MR imaging with ultrasmall superparamagnetic iron oxide particles in experimental soft-tissue infections in rats

Nanomedicine as a promising strategy for the theranostics of infectious diseases

Infectious diseases caused by bacteria, viruses, and fungi and their global spread pose a great threat to human health. The 2019 World Health Organization report predicted that infection-related mortality will be similar to cancer mortality by 2050. Particularly, the global cumulative numbers of the recent outbreak of coronavirus disease (COVID-19) have reached 110.7 million cases and over 2.4 million deaths as of February 23, 2021. Moreover, the crisis of these infectious diseases exposes the many problems of traditional diagnosis, treatment, and prevention, such as time-consuming and unselective detection methods, the emergence of drug-resistant bacteria, serious side effects, and poor drug delivery. There is an urgent need for rapid and sensitive diagnosis as well as high efficacy and low toxicity treatments. The emergence of nanomedicine has provided a promising strategy to greatly enhance detection methods and drug treatment efficacy. Owing to their unique optical, magnetic, and electrical properties, nanoparticles (NPs) have great potential for the fast and selective detection of bacteria, viruses, and fungi. NPs exhibit remarkable antibacterial activity by releasing reactive oxygen species and metal ions, exerting photothermal effects, and causing destruction of the cell membrane. Nano-based delivery systems can further improve drug permeability, reduce the side effects of drugs, and prolong systemic circulation time and drug half-life. Moreover, effective drugs against COVID-19 are still lacking. Recently, nanomedicine has shown great potential to accelerate the development of safe and novel anti-COVID-19 drugs. This article reviews the fundamental mechanisms and the latest developments in the treatment and diagnosis of bacteria, viruses, and fungi and discusses the challenges and perspectives in the application of nanomedicine.

Sensitivity of quantitative relaxometry and susceptibility mapping to microscopic iron distribution

PURPOSE

Determine the impact of the microscopic spatial distribution of iron on relaxometry and susceptibility-based estimates of iron concentration.

METHODS

Monte Carlo simulations and in vitro experiments of erythrocytes were used to create different microscopic distributions of iron. Measuring iron with intact erythrocyte cells created a heterogeneous distribution of iron, whereas lysing erythrocytes was used to create a homogeneous distribution of iron. Multi-echo spin echo and spoiled gradient echo acquisitions were then used to estimate relaxation parameters ( R 2 and R 2 * ) and susceptibility.

RESULTS

Simulations demonstrate that R 2 and R 2 * measurements depend on the spatial distribution of iron even for the same iron concentration and volume susceptibility. Similarly, in vitro experiments demonstrate that R 2 and R 2 * measurements depend on the microscopic spatial distribution of iron whereas the quantitative susceptibility mapping (QSM) susceptibility estimates reflect iron concentration without sensitivity to spatial distribution.

CONCLUSIONS

R 2 and R 2 * for iron quantification depend on the spatial distribution or iron. QSM-based estimation of iron concentration is insensitive to the microscopic spatial distribution of iron, potentially providing a distribution independent measure of iron concentration.

Recent advances in the treatment of pathogenic infections using antibiotics and nano-drug delivery vehicles

The worldwide misuse of antibiotics and the subsequent rise of multidrug-resistant pathogenic bacteria have prompted a paradigm shift in the established view of antibiotic and bacterial-human relations. The clinical failures of conventional antibiotic therapies are associated with lengthy detection methods, poor penetration at infection sites, disruption of indigenous microflora and high potential for mutational resistance. One of the most promising strategies to improve the efficacy of antibiotics is to complex them with micro or nano delivery materials. Such materials/vehicles can shield antibiotics from enzyme deactivation, increasing the therapeutic effectiveness of the drug. Alternatively, drug-free nanomaterials that do not kill the pathogen but target virulent factors such as adhesins, toxins, or secretory systems can be used to minimize resistance and infection severity. The main objective of this review is to examine the potential of the aforementioned materials in the detection and treatment of antibiotic-resistant pathogenic organisms.

MRI detection of bacterial brain abscesses and monitoring of antibiotic treatment using bacCEST

PURPOSE

To develop a new MRI method to detect and characterize brain abscesses using the CEST contrast inherently carried by bacterial cells, namely bacCEST.

METHODS

Bacteria S. aureus (ATCC #49775) and F98 and 9L glioma cells were injected stereotactically in the brains of F344 rats to form abscesses and tumors. The CEST signals of brain abscesses (n = 4) and tumors (n = 7) were acquired using 2 B1 values (i.e., 1 and 3 µT) and compared. The bacCEST signal of the brain abscesses in the rats (n = 3) receiving ampicillin (intraperitoneal injection 40 mg/kg twice daily) was acquired before, 4 and 10 days after the treatment.

RESULTS

The bacCEST signal of S. aureus was characterized in vitro as a strong and broad signal in the range of 1 to 4 ppm, with the maximum contrast occurring at 2.6 ppm. The CEST signal in S. aureus-induced brain abscesses was significantly higher than that of contralateral parenchyma (p = .003). Moreover, thanks to their different B1 independence, brain abscesses and tumors could be effectively differentiated (p = .005) using ΔCEST(2.6 ppm, 3 µT-1 µT), defined by the difference between the CEST signal (offset = 2.6 ppm) acquired using B1  = 3 µT and that of 1 µT. In treated rats, bacCEST MRI could detect the response of bacteria as early as 4 days after the antibiotic treatment (p = .035).

CONCLUSION

BacCEST MRI provides a new imaging method to detect, discriminate, and monitor bacterial infection in deep-seated organs. Because no contrast agent is needed, such an approach has a great translational potential for detecting and monitoring bacterial infection in deep-seated organs.

Nanoparticle-based local antimicrobial drug delivery

Despite the wide success of antibiotics in modern medicine, the treatment of bacterial infections still faces critical challenges, especially due to the rapid emergence of antibiotic resistance. As a result, local antimicrobial treatment aimed at enhancing drug concentration at the site of infection while avoiding systemic exposure is becoming increasingly attractive, as it may alleviate resistance development. Meanwhile, therapeutic nanoparticles, especially liposomes, polymeric nanoparticles, dendrimers, and inorganic nanoparticles, are gaining traction to improve the therapeutic efficacy with many applications specifically focused on local antimicrobial treatment. This review highlights topics where nanoparticle-based strategies hold significant potential to advance treatment against local bacterial infections, including (1) promoting antibiotic localization to the pathogen, (2) modulating drug-pathogen interaction against antibiotic resistance, and (3) enabling novel anti-virulence approaches for 'drug-free' antimicrobial activity. In each area, we highlight the innovative antimicrobial strategies tailored for local applications and review the progress made for the treatment of bacterial infections.

Nano-therapeutics: A revolution in infection control in post antibiotic era

With the arrival of antibiotics 70 years ago, meant a paradigm shift in overcoming infectious diseases. For decades, drugs have been used to treat different infections. However, with time bacteria have become resistant to multiple antibiotics, making some diseases difficult to fight. Nanoparticles (NPs) as antibacterial agents appear to have potential to overcome such problems and to revolutionize the diagnosis and treatment of bacterial infections. Therefore, there is significant interest in the use of NPs to treat variety of infections, particularly caused by multidrug-resistant (MDR) strains. This review begins with illustration of types of NPs followed by the literature of current research addressing mechanisms of NPs antibacterial activity, steps involved in NP mediated drug delivery as well as areas where NPs use has potential to improve the treatment, like NP enabled vaccination. Besides, recently emerged innovative NP platforms have been highlighted and their progress made in each area has been reviewed.

Magnetic resonance imaging of pathogenic protozoan parasite Entamoeba histolytica labeled with superparamagnetic iron oxide nanoparticles

OBJECTIVES

The aim of this study was to establish a noninvasive tracking of the pathogenic parasite Entamoeba histolytica (Eh) after superparamagnetic iron oxide (SPIO) labeling by magnetic resonance imaging (MRI) on a single-cell level in vitro and in vivo in a mouse model for amebic liver abscess (ALA).

MATERIALS AND METHODS

Local institutional review committee on animal care approved all animal experiments. Entamoeba histolytica trophozoites were labeled with SPIO nanoparticles (SPIO-Eh). The uptake of SPIO by Eh was optimized using flow cytometry and visualized by bright field, fluorescence, and transmission electron microscopy. The viability of SPIO-Eh was assessed in vitro by determination of growth and ingestion rate of red blood cells. Migration of SPIO-Eh was proven by in vitro MRI in a preclinical 7 T MRI system using continually repeated MRI scans. In vivo distribution of SPIO-Eh within the mouse liver was assessed qualitatively and quantitatively by serial respiration-triggered T2*-weighted MRI, T2-weighted MRI, and R2* MR relaxometry up to 5 days after injection and correlated with immunohistology of the liver sections after removal.

RESULTS

Entamoeba histolytica can be efficiently labeled with SPIO without influence on parasite growth rate or phagocytic capacity. In vitro dynamic MRI allowed real-time migration monitoring and determination of velocity of single SPIO-Eh. In vivo SPIO-Eh showed signal decrease in T2*-weighted and increase of R2* in ALA formations. Motility of SPIO-Eh was necessary to induce ALA formations.

CONCLUSIONS

The present study demonstrates the feasibility of an efficient magnetic labeling and a noninvasive in vitro and in vivo MR tracking of the pathogenic protozoan Eh in a mouse model for ALA, thus representing in future a noninvasive imaging tool to study parasite, as well as on host-specific pathomechanisms.

Highly monodisperse low-magnetization magnetite nanocubes as simultaneous T(1)-T(2) MRI contrast agents

We report the first study of highly monodisperse and crystalline iron oxide nanocubes with sub-nm controlled size distribution (9.7 ± 0.5 nm in size) that achieve simultaneous contrast enhancement in both T1- and T2-weighted magnetic resonance imaging (MRI). Here, we confirmed the magnetite structure of iron oxide nanocubes by X-ray diffraction (XRD), selected area electron diffraction (SAED) pattern, optical absorption and Fourier transformed infrared (FT-IR) spectra. These magnetite nanocubes exhibit superparamagnetic and paramagnetic behavior simultaneously by virtue of their finely controlled shape and size. The magnetic measurements reveal that the magnetic moment values are favorably much lower because of the small size and cubic shape of the nanoparticles, which results in an enhanced spin canting effect. As a proof-of-concept demonstration, we showed their potential as dual contrast agents for both T1- and T2-weighted MRI via phantom studies, in vivo imaging and relaxivity measurements. Therefore, these low-magnetization magnetite nanocubes, while being non-toxic and bio-compatible, hold great promise as excellent dual-mode T1 and T2 contrast agents for MRI.

Evaluation of [¹¹¹In]-labeled zinc-dipicolylamine tracers for SPECT imaging of bacterial infection

PURPOSE

This study prepared three structurally related zinc-dipicolylamine (ZnDPA) tracers with [(111)In] labels and conducted biodistribution and single-photon emission computed tomography/computed tomography (SPECT/CT) imaging studies of a mouse leg infection model.

PROCEDURES

Two monovalent tracers, ZnDPA-[(111)In]DTPA and ZnDPA-[(111)In]DOTA, each with a single zinc-dipicolylamine targeting unit, and a divalent tracer, Bis(ZnDPA)-[(111)In]DTPA, with two zinc-dipicolylamine units were prepared. Organ biodistribution and SPECT and CT imaging studies were performed on living mice with a leg infection created by injection of clinically relevant Gram positive Streptococcus pyogenes. Fluorescent and luminescent Eu(3+)-labeled versions of these tracers were also prepared and used to measure relative affinity for the exterior membrane surface of bacterial cells and mimics of healthy mammalian cells.

RESULTS

All three (111)In-labeled radiotracers were prepared with a radiopurity of >90 %. The biodistribution studies showed that the two monovalent tracers were cleared from the body through the liver and kidney, with retained percentage injected dose for all organs of <8 % at 20 h and infected leg target to non-target ratio (T/NT) ratio of ≤3.0. Clearance of the divalent tracer from the bloodstream was slower and primarily through the liver, with a retained percentage injected dose for all organs <37 % at 20 h and T/NT ratio rising to 6.2 after 20 h. The SPECT/CT imaging indicated the same large difference in tracer pharmacokinetics and higher accumulation of the divalent tracer at the site of infection.

CONCLUSIONS

All three [(111)In]-ZnDPA tracers selectively targeted the site of a clinically relevant mouse infection model that could not be discerned by visual external inspection of the living animal. The highest target selectivity, observed with a divalent tracer equipped with two zinc-dipicolylamine targeting units, compares quite favorably with the imaging selectivities previously reported for other nuclear tracers that target bacterial cell surfaces. The tracer pharmacokinetics depended heavily on tracer molecular structure suggesting that it may be possible to rapidly fine tune the structural properties for optimized in vivo imaging performance and clinical translation.

Nanoparticle approaches against bacterial infections

Despite the wide success of antibiotics, the treatment of bacterial infections still faces significant challenges, particularly the emergence of antibiotic resistance. As a result, nanoparticle drug delivery platforms including liposomes, polymeric nanoparticles, dendrimers, and various inorganic nanoparticles have been increasingly exploited to enhance the therapeutic effectiveness of existing antibiotics. This review focuses on areas where nanoparticle approaches hold significant potential to advance the treatment of bacterial infections. These areas include targeted antibiotic delivery, environmentally responsive antibiotic delivery, combinatorial antibiotic delivery, nanoparticle-enabled antibacterial vaccination, and nanoparticle-based bacterial detection. In each area we highlight the innovative antimicrobial nanoparticle platforms and review their progress made against bacterial infections.

MRI visualization of Staphyloccocus aureus-induced infective endocarditis in mice

Infective endocarditis (IE) is a severe and often fatal disease, lacking a fast and reliable diagnostic procedure. The purpose of this study was to establish a mouse model of Staphylococcus aureus-induced IE and to develop a MRI technology to characterize and diagnose IE. To establish the mouse model of hematogenous IE, aortic valve damage was induced by placing a permanent catheter into right carotid artery. 24 h after surgery, mice were injected intravenously with either iron particle-labeled or unlabeled S. aureus (strain 6850). To distinguish the effect of IE from mere tissue injury or recruited macrophages, subgroups of mice received sham surgery prior to infection (n = 17), received surgery without infection (n = 8), or obtained additionally injection of free iron particles to label macrophages (n = 17). Cardiac MRI was performed 48 h after surgery using a self-gated ultra-short echo time (UTE) sequence (TR/TE, 5/0.31 ms; in-plane/slice, 0.125/1 mm; duration, 12∶08 min) to obtain high-resolution, artifact-free cinematographic images of the valves. After MRI, valves were either homogenized and plated on blood agar plates for determination of bacterial titers, or sectioned and stained for histology. In the animal model, both severity of the disease and mortality increased with bacterial numbers. Infection with 105 S. aureus bacteria reliably caused endocarditis with vegetations on the valves. Cinematographic UTE MRI visualised the aortic valve over the cardiac cycle and allowed for detection of bacterial vegetations, while mere tissue trauma or labeled macrophages were not detected. Iron labeling of S. aureus was not required for detection. MRI results were consistent with histology and microbial assessment. These data showed that S. aureus-induced IE in mice can be detected by MRI. The established mouse model allows for investigation of the pathophysiology of IE, testing of novel drugs and may serve for the development of a clinical diagnostic strategy.

Determination of liver-specific r2 * of a highly monodisperse USPIO by (59) Fe iron core-labeling in mice at 3 T MRI

Accurate determination of tissue concentration of ultrasmall superparamagnetic iron oxide nanoparticles (USPIO) using T2 * MR relaxometry is still challenging. We present a reliable quantification method for local USPIO amount with the estimation of the liver specific relaxivity r2 * using monodisperse (59) Fe-core-labeled USPIO ((59) FeUSPIO). Dynamic and relaxometric in vivo characteristics of unlabeled monodisperse USPIO were determined in MRI at 3 T. The in vivo MR studies were performed for liver tissue with (59) FeUSPIO using iron dosages of 9 (n = 3), 18 (n = 2) and 27 (n = 3) µmol Fe kg(-1) body weight. The R2 * of the liver before and after USPIO injection (∆R2 *) was measured and correlated with (59) Fe activity measurements of excised organs by a whole body radioactivity counter (HAMCO) to define the dependency of ∆R2 * and (59) FeUSPIO liver concentration and calculate the r2 * of (59) FeUSPIO for the liver. Ultrastructural analysis of liver uptake was performed by histology and transmission electron microscopy. ∆R2 * of the liver revealed a dosage-dependent accumulation of (59) FeUSPIO with a percentage uptake of 70-88% of the injection dose. Hepatic ∆R2 * showed a dose-dependent linear correlation to (59) FeUSPIO activity measurements (r = 0.92) and an r2 * in the liver of 481 ± 74.9 mm(-1) s(-1) in comparison to an in vitro r2 * of 60.5 ± 3.3 mm(-1) s(-1) . Our results indicate that core-labeled (59) FeUSPIO can be used to quantify the local amount of USPIO and to estimate the liver-specific relaxivity r2 *.

Magnetic resonance imaging characterization of microbial infections

The investigation of microbial infections relies to a large part on animal models of infection, if host pathogen interactions or the host response are considered. Especially for the assessment of novel therapeutic agents, animal models are required. Non-invasive imaging methods to study such models have gained increasing importance over the recent years. In particular, magnetic resonance imaging (MRI) affords a variety of diagnostic options, and can be used for longitudinal studies. In this review, we introduce the most important MRI modalities that show how MRI has been used for the investigation of animal models of infection previously and how it may be applied in the future.

MRI confirms loss of blood-brain barrier integrity in a mouse model of disseminated candidiasis

Disseminated candidiasis primarily targets the kidneys and brain in mice and humans. Damage to these critical organs leads to the high mortality associated with such infections, and invasion across the blood-brain barrier can result in fungal meningoencephalitis. Candida albicans can penetrate a brain endothelial cell barrier in vitro through transcellular migration, but this mechanism has not been confirmed in vivo. MRI using the extracellular vascular contrast agent gadolinium diethylenetriaminepentaacetic acid demonstrated that integrity of the blood-brain barrier is lost during C. albicans invasion. Intravital two-photon laser scanning microscopy was used to provide the first real-time demonstration of C. albicans colonizing the living brain, where both yeast and filamentous forms of the pathogen were found. Furthermore, we adapted a previously described method utilizing MRI to monitor inflammatory cell recruitment into infected tissues in mice. Macrophages and other phagocytes were visualized in kidney and brain by the administration of ultrasmall iron oxide particles. In addition to obtaining new insights into the passage of C. albicans across the brain microvasculature, these imaging methods provide useful tools to study further the pathogenesis of C. albicans infections, to define the roles of Candida virulence genes in kidney versus brain infection and to assess new therapeutic measures for drug development.

Cancer and inflammation: differentiation by USPIO-enhanced MR imaging

PURPOSE

To assess ultrasmall superparamagnetic iron oxide particles (USPIO) -enhanced MR imaging for the differentiation of malignant from benign, inflammatory lesions.

MATERIALS AND METHODS

In this study, approved by the local animal care committee, VX2 carcinoma and intramuscular abscesses were implanted into the hind thighs of New Zealand White rabbits. MR imaging was performed pre contrast and serially for 24 h after the injection of USPIO. MR findings were compared with histopathologic results based on Prussian blue stains for the presence of iron.

RESULTS

Twenty-four hours after the Ferumoxtran-injection, no changes were observed in VX2 carcinomas, whereas a mean reduction of the contrast-to-noise ratio (CNR) of approximately 90% was noticed in abscesses as well as in necrotic tumors. On histopathologic examination, abscess and necrotic parts of the tumor were found to include iron-containing monocytes demonstrating that the reduction in CNR was caused by USPIO-tagged monocytes.

CONCLUSION

Our results prove the ability of USPIO-enhanced MRI to differentiate benign, inflammatory from malignant lesions.

Bacteria tracking by in vivo magnetic resonance imaging

Background

Different non-invasive real-time imaging techniques have been developed over the last decades to study bacterial pathogenic mechanisms in mouse models by following infections over a time course. In vivo investigations of bacterial infections previously relied mostly on bioluminescence imaging (BLI), which is able to localize metabolically active bacteria, but provides no data on the status of the involved organs in the infected host organism. In this study we established an in vivo imaging platform by magnetic resonance imaging (MRI) for tracking bacteria in mouse models of infection to study infection biology of clinically relevant bacteria.

Results

We have developed a method to label Gram-positive and Gram-negative bacteria with iron oxide nano particles and detected and pursued these with MRI. The key step for successful labeling was to manipulate the bacterial surface charge by producing electro-competent cells enabling charge interactions between the iron particles and the cell wall. Different particle sizes and coatings were tested for their ability to attach to the cell wall and possible labeling mechanisms were elaborated by comparing Gram-positive and -negative bacterial characteristics. With 5-nm citrate-coated particles an iron load of 0.015 ± 0.002 pg Fe/bacterial cell was achieved for Staphylococcus aureus. In both a subcutaneous and a systemic infection model induced by iron-labeled S. aureus bacteria, high resolution MR images allowed for bacterial tracking and provided information on the morphology of organs and the inflammatory response.

Conclusion

Labeled with iron oxide particles, in vivo detection of small S. aureus colonies in infection models is feasible by MRI and provides a versatile tool to follow bacterial infections in vivo. The established cell labeling strategy can easily be transferred to other bacterial species and thus provides a conceptual advance in the field of molecular MRI.

Assessment and comparison of magnetic nanoparticles as MRI contrast agents in a rodent model of human hepatocellular carcinoma

The purpose of this study was to synthesize, characterize and tailor the surface properties of magnetic nanoparticles with biocompatible copolymer coatings and to evaluate the efficiency of the resulting nanoconjugates as magnetic resonance imaging (MRI) contrast agents for liver imaging. Magnetic nanoparticles with core diameters of 10 and 30 nm were synthesized by pyrolysis and were subsequently coated with a copolymer containing either carboxyl (SHP) or methoxy groups as termini. All four formulas, and ferumoxides (Feridex I.V.(®)), were individually injected intravenously into separate, normal Balb/C mice (at 2.5, 1.0 and 0.56 mg Fe kg(-1)), and the animals underwent T(2)-weighted MRI at multiple time points post injection (p.i.) to evaluate the hepatic uptake and clearance. Furthermore, we compared the abilities of the new formulas and Feridex to detect tumors in an orthotropic Huh7 tumor model. Transmission electron microscopy (TEM) revealed a narrow size distribution of both the 10 and 30 nm nanoparticles, in contrast to a wide size distribution of Feridex. MTT, apoptosis and cyclin/DNA flow cytometry assays showed that the polymer coated nanoparticles had no adverse effect on cell growth. Among all the tested formulas, including Feridex, SHP-30 showed the highest macrophage uptake at the in vitro level. In vivo MRI studies on normal mice confirmed the superiority of SHP-30 in inducing hypointensities in the liver tissue, especially at clinical dose (0.56 mg Fe kg(-1)) and 3 T field. SHP-30 showed better contrast-to-noise ratio than Feridex on the orthotropic Huh7 tumor model. SHP-30 was found to be an efficient contrast agent for liver MR imaging. The success of this study suggests that, by improving the synthetic approach and by tuning the surface properties of IONPs, one can arrive at better formulas than Feridex for clinical practice.

Magnetic resonance imaging of soft tissue infection with iron oxide labeled granulocytes in a rat model

OBJECT

We sought to detect an acute soft tissue infection in rats by magnetic resonance imaging (MRI) using granulocytes, previously labeled with superparamagnetic particles of iron oxide (SPIO).

MATERIALS AND METHODS

Parasternal infection was induced by subcutaneous inoculation of Staphylococcus aureus suspension in rats. Granulocytes isolated from isogenic donor rats were labeled with SPIO. Infected rats were imaged by MRI before, 6 and 12 hours after intravenous injection of SPIO-labeled or unlabeled granulocytes. MR findings were correlated with histological analysis by Prussian blue staining and with re-isolated SPIO-labeled granulocytes from the infectious area by magnetic cell separation.

RESULTS

Susceptibility effects were present in infected sites on post-contrast T2*-weighted MR images in all animals of the experimental group. Regions of decreased signal intensity (SI) in MRI were detected at 6 hours after granulocyte administration and were more pronounced at 12 hours. SPIO-labeled granulocytes were identified by Prussian blue staining in the infected tissue and could be successfully re-isolated from the infected area by magnetic cell separation.

CONCLUSION

The application of SPIO-labeled granulocytes in MRI offers new perspectives in diagnostic specificity and sensitifity to detect early infectious processes.

Septic arthritis: monitoring with USPIO-enhanced macrophage MR imaging

PURPOSE

To prospectively evaluate in vivo noninvasive monitoring of antibiotic therapy in experimental infectious arthritis by imaging macrophages by using magnetic resonance (MR) imaging enhanced with ultrasmall superparamagnetic iron oxide (USPIO) particles.

MATERIALS AND METHODS

The institutional review committee on animal care approved the experimental protocol. Unilateral knee infection was induced by intra-articular injection of Staphylococcus aureus in 12 rabbits. Each rabbit underwent MR imaging before and after injection of USPIO particles, as well as before and after injection of gadoterate meglumine. All 12 of the animals were imaged during the acute phase of infection. Half were then sacrificed to obtain histopathologic samples, and the other half were imaged a second time after antibiotic treatment. MR imaging data were analyzed and compared with bacteriologic and histopathologic findings.

RESULTS

In acute infections, intense synovitis with marked signal intensity increase of the synovium on gadoterate dimeglumine-enhanced fat-suppressed T1-weighted images was observed in all animals and was associated with areas of signal intensity loss within the infected synovium on USPIO-enhanced T2*-weighted gradient-echo images, reflecting an intense infiltration of USPIO-loaded macrophages. After antibiotic treatment and histologic evidence of healing infection, less synovial signal intensity loss was seen (P = .03). In contradistinction, the signal intensity increase on gadoterate dimeglumine-enhanced fat-suppressed T1-weighted images remained unchanged.

CONCLUSION

In contrast to conventional MR imaging performed by using extracellular contrast agents, USPIO-enhanced macrophage MR imaging can demonstrate resolution of experimental bacterial joint infection.

Visualization of abscess formation in a murine thigh infection model of Staphylococcus aureus by 19F-magnetic resonance imaging (MRI)

BACKGROUND

During the last years, (19)F-MRI and perfluorocarbon nanoemulsion (PFC) emerged as a powerful contrast agent based MRI methodology to track cells and to visualize inflammation. We applied this new modality to visualize deep tissue abscesses during acute and chronic phase of inflammation caused by Staphylococcus aureus infection.

METHODOLOGY AND PRINCIPAL FINDINGS

In this study, a murine thigh infection model was used to induce abscess formation and PFC or CLIO (cross linked ironoxides) was administered during acute or chronic phase of inflammation. 24 h after inoculation, the contrast agent accumulation was imaged at the site of infection by MRI. Measurements revealed a strong accumulation of PFC at the abscess rim at acute and chronic phase of infection. The pattern was similar to CLIO accumulation at chronic phase and formed a hollow sphere around the edema area. Histology revealed strong influx of neutrophils at the site of infection and to a smaller extend macrophages during acute phase and strong influx of macrophages at chronic phase of inflammation.

CONCLUSION AND SIGNIFICANCE

We introduce (19)F-MRI in combination with PFC nanoemulsions as a new platform to visualize abscess formation in a murine thigh infection model of S. aureus. The possibility to track immune cells in vivo by this modality offers new opportunities to investigate host immune response, the efficacy of antibacterial therapies and the influence of virulence factors for pathogenesis.

Combined off-resonance imaging and T2 relaxation in the rotating frame for positive contrast MR imaging of infection in a murine burn model

PURPOSE

To develop novel magnetic resonance (MR) imaging methods to monitor accumulation of macrophages in inflammation and infection. Positive-contrast MR imaging provides an alternative to negative-contrast MRI, exploiting the chemical shift induced by ultra-small superparamagnetic iron-oxide (USPIO) nanoparticles to nearby water molecules. We introduce a novel combination of off-resonance (ORI) positive-contrast MRI and T(2ρ) relaxation in the rotating frame (ORI-T(2ρ)) for positive-contrast MR imaging of USPIO.

MATERIALS AND METHODS

We tested ORI-T(2ρ) in phantoms and imaged in vivo the accumulation of USPIO-labeled macrophages at the infection site in a mouse model of burn trauma and infection with Pseudomonas aeruginosa (PA). PA infection is clinically important. The USPIO nanoparticles were injected directly in the animals in solution, and macrophage labeling occurred in vivo in the animal model.

RESULTS

We observed a significant difference between ORI-T(2ρ) and ORI, which leads us to suggest that ORI-T(2ρ) is more sensitive in detecting USPIO signal. To this end, the ORI-T(2ρ) positive contrast method may prove to be of higher utility in future research.

CONCLUSION

Our results may have direct implications in the longitudinal monitoring of infection, and open perspectives for testing novel anti-infective compounds.

Dynamics and fate of USPIO in the central nervous system in experimental autoimmune encephalomyelitis

Signal loss observed in the brain by MRI following the administration of ultrasmall superparamagnetic particles of iron oxide (USPIO) has been correlated with immune cell activity in inflammatory areas during multiple sclerosis. Uptake of USPIO by circulating monocytes and their migration towards inflammatory areas have been considered as the most important mechanism for USPIO uptake by the brain parenchyma. However, the involvement of a damaged blood-brain barrier is also debated as a possible mechanism for cerebral USPIO uptake. Compared with these uptake-associated issues, little is known about the clearance of USPIO from the brain. The acute uptake and chronic clearance of USPIO in the brain were therefore studied with MRI in an animal model of multiple sclerosis. Lewis Hannover rats with acute experimental autoimmune encephalomyelitis received a single intravenous injection of USPIO (300 µmol Fe/kg), and repetitive MRI of the brain and cervical lymph nodes, a possible drainage pathway, was performed. USPIO were detected in the brain within 1 h after injection independent of the severity of experimental autoimmune encephalomyelitis, and histological analysis revealed extracellular iron clusters colocalising with a leaky blood-brain barrier. Loss of signal was not present 72 h after USPIO injection, irrespective of the disease state. MR images of cervical lymph nodes showed USPIO accumulation at 24 h after administration, which stabilised at 72 h. Histological analyses revealed that USPIO accumulated in infiltrated macrophages in the medulla and subcapsular sinus. The current study demonstrates that USPIO enter the central nervous system directly after administration, pointing to the involvement of a damaged blood-brain barrier in the appearance of USPIO-associated MR abnormalities. Furthermore, a possible role of the cervical lymph nodes as a drainage pathway of USPIO is suggested. These data shed new light on the use of USPIO in neuroinflammatory diseases, identifying USPIO as a marker for both cellular infiltration and blood-brain barrier damage.

USPIO-enhanced magnetic resonance imaging of the knee in asymptomatic volunteers

The aim of this study was to compare signal characteristics of the synovium in knees of asymptomatic volunteers before and after intravenous administration of ultrasmall superparamagnetic iron oxide particles (USPIO). Ten knees of 10 asymptomatic volunteers were examined before and 36 h after intravenous administration of USPIO on a 1.5-T MR system using T1-weighted spin-echo, T2-weighted fast spin-echo, T2*-weighted gradient-echo (GRE), and short inversion time inversion-recovery sequences. In addition, synovial perfusion was measured using Gd-enhanced GRE imaging during the first imaging session. Images were analyzed qualitatively for any visual changes before and after USPIO administration. Signal-to-noise ratios (SNR) of the synovium were determined on unenhanced and USPIO-enhanced sequences. All MR images were reviewed for presence of any degenerative changes. Qualitative image analysis revealed no visually detectable changes of any knee joint before and after USPIO administration. The SNR values of the synovium on T1w, T2w, and T2*w images before and after USPIO administration showed no significant difference (T1, P = 0.86; T2, P = 0.95; T2*, P = 0.86). None of the volunteers showed any relevant degenerative changes of the knee and synovial perfusion was within normal limits. In knees of asymptomatic volunteers without any relevant degenerative changes and normal synovial perfusion neither visual changes nor changes of SNR values of the synovium can be depicted after USPIO administration. This means that USPIO-enhanced MRI may be used for assessment of knee disorders with increased macrophage activity.

Macrophage imaging by USPIO-enhanced MR for the differentiation of infectious osteomyelitis and aseptic vertebral inflammation

The purpose of this study was to prospectively evaluate USPIO-enhanced MR imaging for the differentiation of vertebral infectious osteomyelitis and sterile inflammation. Vertebral osteomyelitis and sterile vertebral inflammation were induced in two groups of six rabbits each. MRI examinations were performed including unenhanced and gadolinium-enhanced fat-saturated SE T1w sequences. Once endplate enhancement was observed on the T1 gadolinium-enhanced MR sequence, a second MRI examination (SE T1w sequence) was performed 24 h after USPIO administration (45 micromol Fe/kg). MR imaging was correlated with histopathological findings (macrophage immunostaining and Perls Prussian blue staining). On gadolinium-enhanced T1 sequences, a significant SNR increase in vertebral endplates was present in both groups without significant difference between the two groups (P = 0.26). On USPIO-enhanced T1 sequences, a significant SNR increase was only observed in the infection group (P = 0.03) with a significant difference in SNR between the infection and the sterile-inflammation groups (P = 0.002). Infected areas presented replacement of bone marrow by an intense macrophage infiltration, some being iron-loaded. Sterile inflammation showed a replacement of bone marrow by inflammatory tissue with only rare macrophages without any Perls blue staining. USPIO-enhanced MR imaging can distinguish infectious osteomyelitis from sterile vertebral inflammation due to different macrophage distributions in the two lesions.

Macrophage activity in infected areas of an experimental vertebral osteomyelitis model: USPIO-enhanced MR imaging--feasibility study

PURPOSE

To prospectively evaluate ultrasmall superparamagnetic iron oxide (USPIO) magnetic resonance (MR) imaging for the depiction of macrophages in infected areas of an experimental rabbit vertebral osteomyelitis model.

MATERIALS AND METHODS

Lumbar vertebral osteomyelitis was induced in 10 rabbits with intradiscal injection of bacteria in a vertebral disk (test level) versus saline injection in another disk (control level). After a mean interval of 12 days, rabbits were imaged prior to and 24 hours after administration of USPIO. The MR imaging protocol included T1-weighted spin-echo, T2-weighted fast spin-echo, and T2*-weighted gradient-echo sequences. MR findings were compared with histologic findings (macrophage immunostaining and Perls Prussian blue staining). A Wilcoxon signed rank test was used to compare signal-to-noise ratio (SNR) results before and after USPIO administration.

RESULTS

T1-weighted MR images of infected vertebral test levels obtained 24 hours after USPIO administration showed a significant increase in SNR (P = .005), whereas T2- and T2*-weighted images showed no significant changes in SNR (P = .14 and P = .87, respectively). Histologic examination results of infected areas demonstrated complete replacement of hematopoietic bone marrow by macrophage infiltration. Perls Prussian blue staining showed that some macrophages were iron loaded. T1- (P = .02), T2- (P = .04), and T2*-weighted (P = .04) images of control vertebrae showed a significant decrease in SNR. Histologic examination results confirmed the persistence of normal hematopoietic bone marrow without macrophage infiltration, which was reflected by more intensive Perls Prussian blue staining compared with that in infected areas.

CONCLUSION

MR imaging can depict USPIO-loaded macrophage infiltration present in infected areas in an experimental rabbit model of vertebral osteomyelitis.

Synthesis and characterization of PVP-coated large core iron oxide nanoparticles as an MRI contrast agent

The purpose of this study was to synthesize biocompatible polyvinylpyrrolidone (PVP)-coated iron oxide (PVP-IO) nanoparticles and to evaluate their efficacy as a magnetic resonance imaging (MRI) contrast agent. The PVP-IO nanoparticles were synthesized by a thermal decomposition method and characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), dynamic light scattering (DLS), and a superconducting quantum interface device (SQUID). The core size of the particles is about 8-10 nm and the overall size is around 20-30 nm. The measured r(2) (reciprocal of T(2) relaxation time) and r2∗ (reciprocal of T2∗ relaxation time) are 141.2 and 338.1 (s mM)(-1), respectively. The particles are highly soluble and stable in various buffers and in serum. The macrophage uptake of PVP-IO is comparable to that of Feridex as measured by a Prussian blue iron stain and phantom study. The signal intensity of a rabbit liver was effectively reduced after intravenous administration of PVP-IO. Therefore PVP-IO nanoparticles are potentially useful for T(2)-weighted MR imaging.

Ultrasensitive optical biodiagnostic methods using metallic nanoparticles

Dramatic progress has been made over the recent decade in the applications of metallic nanoparticles in the field of biomolecule detection. The useful physical and chemical properties (e.g., availability of various synthetic methods of size- and shape-controlled nanoparticles, size- and shape-dependent optical properties, availability of various surface chemistries and biocompatibility) of metallic nanoparticles have brought development to the ultrasensitive detection of biomolecules at the attomolar level and this sensitivity enables the diagnosis of otherwise undetectable biomarkers of many fatal diseases, including Alzheimer’s disease. Furthermore, coupled with the strong physical properties and biocompatible nature of gold nanoparticles in in vivo conditions, the scope of applications for these particles have been broadened into the field of in vivo imaging, such as X-ray contrasting agents, and also cellular tracking. Here, we review synthetic methods and optical properties of metallic nanoparticles and their use in ultrasensitive, in vitro and in vivo biodiagnostic methods.

[Molecular and parametric imaging with iron oxides]

Superparamagnetic iron oxide (SPIO) contrast agents, clinically established for high resolution magnetic resonance imaging of reticuloendothelial system containing anatomical structures, can additionally be exploited for the non-invasive characterization and quantification of pathology down to the molecular level. In this context, SPIOs can be applied for non-invasive cell tracking, quantification of tissue perfusion and target specific imaging, as well as for the detection of gene expression. This article provides an overview of new applications for clinically approved iron oxides as well of new, modified SPIO contrast agents for parametric and molecular imaging.

[Macrophage specific MRI imaging for antigen induced arthritides. A potential new strategy for the diagnosis of rheumatoid arthritis]

PURPOSE

The present work describes the potential of iron oxides for the detection of macrophages in synovitis in experimental, antigen-induced arthritis.

METHODS

The pivotal role of macrophages in rheumatoid arthritis (RA) in humans and in antigen-induced arthritis (AIA) in animal models is discussed. The latter appear to be very similar in many aspects to the human RA. We show the potential for iron oxide-enhanced magnetic resonance imaging (MRI) to determine the macrophage content in the arthritic synovial membranes. The results of our own research, as well as those of other research groups, are presented and discussed.

RESULTS

MRI after the intravenous (i.v.) administration of iron oxides enables the depiction of macrophage content in arthritic synovial membranes in AIA through the effects of the intracellular compartmentalisation of iron oxide particles. These effects can be demonstrated in 24-h delayed images after i.v. contrast application, on T2-weighted spin-echo or turbo-spin-echo sequences, and especially on T2*-weighted gradient-echo sequences. The signal effects are not only apparent in high field strength (4.7 Tesla) but also on 1.5 Tesla clinical scanners.

CONCLUSIONS AND PERSPECTIVES

The use of iron oxides enables the determination of the macrophage content in synovitis in animals with AIA. This parameter represents a potential marker to determine disease activity, and possibly represents a marker to evaluate the effectiveness of specific therapies in human RA. Current knowledge of iron oxide-enhanced MRI is limited to animal models. The clinical evaluation of this new method in patients with RA has not yet been performed. However, based on the considerations presented here, significant progress in the diagnostic work-up of RA can be expected.

Assessment of Musculoskeletal Infection in Rats to Determine Usefulness of SPIO-Enhanced MRI

OBJECTIVE

The objective of our study was to evaluate the usefulness of superparamagnetic iron oxide (SPIO)-enhanced MRI in experimental models of infectious disease and to analyze the intracellular uptake of SPIO.

MATERIALS AND METHODS

Nine rats with infectious arthritis of the knee or soft-tissue infection were imaged on an MRI unit on days 4-6 after i.v. injection of a bacterial suspension. All animals were imaged on a T2-weighted fast spin-echo sequence before and 24 hours after administration of SPIO. The nine rats were classified into two groups according to the dose of SPIO. We calculated the relative signal-to-noise ratio (SNR) change and compared the relative SNR change with the histologic findings. We analyzed iron-loaded cells and the intracellular uptake of iron particles according to the dose of SPIO.

RESULTS

The SNR value decreased in proportion to the increase in the number of iron-laden macrophages or fibroblasts in the wall of the soft-tissue abscess (p < 0.01). The intracellular uptake of iron particles was shown in fibroblasts as well as in macrophages, and their uptake in the fibroblasts was greater than that in the macrophages (p < 0.05). There was no statistically significant difference in the intracellular uptake of iron particles according to the dose of SPIO (p > 0.1).

CONCLUSION

SPIO-enhanced MRI can be useful in evaluating infectious disease of the joint or soft tissue and is influenced by the uptake of iron particles in fibroblasts as well as macrophages.

USPIO-enhanced MR imaging for visualization of synovial hyperperfusion and detection of synovial macrophages: preliminary results in an experimental model of antigen-induced arthritis

PURPOSE

To evaluate whether ultrasmall superparamagnetic iron particles (USPIO)-enhanced MRI is capable of assessing both synovial perfusion characteristics and the presence of synovial macrophages in a model of antigen-induced arthritis.

MATERIALS AND METHODS

Unilateral arthritis was induced in six knees of six rabbits. The contralateral knees of the rabbits served as control knees. After onset of arthritis, all 12 knees were scanned prior to and immediately following intravenous administration of USPIO using a multiphase T1-weighted (T1w) fast gradient-echo (FGRE) sequence, and T1w spin-echo (SE), T2-weighted (T2w) FSE, T2*w GRE, and short-tau inversion recovery (STIR) sequences prior to and 24 hours following USPIO administration. SI-vs.-time curves (STCs) and the early enhancement rate during the first 56 seconds (REE(56)) were calculated from SI measurements within the synovial tissue of all knees on dynamic T1w images. MR findings were correlated to histopathology.

RESULTS

REE(56) was significantly higher in the synovial tissue of arthritic knees than in the control knees (P < 0.01). Significant T1-, T2-, and T2* effects (P = 0.03-0.04) and multiple synovial vessels were visually detectable within the arthritic synovial tissue 24 hours after administration of USPIO, whereas no signal changes or synovial vessels were seen in the control knees. Histopathology revealed widened synovial blood vessels in the arthritic knees, and confirmed iron uptake by macrophages in the arthritic knees.

CONCLUSION

USPIO-enhanced MRI is capable of both assessing synovial hyperperfusion and detecting macrophages in antigen-induced arthritis in rabbits.

MR Imaging of in Vivo Recruitment of Iron Oxide–labeled Macrophages in Experimentally Induced Soft-Tissue Infection in Mice

PURPOSE

To evaluate the feasibility of magnetic resonance (MR) imaging in depicting in vivo recruitment of iron oxide-labeled macrophages in experimentally induced soft-tissue infection.

MATERIALS AND METHODS

The study was performed according to the guidelines of the U.S. National Institutes of Health and recommendations of the committee on animal research. The protocol was approved by the local institutional review committee on animal care. Experimental soft-tissue infection in 12 mice was induced by inoculation with a 5 x 10(7) colony-forming units of Staphylococcus aureus into the left calf. Peritoneal macrophages were harvested from thioglycollate-treated mice, cultured, and labeled with iron oxide in vitro. The iron oxide-labeled macrophage (macrophage group, n = 6) or iron oxide solution (control group, n = 6) was administered through the tail vein. The left calf of the mice was imaged on days 2 and 3 with a 4.7-T MR unit. Changes in relative signal intensity (SI) and pattern of contrast material enhancement (macrophage distribution) were analyzed and compared with histopathologic findings. Statistical analysis was performed with the Wilcoxon matched-pairs signed rank test.

RESULTS

On MR images obtained 24 hours after administration of macrophage labeled with iron oxide, a band-shaped lower SI zone was noted in the abscess wall, which corresponded to the distribution of the iron oxide-labeled macrophages at histopathologic examination. The relative SI of the abscess wall significantly decreased after injection of iron oxide-labeled macrophages (median, 0.42) compared with that before injection (median, 1.23) (P = .031). In the control group, the SI change after administration of iron oxide solution was not significant (P = .688).

CONCLUSION

Homing of intravenously administered iron oxide-labeled macrophages can be monitored with MR imaging and may provide a tool to investigate interactions between macrophages and the invading pathogens.

MRI of arthritis: comparison of ultrasmall superparamagnetic iron oxide vs. Gd-DTPA

PURPOSE

To compare the ability of the ultrasmall superparamagnetic iron oxide (USPIO) SHU555C vs. gadopentetate dimeglumine (Gd-DTPA) to detect antigen-induced monoarthritis with MRI.

MATERIALS AND METHODS

Twelve seven-week-old female rats with an antigen-induced monoarthritis of the right knee were randomly assigned to two groups. Animals in group I (N = 6) underwent MRI using T1-weighted gradient-echo sequences before injection and at 2, 9, 17, 25, 33, 40, 47, 55, and 63 minutes postinjection (p.i.) of Gd-DTPA on day 1, and before injection and at 3, 23, 43, and 123 minutes p.i. of SHU555C on day 2. Animals in group II (N = 6) were imaged before injection and at 3, 23, 43, and 123 minutes p.i. using identical sequences. Signal-to-noise ratios (SNRs) and relative enhancement (DeltaSI%) of arthritic and normal synovium were determined from region-of-interest (ROI) measurements in consensus reading by two experienced radiologists. Data were tested for significant differences between the two agents and between the arthritic and normal knees using a mixed-effect model and F-tests (P < 0.05). Joints were processed for histopathology as the gold standard.

RESULTS

USPIO and Gd-DTPA showed significant enhancement differences (P < 0.001). USPIO provided a progressive and persistent enhancement of arthritic joints while Gd-DTPA provided an early and rapidly declining enhancement. Maximal enhancement in synovitis was 400% at 40-120 minutes p.i. of USPIO vs. 300% at two minutes p.i. of Gd-DTPA. USPIO provided a significant higher difference in enhancement between the arthritic and normal synovium than Gd-DTPA (P < 0.001). Histopathology confirmed marked inflammatory synovial changes in all arthritis-induced right knee joints and normal synovium in all left knee joints.

CONCLUSION

Both USPIO and Gd-DTPA detect arthritis by positive T1-enhancement. Compared to standard Gd-DTPA, the USPIO SHU555C provides a comparable maximal T1-enhancement (at two minutes p.i for Gd-DTPA and between 43 and 123 minutes p.i. for SHU555C), but in addition it provides a prolonged T1-enhancement of synovitis and a higher difference between the relative enhancement of arthritic and normal synovium.

Parameters influencing the stealthiness of colloidal drug delivery systems

Over the last few decades, colloidal drug delivery systems (CDDS) such as nano-structures have been developed in order to improve the efficiency and the specificity of drug action. Their small size permits them to be injected intravenously in order to reach target tissues. However, it is known that they can be rapidly removed from blood circulation by the immune system. CDDS are removed via the complement system and via the cells of the mononuclear phagocyte system (MPS), after their recognition by opsonins and/or receptors present at the cell surface. This recognition is dependent on the physicochemical characteristics of the CDDS. In this study, we will focus on parameters influencing the interactions of opsonins and the macrophage plasma membrane with the surface of CDDS, whereby parameters of the polymer coating become necessary to provide good protection.

Ultrasmall supraparamagnetic iron oxide-enhanced magnetic resonance imaging of antigen-induced arthritis: a comparative study between SHU 555 C, ferumoxtran-10, and ferumoxytol

OBJECTIVES

We sought to compare the ability of 3 ultrasmall superparamagnetic iron oxides (USPIOs) to detect and characterize antigen-induced arthritis with MR imaging.

MATERIALS AND METHODS

A monoarthritis was induced in the right knee of 18 rats. The left knee served as a normal control. Knees underwent magnetic resonance (MR) imaging before, up to 2 hours, and 24 hours after injection (p.i.) of 200 mumol Fe/kg SHU 555 C (n= 6), ferumoxtran-10 (n = 6), or ferumoxytol (n = 6), using T2-2D-SE 100/20,40,60,80/90 (TR/TE/flipangle), T2*-3D-spoiled gradient recalled (SPGR) 100/15/38, and T1-3D-SPGR 50/1,7/60 sequences.

RESULTS

Quantitative signal to noise ratio and DeltaSI data of arthritic knees on T1- and T2*-weighted MR images showed no significant differences between the 3 USPIOs (P > 0.05). At 2 hours p.i., SNR and DeltaSI data were significantly increased from baseline on T1-weighted images and significantly decreased on T2*-weighted images (P < 0.001). At 24 hours p.i., the T1-enhancement returned to baseline, whereas the T2*-enhancement remained significantly elevated (P < 0.001). Immunostains demonstrated an USPIO compartmentalization in macrophages in the arthritic synovium.

CONCLUSIONS

Based on the relatively small number of animals in our study group, inflammation in antigen-induced arthritis can be equally detected and characterized with any of the three USPIOs evaluated.

Nanoparticles for the optical imaging of tumor E-selectin

We designed a fluorescent peptide-magnetic nanoparticle conjugate that images E-selectin expression in mouse xenograft models of Lewis lung carcinoma (LLC) by fluorescence reflectance imaging. It was synthesized by attaching the E-selectin-binding peptide (ESBP; CDSDSDITWDQLWDLMK) to a CLIO(Cy5.5) nanoparticle to yield ESBP-CLIO(Cy5.5). Internalization by activated human umbilical vein endothelial cells (HUVECs) was rapid and mediated by E-selectin, indicated by the lack of uptake of nanoparticles bearing similar numbers of a scrambled peptide (Scram). To demonstrate the specificity of E-selectin targeting to ESBP-CLIO(Cy5.5) in vivo, we coinjected ESBP-CLIO(Cy5.5) and Scram-CLIO(Cy3.5) and demonstrated a high Cy5.5/Cy3.5 fluorescence ratio using the LLC. Histology showed that ESBP-CLIO was associated with tumor cells as well as endothelial cells, but fluorescence-activated cell sorter analysis showed a far less expression of E-selectin on LLC than on HUVECs. Using immunohistochemistry, we demonstrated E-selectin expression in both endothelial cells and cancer cells in human prostate cancer specimens. We conclude that ESBP-CLIO(Cy5.5) is a useful probe for imaging E-selectin associated with the LLC tumor, and that E-selectin is expressed not only on endothelial cells but also on LLC cells and human prostate cancer specimens.

T1 and T2 relaxivity of intracellular and extracellular USPIO at 1.5T and 3T clinical MR scanning

In this study we evaluated the effects of intracellular compartmentalization of the ultrasmall superparamagnetic iron oxide (USPIO) ferumoxtran-10 on its proton T1 and T2 relaxivities at 1.5 and 3T. Monocytes were labeled with ferumoxtran-10 by simple incubation. Decreasing quantities of ferumoxtran-10-labeled cells (2.5x10(7)-0.3x10(7) cells/ml) and decreasing concentrations of free ferumoxtran-10 (without cells) in Ficoll solution were evaluated with 1.5 and 3T clinical magnetic resonance (MR) scanners. Pulse sequences comprised axial spin echo (SE) sequences with multiple TRs and fixed TE and SE sequences with fixed TR and increasing TEs. Signal intensity measurements were used to calculate T1 and T2 relaxation times of all samples, assuming a monoexponential signal decay. The iron content in all samples was determined by inductively coupled plasma atomic emission spectrometry and used for calculating relaxivities. Measurements at 1.5T and 3T showed higher T1 and T2 relaxivity values of free extracellular ferumoxtran-10 as opposed to intracellularly compartmentalized ferumoxtran-10, under the evaluated conditions of homogeneously dispersed contrast agents/cells in Ficoll solution and a cell density of up to 2.5x10(7) cells/ml. At 3T, differences in T1-relaxivities between intra- and extracellular USPIO were smaller, while differences in USPIO T2-relaxivities were similar compared with 1.5T. In conclusion, cellular compartmentalization of ferumoxtran-10 changes proton relaxivity.

Ultrasmall superparamagnetic iron-oxide-enhanced MR imaging of normal bone marrow in rodents: original research original research

RATIONALE AND OBJECTIVES

The objective is to compare three different ultrasmall superparamagnetic iron oxides (USPIOs) for magnetic resonance (MR) imaging of normal bone marrow in rodents.

MATERIALS AND METHODS

Femoral bone marrow in 18 Sprague-Dawley rats was examined by using MR imaging before and up to 2 and 24 hours postinjection (PI) of 200 mumol of Fe/kg of SHU555C (n = 6), ferumoxtran-10 (n = 6), or ferumoxytol (n = 6), using T1-weighted (50 ms/1.7 ms/60 degrees = repetition time [TR]/echo time [TE]/flip angle) and T2*-weighted (100 ms/15 ms/38 degrees = TR/TE/flip angle) three-dimensional spoiled gradient recalled echo sequences. USPIO-induced bone marrow was evaluated qualitatively and quantified as signal-to-noise ratio (SNR) and change in signal intensity (DeltaSI) values. A mixed-effect model was fitted to the SNR and DeltaSI values, and differences among USPIOs were tested for significance by using F tests.

RESULTS

At 2 hours PI, all three USPIOs showed marked positive signal enhancement on T1-weighted images and a corresponding marked signal loss on T2*-weighted images. At 24 hours PI, the T1 effect of all three USPIOs disappeared, whereas T2*-weighted images showed persistent signal loss on SHU555C and ferumoxytol-enhanced MR images, but not ferumoxtran-10-enhanced MR images. Corresponding SNR and DeltaSI values on T2*-weighted MR images at 24 hours PI were significantly different from baseline for SHU555C and ferumoxytol, but not ferumoxtran-10.

CONCLUSION

All three USPIO contrast agents, ferumoxtran-10, ferumoxytol, and SHU555C, can be applied for MR imaging of bone marrow. Ferumoxtran-10 apparently reveals a different kinetic behavior in bone marrow than ferumoxytol and SHU555C.

Imaging of Macrophages in Soft-Tissue Infection in Rats: Relationship between Ultrasmall Superparamagnetic Iron Oxide Dose and MR Signal Characteristics

PURPOSE

To describe dose-dependent signal intensity (SI) characteristics of experimentally induced soft-tissue abscesses on 1.5-T T1- and T2*-weighted magnetic resonance (MR) images obtained 24 hours after administration of ultrasmall superparamagnetic iron oxide (USPIO) and to describe the relationship between SI and amount of USPIO uptake and macrophage iron content.

MATERIALS AND METHODS

Local institutional review committee on animal care approved the experiments, which were performed according to the guidelines of the National Institutes of Health and the committee on animal research at our institution. Unilateral calf muscle abscesses were induced in 21 rats with an injection of a Staphylococcus aureus suspension. The rats were divided into three groups of seven animals each: low USPIO dose (50 micromol of iron per kilogram of body weight), high USPIO dose (150 micromol Fe/kg), and control (saline solution). All rats were imaged before and 24 hours after USPIO administration at 1.5 T (transverse T1-weighted spin-echo, T2*-weighted fast gradient-echo, and short inversion time inversion-recovery sequences). Images were analyzed quantitatively and qualitatively with regard to SI and signal pattern. Temporal variation of calculated contrast-to-noise ratios was analyzed with the Wilcoxon signed rank test. MR findings were correlated with histopathologic findings, including those of electron microscopy.

RESULTS

Twenty-four hours after USPIO administration in the high-dose group, susceptibility effects were present in abscess periphery on postcontrast T2*-weighted images (P=.04), and SI enhancement was noted on postcontrast T1-weighted images within both abscess wall and abscess center (P=.04 for both). In the low-dose group, SI enhancement was noted in entire abscess on T1-weighted postcontrast images (P=.03). Neither significant SI loss (P=.09) nor susceptibility effects were detected in periphery or center of any abscess on postcontrast T2*-weighted images. There was no obvious difference in total amount of macrophages among the groups, but there was a clear difference with regard to individual iron content of iron-positive macrophages between the USPIO dose groups.

CONCLUSION

At 1.5 T, SI characteristics of abscesses on T1- and T2*-weighted images obtained 24 hours after USPIO injection strongly depend on administered dose of the contrast agent. At low doses, T1 effects were stronger than T2* effects.

Molecular Imaging in Drug Discovery and Development: Potential and Limitations of Nonnuclear Methods

Noninvasive conventional imaging methods are established technologies in modern drug discovery and development providing valuable morphological, physiological, and metabolic information to characterize disease phenotypes, to evaluate the efficacy of therapy and to identify and develop potential biomarkers for clinical drug evaluation. The development of target-specific or molecular imaging has added a new dimension: molecular events such as the target expression, the drug-target interaction, or the activation of signal transduction pathways can be studied in the intact organism with high spatial and temporal resolution. Molecular imaging is inherently a multimodality approach. In this article, we review the role of molecular imaging for drug discovery and development focusing on nonnuclear imaging methods, i.e., magnetic resonance imaging (MRI) and optical imaging techniques based on fluorescence and bioluminescence readouts. Examples discussed are direct visualization of target expression using target-specific ligands or reporter genes, pathway imaging, and cell-trafficking studies.

The utility of superparamagnetic contrast agents in MRI: theoretical consideration and applications in the cardiovascular system

This review will discuss the in vivo physical chemical relaxation properties of superparamagnetic iron oxide particles. Various parameters such as size, magnetization, compartmentalization and water exchange effects and how these alter the behavior of the iron oxide particles in an in vitro vs an in vivo situation with special reference to the cardiovascular system will be exemplified. Furthermore, applications using iron oxide particles for vascular, perfusion and viability imaging as well as assessment of the inflammatory status of a given tissue will be discussed.