XRA (kappa = 0.82) and MRA (kappa = 0.78). The flow model showed that collateral vessels with diameters > 0.3 mm scored by XRA as well as MRA represent nearly 100% of the total blood flow, whereas smaller (0.1-0.3 mm diameter) vessels that can only be detected with XRA contribute little to the blood flow. Serial contrast-enhanced MRA can non-invasively visualize sub-millimeter collateral vessels that represent nearly 100% of the restored blood flow, in a femoral artery ligation model.

PAMAM Dendrimer Based Macromolecules as Improved Contrast Agents

Dendrimers are an attractive platform for macromolecular imaging due to the presence of multiple terminal groups on the exterior of the molecule. Through application of appropriate metal ion chelates, large numbers of metal ions for imaging (paramagnetic or radioopaque) and therapy (radioactive particle emitters) may be conjugated to the dendrimer in combination with a targeting vector, through classic organic synthetic techniques. Thus, a large amount of these metal ions potentially may be site specifically delivered directly into the body with the dendrimer as the vehicle with the targeting vector directing the modified dendrimer. The development of targeted macromolecular agents with acceptable blood retention times and selective organ uptake then has the potential for various biological applications. A review of comparative studies of dendrimers with various externally appended imaging and targeting agents is presented herein.