Perfluoro-15-crown-5-ether labelled macrophages in adoptive transfer experimental allergic encephalomyelitis

Opportunities for Molecular Imaging in Multiple Sclerosis Management: Linking Probe to Treatment

Imaging has been a critical component of multiple sclerosis (MS) management for nearly 40 years. The visual information derived from structural MRI, that is, signs of blood-brain barrier disruption, inflammation and demyelination, and brain and spinal cord atrophy, are the primary metrics used to evaluate therapeutic efficacy in MS. The development of targeted imaging probes has expanded our ability to evaluate and monitor MS and its therapies at the molecular level. Most molecular imaging probes evaluated for MS applications are small molecules initially developed for PET, nearly half of which are derived from U.S. Food and Drug Administration-approved drugs and those currently undergoing clinical trials. Superparamagnetic and fluorinated particles have been used for tracking circulating immune cells (in situ labeling) and immunosuppressive or remyelinating therapeutic stem cells (ex vivo labeling) clinically using proton (hydrogen 1 [¹H]) and preclinically using fluorine 19 MRI. Translocator protein PET and ¹H MR spectroscopy have been demonstrated to complement imaging metrics from structural (gadolinium-enhanced) MRI in nine and six trials for MS disease-modifying therapies, respectively. Still, despite multiple demonstrations of the utility of molecular imaging probes to evaluate the target location and to elucidate the mechanisms of disease-modifying therapies for MS applications, their use has been sparse in both preclinical and clinical settings.

Click-Ready Perfluorocarbon Nanoemulsion for 19F MRI and Multimodal Cellular Detection

We describe an in vivo imaging probe platform that is readily modifiable to accommodate binding of different molecular targeting moieties and payloads for multimodal image generation. In this work, we demonstrate the utility of perfluorocarbon (PFC) nanoemulsions incorporating dibenzocyclooctyne (DBCO) by enabling postemulsification functionalization via a click reaction with azide-containing ligands. The addition of DBCO-lipid to the surfactant in PFC nanoemulsions did not affect nanoemulsion size or nanoemulsion stability. As proof-of-concept, fluorescent dye-azides were conjugated to PFC nanoemulsions, demonstrating the feasibility of functionalization the by click reaction. Uptake of the fluorescent PFC by macrophages was demonstrated both in vitro in cultured macrophages and in situ in an acute inflammation mouse model, where fluorescence imaging and ¹H/¹⁹F magnetic resonance imaging (MRI) were used for in vivo detection. Overall, these data demonstrate the potential of PFC nanoemulsions incorporating DBCO as a versatile platform for generating functionalized probes.

Pentafluorosulfanyl (SF5) as a Superior 19F Magnetic Resonance Reporter Group: Signal Detection and Biological Activity of Teriflunomide Derivatives

Fluorine (19F) magnetic resonance imaging (MRI) is severely limited by a low signal-to noise ratio (SNR), and tapping it for 19F drug detection in vivo still poses a significant challenge. However, it bears the potential for label-free theranostic imaging. Recently, we detected the fluorinated dihydroorotate dehydrogenase (DHODH) inhibitor teriflunomide (TF) noninvasively in an animal model of multiple sclerosis (MS) using 19F MR spectroscopy (MRS). In the present study, we probed distinct modifications to the CF3 group of TF to improve its SNR. This revealed SF5 as a superior alternative to the CF3 group. The value of the SF5 bioisostere as a 19F MRI reporter group within a biological or pharmacological context is by far underexplored. Here, we compared the biological and pharmacological activities of different TF derivatives and their 19F MR properties (chemical shift and relaxation times). The 19F MR SNR efficiency of three MRI methods revealed that SF5-substituted TF has the highest 19F MR SNR efficiency in combination with an ultrashort echo-time (UTE) MRI method. Chemical modifications did not reduce pharmacological or biological activity as shown in the in vitro dihydroorotate dehydrogenase enzyme and T cell proliferation assays. Instead, SF5-substituted TF showed an improved capacity to inhibit T cell proliferation, indicating better anti-inflammatory activity and its suitability as a viable bioisostere in this context. This study proposes SF5 as a novel superior 19F MR reporter group for the MS drug teriflunomide.

Imaging of Inflammation in Spinal Cord Injury: Novel Insights on the Usage of PFC-Based Contrast Agents

Labeling of macrophages with perfluorocarbon (PFC)-based compounds allows the visualization of inflammatory processes by ¹⁹F-magnetic resonance imaging (¹⁹F-MRI), due to the absence of endogenous background. Even if PFC-labeling of monocytes/macrophages has been largely investigated and used, information is lacking about the impact of these agents over the polarization towards one of their cell subsets and on the best way to image them. In the present work, a PFC-based nanoemulsion was developed to monitor the course of inflammation in a model of spinal cord injury (SCI), a pathology in which the understanding of immunological events is of utmost importance to select the optimal therapeutic strategies. The effects of PFC over macrophage polarization were studied in vitro, on cultured macrophages, and in vivo, in a mouse SCI model, by testing and comparing various cell tracking protocols, including single and multiple administrations, the use of MRI or Point Resolved Spectroscopy (PRESS), and application of pre-saturation of Kupffer cells. The blood half-life of nanoemulsion was also investigated by ¹⁹F Magnetic Resonance Spectroscopy (MRS). In vitro and in vivo results indicate the occurrence of a switch towards the M2 (anti-inflammatory) phenotype, suggesting a possible theranostic function of these nanoparticles. The comparative work presented here allows the reader to select the most appropriate protocol according to the research objectives (quantitative data acquisition, visual monitoring of macrophage recruitment, theranostic purpose, rapid MRI acquisition, etc.). Finally, the method developed here to determine the blood half-life of the PFC nanoemulsion can be extended to other fluorinated compounds.

Hot spot 19 F magnetic resonance imaging of inflammation

Among the preclinical molecular imaging approaches, lately fluorine (¹⁹ F) magnetic resonance imaging (MRI) has garnered significant scientific interest in the biomedical research community, due to the unique properties of fluorinated materials and the ¹⁹ F nucleus. Fluorine is an intrinsically sensitive nucleus for MRI-there is negligible endogenous ¹⁹ F in the body and, thus, no background signal which allows the detection of fluorinated materials as "hot spots" by combined ¹ H/¹⁹ F MRI and renders fluorine-containing molecules as ideal tracers with high specificity. In addition, perfluorocarbons are a family of compounds that exhibit a very high fluorine payload and are biochemically as well as physiologically inert. Perfluorocarbon nanoemulsions (PFCs) are well known to be readily taken up by immunocompetent cells, which can be exploited for the unequivocal identification of inflammatory foci by tracking the recruitment of PFC-loaded immune cells to affected tissues using ¹ H/¹⁹ F MRI. The required ¹⁹ F labeling of immune cells can be accomplished either ex vivo by PFC incubation of isolated endogenous immune cells followed by their re-injection or by intravenous application of PFCs for in situ uptake by circulating immune cells. With both approaches, inflamed tissues can unambiguously be detected via background-free ¹⁹ F signals due to trafficking of PFC-loaded immune cells to affected organs. To extend ¹⁹ F MRI tracking beyond cells with phagocytic properties, the PFC surface can further be equipped with distinct ligands to generate specificity against epitopes and/or types of immune cells independent of phagocytosis. Recent developments also allow for concurrent detection of different PFCs with distinct spectral signatures allowing the simultaneous visualization of several targets, such as various immune cell subtypes labeled with these PFCs. Since ligands and targets can easily be adapted to a variety of problems, this approach provides a general and versatile platform for inflammation imaging which will strongly extend the frontiers of molecular MRI. This article is categorized under: Diagnostic Tools > in vivo Nanodiagnostics and Imaging Therapeutic Approaches and Drug Discovery > Emerging Technologies Therapeutic Approaches and Drug Discovery > Nanomedicine for Cardiovascular Disease.

Fluorine-19 MRI at 21.1 T: enhanced spin-lattice relaxation of perfluoro-15-crown-5-ether and sensitivity as demonstrated in ex vivo murine neuroinflammation

OBJECTIVE

Fluorine MR would benefit greatly from enhancements in signal-to-noise ratio (SNR). This study examines the sensitivity gain of 19F MR that can be practically achieved when moving from 9.4 to 21.1 T.

MATERIALS AND METHODS

We studied perfluoro-15-crown-5-ether (PFCE) at both field strengths (B0), as a pure compound, in the form of nanoparticles (NP) as employed to study inflammation in vivo, as well as in inflamed tissue. Brains, lymph nodes (LNs) and spleens were obtained from mice with experimental autoimmune encephalomyelitis (EAE) that had been administered PFCE NPs. All samples were measured at both B0 with 2D-RARE and 2D-FLASH using 19F volume radiofrequency resonators together. T1 and T2 of PFCE were measured at both B0 strengths.

RESULTS

Compared to 9.4 T, an SNR gain of > 3 was observed for pure PFCE and > 2 for PFCE NPs at 21.1 T using 2D-FLASH. A dependency of 19F T1 and T2 relaxation on B0 was demonstrated. High spatially resolved 19F MRI of EAE brains and LNs at 21.1 T revealed signals not seen at 9.4 T.

DISCUSSION

Enhanced SNR and T1 shortening indicate the potential benefit of in vivo 19F MR at higher B0 to study inflammatory processes with greater detail.

Perfluorocarbon Labeling of Human Glial-Restricted Progenitors for 19 F Magnetic Resonance Imaging

One of the fundamental limitations in assessing potential efficacy in Central Nervous System (CNS) transplantation of stem cells is the capacity for monitoring cell survival and migration noninvasively and longitudinally. Human glial‐restricted progenitor (hGRP) cells (Q‐Cells) have been investigated for their utility in providing neuroprotection following transplantation into models of amyotrophic lateral sclerosis (ALS) and have been granted a Food and Drug Administration (FDA) Investigational New Drug (IND) for intraspinal transplantation in ALS patients. Furthermore, clinical development of these cells for therapeutic use will rely on the ability to track the cells using noninvasive imaging methodologies as well as the verification that the transplanted GRPs have disease‐relevant activity. As a first step in development, we investigated the use of a perfluorocarbon (PFC) dual‐modal (¹⁹F magnetic resonance imaging [MRI] and fluorescence) tracer agent to label Q‐Cells in culture and following spinal cord transplantation. PFCs have a number of potential benefits that make them appealing for clinical use. They are quantitative, noninvasive, biologically inert, and highly specific. In this study, we developed optimized PFC labeling protocols for Q‐Cells and demonstrate that PFCs do not significantly alter the glial identity of Q‐Cells. We also show that PFCs do not interfere with the capacity for differentiation into astrocytes either in vitro or following transplantation into the ventral horn of the mouse spinal cord, and can be visualized in vivo by hot spot ¹⁹F MRI. These studies provide a foundation for further preclinical development of PFCs within the context of evaluating Q‐Cell transplantation in the brain and spinal cord of future ALS patients using ¹⁹F MRI. stem cells translational medicine2019;8:355–365

In Vivo Quantification of Inflammation in Experimental Autoimmune Encephalomyelitis Rats Using Fluorine-19 Magnetic Resonance Imaging Reveals Immune Cell Recruitment outside the Nervous System

Progress in identifying new therapies for multiple sclerosis (MS) can be accelerated by using imaging biomarkers of disease progression or abatement in model systems. In this study, we evaluate the ability to noninvasively image and quantitate disease pathology using emerging “hot-spot” ¹⁹F MRI methods in an experimental autoimmune encephalomyelitis (EAE) rat, a model of MS. Rats with clinical symptoms of EAE were compared to control rats without EAE, as well as to EAE rats that received daily prophylactic treatments with cyclophosphamide. Perfluorocarbon (PFC) nanoemulsion was injected intravenously, which labels predominately monocytes and macrophages in situ. Analysis of the spin-density weighted ¹⁹F MRI data enabled quantification of the apparent macrophage burden in the central nervous system and other tissues. The in vivo MRI results were confirmed by extremely high-resolution ¹⁹F/¹H magnetic resonance microscopy in excised tissue samples and histopathologic analyses. Additionally, ¹⁹F nuclear magnetic resonance spectroscopy of intact tissue samples was used to assay the PFC biodistribution in EAE and control rats. In vivo hot-spot ¹⁹F signals were detected predominantly in the EAE spinal cord, consistent with the presence of inflammatory infiltrates. Surprising, prominent ¹⁹F hot-spots were observed in bone-marrow cavities adjacent to spinal cord lesions; these were not observed in control animals. Quantitative evaluation of cohorts receiving cyclophosphamide treatment displayed significant reduction in ¹⁹F signal within the spinal cord and bone marrow of EAE rats. Overall, ¹⁹F MRI can be used to quantitatively monitored EAE disease burden, discover unexpected sites of inflammatory activity, and may serve as a sensitive biomarker for the discovery and preclinical assessment of novel MS therapeutic interventions.

Theranostic nanoparticles for cancer and cardiovascular applications

Theranostics have received enormous attentions for individualized diagnosis and treatment in the past few years. Especially, the availability of various nanoplatforms provides great potentials for designing of sophisticated theranostic agents including imaging, targeting and therapeutic functions. Numerous reports have been published on how to construct multifunctional nanoparticles for the targeted diagnosis and therapy simultaneously since the concept of "theranostics". This review presents recent advances of molecular imaging and nanoplatform technology, and their applications in drug discovery and development. Applications of nanoplatform-based theranostics in cancer and cardiovascular diseases will also be covered including diagnosis, assessment of drug biodistribution, and visualization of drug release from nanoparticles, as well as monitoring of therapeutic effects.

Tracking immune cells in vivo using magnetic resonance imaging

The increasing complexity of in vivo imaging technologies, coupled with the development of cell therapies, has fuelled a revolution in immune cell tracking in vivo. Powerful magnetic resonance imaging (MRI) methods are now being developed that use iron oxide- and ¹⁹F-based probes. These MRI technologies can be used for image-guided immune cell delivery and for the visualization of immune cell homing and engraftment, inflammation, cell physiology and gene expression. MRI-based cell tracking is now also being applied to evaluate therapeutics that modulate endogenous immune cell recruitment and to monitor emerging cellular immunotherapies. These recent uses show that MRI has the potential to be developed in many applications to follow the fate of immune cells in vivo.

In vivo MRI cell tracking using perfluorocarbon probes and fluorine-19 detection

This article presents a brief review of preclinical in vivo cell-tracking methods and applications using perfluorocarbon (PFC) probes and fluorine-19 ((19) F) MRI detection. Detection of the (19) F signal offers high cell specificity and quantification ability in spin density-weighted MR images. We discuss the compositions of matter, methods and applications of PFC-based cell tracking using ex vivo and in situ PFC labeling in preclinical studies of inflammation and cellular therapeutics. We also address the potential applicability of (19) F cell tracking to clinical trials.

Accelerated fluorine-19 MRI cell tracking using compressed sensing

Cell tracking using perfluorocarbon labels and fluorine-19 (19F) MRI is a noninvasive approach to visualize and quantify cell populations in vivo. In this study, we investigated three-dimensional compressed sensing methods to accelerate 19F MRI data acquisition for cell tracking and evaluate the impact of acceleration on 19F signal quantification. We show that a greater than 8-fold reduction in imaging time was feasible without pronounced image degradation and with minimal impact on the image signal-to-noise ratio and 19F quantification accuracy. In 19F phantom studies, we show that apparent feature topology is maintained with compressed sensing reconstruction, and false positive signals do not appear in areas devoid of fluorine. We apply the three-dimensional compressed sensing 19F MRI methods to quantify the macrophage burden in a localized wounding-inflammation mouse model in vivo; at 8-fold image acceleration, the 19F signal distribution was accurately reproduced, with no loss in signal-to-noise ratio. Our results demonstrate that three-dimensional compressed sensing methods have potential for advancing in vivo 19F cell tracking for a wide range of preclinical and translational applications.

Assaying macrophage activity in a murine model of inflammatory bowel disease using fluorine-19 MRI

Macrophages have an important role in the pathogenesis of most chronic inflammatory diseases. A means of non-invasively quantifying macrophage migration would contribute significantly towards our understanding of chronic inflammatory processes and aid the evaluation of novel therapeutic strategies. We describe the use of a perfluorocarbon tracer reagent and in vivo (19)F magnetic resonance imaging (MRI) to quantify macrophage burden longitudinally. We apply these methods to evaluate the severity and three-dimensional distribution of macrophages in a murine model of inflammatory bowel disease (IBD). MRI results were validated by histological analysis, immunofluorescence and quantitative real-time polymerase chain reaction. Selective depletion of macrophages in vivo was also performed, further validating that macrophage accumulation of perfluorocarbon tracers was the basis of (19)F MRI signals observed in the bowel. We tested the effects of two common clinical drugs, dexamethasone and cyclosporine A, on IBD progression. Whereas cyclosporine A provided mild therapeutic effect, unexpectedly dexamethasone enhanced colon inflammation, especially in the descending colon. Overall, (19)F MRI can be used to evaluate early-stage inflammation in IBD and is suitable for evaluating putative therapeutics. Due to its high macrophage specificity and quantitative ability, we envisage (19)F MRI having an important role in evaluating a wide range of chronic inflammatory conditions mediated by macrophages.

In vivo Noninvasive Small Animal Molecular Imaging

The remarkable efforts that are made on molecular imaging technologies demonstrate its potential importance and range of applications. The generation of disease-specific animal models, and the developments of target-specific probes and genetically encoded reporters are another important component. Continued improvements in the instrumentation, the identification of novel targets and genes, and the availability of improved imaging probes should be made. Multimodal imaging probes should provide easier transitions between laboratory studies, including small animal studies and clinical applications. Here, we reviewed basic strategies of noninvasive in vivo imaging methods in small animals to introducing the concept of molecular imaging.

Rapid quantification of inflammation in tissue samples using perfluorocarbon emulsion and fluorine-19 nuclear magnetic resonance

Quantification of inflammation in tissue samples can be a time-intensive bottleneck in therapeutic discovery and preclinical endeavors. We describe a versatile and rapid approach to quantitatively assay macrophage burden in intact tissue samples. Perfluorocarbon (PFC) emulsion is injected intravenously, and the emulsion droplets are effectively taken up by monocytes and macrophages. These 'in situ' labeled cells participate in inflammatory events in vivo resulting in PFC accumulation at inflammatory loci. Necropsied tissues or intact organs are subjected to conventional fluorine-19 ((19)F) NMR spectroscopy to quantify the total fluorine content per sample, proportional to the macrophage burden. We applied these methods to a rat model of experimental allergic encephalomyelitis (EAE) exhibiting extensive inflammation and demyelination in the central nervous system (CNS), particularly in the spinal cord. In a cohort of EAE rats, we used (19)F NMR to derive an inflammation index (IFI) in intact CNS tissues. Immunohistochemistry was used to confirm intracellular colocalization of the PFC droplets within CNS CD68+ cells having macrophage morphology. The IFI linearly correlated to mRNA levels of CD68 via real-time PCR analysis. This (19)F NMR approach can accelerate tissue analysis by at least an order of magnitude compared with histological approaches.

Fluorine (19F) MRS and MRI in biomedicine

Shortly after the introduction of (1)H MRI, fluorinated molecules were tested as MR-detectable tracers or contrast agents. Many fluorinated compounds, which are nontoxic and chemically inert, are now being used in a broad range of biomedical applications, including anesthetics, chemotherapeutic agents, and molecules with high oxygen solubility for respiration and blood substitution. These compounds can be monitored by fluorine ((19)F) MRI and/or MRS, providing a noninvasive means to interrogate associated functions in biological systems. As a result of the lack of endogenous fluorine in living organisms, (19)F MRI of 'hotspots' of targeted fluorinated contrast agents has recently opened up new research avenues in molecular and cellular imaging. This includes the specific targeting and imaging of cellular surface epitopes, as well as MRI cell tracking of endogenous macrophages, injected immune cells and stem cell transplants.

Ultrasound-mediated tumor imaging and nanotherapy using drug loaded, block copolymer stabilized perfluorocarbon nanoemulsions

Perfluorocarbon nanoemulsions can deliver lipophilic therapeutic agents to solid tumors and simultaneously provide for monitoring nanocarrier biodistribution via ultrasonography and/or (19)F MRI. In the first generation of block copolymer stabilized perfluorocarbon nanoemulsions, perfluoropentane (PFP) was used as the droplet forming compound. Although manifesting excellent therapeutic and ultrasound imaging properties, PFP nanoemulsions were unstable at storage, difficult to handle, and underwent hard to control phenomenon of irreversible droplet-to-bubble transition upon injection. To solve the above problems, perfluoro-15-crown-5-ether (PFCE) was used as a core forming compound in the second generation of block copolymer stabilized perfluorocarbon nanoemulsions. PFCE nanodroplets manifest both ultrasound and fluorine ((19)F) MR contrast properties, which allows using multimodal imaging and (19)F MR spectroscopy for monitoring nanodroplet pharmacokinetics and biodistribution. In the present paper, acoustic, imaging, and therapeutic properties of unloaded and paclitaxel (PTX) loaded PFCE nanoemulsions are reported. As manifested by the (19)F MR spectroscopy, PFCE nanodroplets are long circulating, with about 50% of the injected dose remaining in circulation 2h after the systemic injection. Sonication with 1-MHz therapeutic ultrasound triggered reversible droplet-to-bubble transition in PFCE nanoemulsions. Microbubbles formed by acoustic vaporization of nanodroplets underwent stable cavitation. The nanodroplet size (200nm to 350nm depending on a type of the shell and conditions of emulsification) as well as long residence in circulation favored their passive accumulation in tumor tissue that was confirmed by ultrasonography. In the breast and pancreatic cancer animal models, ultrasound-mediated therapy with paclitaxel-loaded PFCE nanoemulsions showed excellent therapeutic properties characterized by tumor regression and suppression of metastasis. Anticipated mechanisms of the observed effects are discussed.

Fluorine-containing nanoemulsions for MRI cell tracking

In this article we review the chemistry and nanoemulsion formulation of perfluorocarbons used for in vivo(19)F MRI cell tracking. In this application, cells of interest are labeled in culture using a perfluorocarbon nanoemulsion. Labeled cells are introduced into a subject and tracked using (19)F MRI or NMR spectroscopy. In the same imaging session, a high-resolution, conventional ((1)H) image can be used to place the (19)F-labeled cells into anatomical context. Perfluorocarbon-based (19)F cell tracking is a useful technology because of the high specificity for labeled cells, ability to quantify cell accumulations, and biocompatibility. This technology can be widely applied to studies of inflammation, cellular regenerative medicine, and immunotherapy.

Perfluorocarbon-loaded Shell Crosslinked Knedel-like Nanoparticles: Lessons regarding polymer mobility and self assembly

Reversible addition-fragmentation chain transfer polymerization was employed to synthesize a set of copolymers of styrene (PS) and 2,3,4,5,6-pentafluorostyrene (PPFS), as well as block copolymers with tert-butyl acrylate (PtBA)-b-PS-co-PPFS, with control over molecular weight and polydispersity. It was found that the copolymerization of styrene and PFS allowed for the preparation of gradient copolymers with opposite levels of monomer consumption, depending on the feed ratio. Conversion to amphiphilic block copolymers, PAA-b-(PS-co-PPFS), by removing the protecting groups was followed by fitting with monomethoxy poly(ethylene glycol) chains. Solution-state assembly and intramicellar crosslinking afforded shell crosslinked (SCK) block copolymer nanoparticles. These fluorinated nanoparticles (ca. 20 nm diameters) were studied as potential magnetic resonance imaging (MRI) contrast agents based on the (19)F-nuclei, however, it was found that packaging of the hydrophobic fluorinated polymers into the core domain restricted the mobility of the chains and prohibited (19)F-NMR spectroscopy when the particles were dispersed in water without an organic cosolvent. Packing of perflouro-15-crown-5-ether (PFCE) into the polymer micelle was demonstrated with good uptake efficiency, however, it was necessary to swell the core with a good solvent (DMSO) to increase the mobility and observe the (19)F-NMR signal of the PFCE.

In vivo "hot spot" MR imaging of neural stem cells using fluorinated nanoparticles

To optimize (19)F MR tracking of stem cells, we compared cellular internalization of cationic and anionic perfluoro-15-crown-5-ether (PFCE) nanoparticles using cell culture plates with different surface coatings. The viability and proliferation of anionic and cationic PFCE-labeled neural stem cells (NSCs) did not differ from unlabeled cells. Cationic PFCE nanoparticles ((19)F T1/T2 = 580/536 ms at 9.4 Tesla) were superior to anionic particles for intracellular fluorination. Best results were obtained with modified polystyrene culture dishes coated with both carboxylic and amino groups rather than conventional carboxyl-coated dishes. After injecting PFCE-labeled NSCs into the striatum of mouse brain, cells were readily identified in vivo by (19)F MRI without changes in signal or viability over a 2-week period after grafting. These results demonstrate that neural stem cells can be efficiently fluorinated with cationic PFCE nanoparticles without using transfection agents and visualized in vivo over prolonged periods with an MR sensitivity of approximately 140 pmol of PFCE/cell.

Self-delivering nanoemulsions for dual fluorine-19 MRI and fluorescence detection

We report the design, synthesis, and biological testing of highly stable, nontoxic perfluoropolyether (PFPE) nanoemulsions for dual 19F MRI-fluorescence detection. A linear PFPE polymer was covalently conjugated to common fluorescent dyes (FITC, Alexa647 and BODIPy-TR), mixed with pluronic F68 and linear polyethyleneimine (PEI), and emulsified by microfluidization. Prepared nanoemulsions (<200 nm) were readily taken up by both phagocytic and non-phagocytic cells in vitro after a short (approximately 3 h) co-incubation. Following cell administration in vivo, 19F MRI selectively visualizes cell migration. Exemplary in vivo MRI images are presented of T cells labeled with a dual-mode nanoemulsion in a BALB/c mouse. Fluorescence detection enables fluorescent microscopy and FACS analysis of labeled cells, as demonstrated in several immune cell types including Jurkat cells, primary T cells and dendritic cells. The intracellular fluorescence signal is directly proportional to the 19F NMR signal and can be used to calibrate cell loading in vitro.

Neuroimaging of demyelination and remyelination models

Small-animal magnetic resonance imaging is becoming an increasingly utilized noninvasive tool in the study of animal models of MS including the most commonly used autoimmune, viral, and toxic models. Because most MS models are induced in rodents with brains and spinal cords of a smaller magnitude than humans, small-animal MRI must accomplish much higher resolution acquisition in order to generate useful data. In this review, we discuss key aspects and important differences between high field strength experimental and human MRI. We describe the role of conventional imaging sequences including T1, T2, and proton density-weighted imaging, and we discuss the studies aimed at analyzing blood-brain barrier (BBB) permeability and acute inflammation utilizing gadolinium-enhanced MRI. Advanced MRI methods, including diffusion-weighted and magnetization transfer imaging in monitoring demyelination, axonal damage, and remyelination, and studies utilizing in vivo T1 and T2 relaxometry, provide insight into the pathology of demyelinating diseases at previously unprecedented details. The technical challenges of small voxel in vivo MR spectroscopy and the biologically relevant information obtained by analysis of MR spectra in demyelinating models is also discussed. Novel cell-specific and molecular imaging techniques are becoming more readily available in the study of experimental MS models. As a growing number of tissue restorative and remyelinating strategies emerge in the coming years, noninvasive monitoring of remyelination will be an important challenge in small-animal imaging. High field strength small-animal experimental MRI will continue to evolve and interact with the development of new human MR imaging and experimental NMR techniques.

In Vivo Imaging of Autoimmune Disease in Model Systems

Autoimmune diseases are characterized by infiltration of the target tissue with specific immune cells that ultimately leads to the destruction of normal tissue and the associated disease. There is a need for imaging tools that allow the monitoring of ongoing inflammatory disease as well as the response to therapy. We discuss new magnetic resonance imaging-based technologies that have been used to monitor inflammation and disease progression in animal models of type 1 diabetes, multiple sclerosis, and rheumatoid arthritis. Therapeutic strategies for these diseases include the transfer of immune cells, such as dendritic cells, with the aim of preventing or halting the disease course. We discuss several new MRI labeling techniques developed to allow tracking of immune cells in vivo. These include direct ex vivo labeling techniques as well as the genetic modification of cells to allow them to produce their own contrast agents. This is an area of intense recent research and can be expanded to other conditions such as cancer.

19F-MRI in vivo determination of the partial oxygen pressure in perfluorocarbon-loaded alginate capsules implanted into the peritoneal cavity and different tissues

Semipermeable hydrogels formed with a biocompatible alginate solution and Ba(2+) ions protect encapsulated cells and tissues from a foreign immune system. For the viability and metabolic activity of the encapsulated materials, a sufficient oxygen supply inside the capsules is necessary. Quantitative (19)F-MRI was performed on perfluorocarbon-loaded alginate capsules implanted into the peritoneal cavity, the musculus quadriceps femoris, and beneath the kidney capsule of rats, in order to determine in vivo the partial oxygen pressure (pO(2)) inside the capsules at these implantation sites. The temporal behavior of the pO(2) values was observed for at least 3 months. The most stable values over time were observed in the kidney, where inter-rat pO(2) differences were considerable. In the muscle, the values were very high directly after implantation and decreased to nearly zero after 2 weeks. In the peritoneal cavity, values changed randomly over a wide range between different rats and over time. Magn Reson Med 42:1039-1047, 1999.

Correlation between MRI and clinico-pathological manifestations in Lewis rats protected from experimental allergic encephalomyelitis by acylated synthetic peptide of myelin basic protein

Experimental allergic encephalomyelitis (EAE) is an autoimmune disease of the central nervous system which constitutes an accepted animal model for multiple sclerosis (MS). The disease can take an acute or chronic form depending on the injection route, animal strain and nature of the disease-inducing antigen administered. The neuroinflammation associated with the acute form can be detected with T2-weighted, T1-weighted and diffusion MRI, and blood-brain barrier changes can be investigated with Gd-DTPA-enhanced T1-weighted imaging, similar to that of MS patients. A synthetic peptide of myelin basic protein (MBP) encephalitogenic for the Lewis rat (MBP 68-86) was acylated by the attachment of a palmitoyl residue (PAL68-86), and was shown to confer almost complete protection against EAE, when administered to rats before and after an encephalitogenic challenge. In this study, treatment of Lewis rats with PAL68-86 prevented the appearance of clinical signs (p < 0.0001) after challenge with the native peptide (p68-86) in complete Freund's adjuvant (CFA), and reduced considerably the MRI and histopathological signs of the disease (p < 0.0001). Measurement of the gadolinium leakage due to neuroinflammation revealed a significant decrease in permeability from 4.09 +/- 2.1 to 2.95 +/- 1.79% pixels > mean + 2 SD (p = 0.011). Therefore, quantitative MRI measurements correlate very well with the reduced cellular infiltration in the CNS and the absence of clinical signs in the EAE-protected animal.