Mapping the acute time course of immune cell infiltration into an ECM hydrogel in a rat model of stroke using 19F MRI

Extracellular matrix (ECM) hydrogel implantation into a stroke-induced tissue cavity invokes a robust cellular immune response. However, the spatio-temporal dynamics of immune cell infiltration into peri-infarct brain tissues versus the ECM-bioscaffold remain poorly understood. We here tagged peripheral immune cells using perfluorocarbon (PFC) nanoemulsions that afford their visualization by ¹⁹F magnetic resonance imaging (MRI). Prior to ECM hydrogel implantation, only blood vessels could be detected using ¹⁹F MRI. Using "time-lapse" ¹⁹F MRI, we established the infiltration of immune cells into the peri-infarct area occurs 5-6 h post-ECM implantation. Immune cells also infiltrated through the stump of the MCA, as well as a hydrogel bridge that formed between the tissue cavity and the burr hole in the skull. Tissue-based migration into the bioscaffold was observed between 9 and 12 h with a peak signal measured between 12 and 18 h post-implantation. Fluorescence-activated cell sorting of circulating immune cells revealed that 9% of cells were labeled with PFC nanoemulsions, of which the vast majority were neutrophils (40%) or monocytes (48%). Histology at 24 h post-implantation, in contrast, indicated that macrophages (35%) were more numerous in the peri-infarct area than neutrophils (11%), whereas the vast majority of immune cells within the ECM hydrogel were neutrophils (66%). Only a small fraction (12%) of immune cells did not contain PFC nanoemulsions, indicating a low type II error for ¹⁹F MRI. ¹⁹F MRI hence provides a unique tool to improve our understanding of the spatio-temporal dynamics of immune cells invading bioscaffolds and effecting biodegradation.

In vivo clearance of 19F MRI imaging nanocarriers is strongly influenced by nanoparticle ultrastructure

Perfluorocarbons hold great promise both as imaging agents, particularly for ¹⁹F MRI, and in therapy, such as oxygen delivery. ¹⁹F MRI is unique in its ability to unambiguously track and quantify a tracer while maintaining anatomic context, and without the use of ionizing radiation. This is particularly well-suited for inflammation imaging and quantitative cell tracking. However, perfluorocarbons, which are best suited for imaging - like perfluoro-15-crown-5 ether (PFCE) - tend to have extremely long biological retention. Here, we showed that the use of a multi-core PLGA nanoparticle entrapping PFCE allows for a 15-fold reduction of half-life in vivo compared to what is reported in literature. This unexpected rapid decrease in ¹⁹F signal was observed in liver, spleen and within the infarcted region after myocardial infarction and was confirmed by whole body NMR spectroscopy. We demonstrate that the fast clearance is due to disassembly of the ~200 nm nanoparticle into ~30 nm domains that remain soluble and are cleared quickly. We show here that the nanoparticle ultrastructure has a direct impact on in vivo clearance of its cargo i.e. allowing fast release of PFCE, and therefore also bringing the possibility of multifunctional nanoparticle-based imaging to translational imaging, therapy and diagnostics.

Nanoparticles for "two color" 19F magnetic resonance imaging: Towards combined imaging of biodistribution and degradation

The use of polymeric nanoparticles (NPs) as therapeutics has been steadily increasing over past decades. In vivo imaging of NPs is necessary to advance the therapeutic performance. 19F Magnetic Resonance Imaging (19F MRI) offers multiple advantages for in vivo imaging. However, design of a probe for both biodistribution and degradation has not been realized yet. We developed polymeric NPs loaded with two fluorocarbons as promising imaging tools to monitor NP biodistribution and degradation by 19F MRI. These 200 nm NPs consist of poly(lactic-co-glycolic acid) (PLGA) loaded with perfluoro-15-crown-5 ether (PFCE) and PERFECTA. PERFECTA/PFCE-PLGA NPs have a fractal sphere structure, in which both fluorocarbons are distributed in the polymeric matrix of the fractal building blocks, which differs from PFCE-PLGA NPs and is unique for fluorocarbon-loaded colloids. This structure leads to changes of magnetic resonance properties of both fluorocarbons after hydrolysis of NPs. PERFECTA/PFCE-PLGA NPs are colloidally stable in serum and biocompatible. Both fluorocarbons show a single resonance in 19F MRI that can be imaged separately using different excitation pulses. In the future, these findings may be used for biodistribution and degradation studies of NPs by 19F MRI in vivo using "two color" labeling leading to improvement of drug delivery agents.

Small-scale preparation of perfluorocarbon-nanoemulsions utilizing dual centrifugation

BACKGROUND

Perfluorocarbon-nanoemulsions (PFC-NE) made of PFC and phospholipids (PL) by homogenization are optimal for in vivo-¹⁹F labelling of monocytes and subsequently of inflamed tissues in magnetic resonance imaging (MRI). Necessary requirements for in vivo use of PFC-NE are sterility, suitable droplet sizes and the absence of immune activating liposomes, which are a typical byproduct of the homogenization process.

METHODS AND RESULTS

To meet these requirements, we developed an aseptic in-vial preparation technique for PFC-NE based on dual centrifugation (DC) by testing different PFC/phospholipid ratios as well as the application of additives. Two different PFC, perfluorooctylbromide (PFOB) and perfluoro-15-crown-5-ether (PFCE), were investigated. Particle sizes were assessed by dynamic light scattering and NE morphology by cryoTEM. DC homogenization was optimal when using an excess of PL (8.7 % (m/m) of utilized PFC, z-ave: 180 nm, pdi: 0.2). A purification approach by centrifugation was implemented to remove liposomes formed from the excess of PL during homogenization. The purification success was proven by phospholipid assay and PFC quantification via density and sound velocity measurements.

CONCLUSION

DC in combination with a short centrifugation is a fast and reliable way of small-scale aseptic PFC-NE production for ¹⁹F MRI passive-targeting experiments of monocytes and inflamed tissues.

Real time MRI to elucidate the functionality of coating films intended for modified release

Polymer films based on mixtures of ethyl cellulose (EC) and hydroxypropyl cellulose (HPC) have been widely used to coat pellets and tablets to modify the release profile of drugs. For three different EC/HPC films we used ¹H and ¹⁹F MRI in combination with a designed release cell to monitor the drug, polymer and water in 5 dimensional (5D) datasets; three spatial, one diffusion or relaxation and a temporal dimension, in real time. We observed that the water inflow through the films correlated with the initiation of the dissolution of the drug in the tablet beneath the film. Leaching of the pore forming HPC further accelerated water penetration and resulted in a drug release onset after a hydrostatic pressure was generated below the film indicated by positional changes of the film. For the more permeable film, both water ingress and drug egress showed a large variability of release over the film surface indicating the heterogeneity of the system. Furthermore, the ¹H diffusion dataset revealed the formation of a gel layer of HPC at the film surface. We conclude that the setup presented provides a significant level of details, which are not achieved with traditional methods.

A systematic optimization of 19F MR image acquisition to detect macrophage invasion into an ECM hydrogel implanted in the stroke-damaged brain

¹⁹F-MR imaging of perfluorocarbon (PFC)-labeled macrophages can provide a unique insight into their participation and spatio-temporal dynamics of inflammatory events, such as the biodegradation of an extracellular matrix (ECM) hydrogel implanted into a stroke cavity. To determine the most efficient acquisition strategy for ¹⁹F-MR imaging, five commonly used sequences were optimized using a design of experiment (DoE) approach and compared based on their signal-to-noise ratio (SNR). The fast imaging with steady-state precession (FISP) sequence produced the most efficient detection of a ¹⁹F signal followed by the rapid acquisition with relaxation enhancement (RARE) sequence. The multi-slice multi-echo (MSME), fast low angle shot (FLASH), and zero echo time (ZTE) sequences were significantly less efficient. Imaging parameters (matrix/voxel size; slice thickness, number of averages) determined the accuracy (i.e. trueness and precision) of object identification by reducing partial volume effects, as determined by analysis of the point spread function (PSF). A 96 × 96 matrix size (0.35 mm³) produced the lowest limit of detection (LOD) for RARE (2.85 mM PFPE; 119 mM ¹⁹F) and FISP (0.43 mM PFPE; 18.1 mM ¹⁹F), with an SNR of 2 as the detection threshold. Imaging of a brain phantom with PFC-labeled macrophages invading an ECM hydrogel further illustrated the impact of these parameter changes. The systematic optimization of sequence and imaging parameters provides the framework for an accurate visualization of ¹⁹F-labeled macrophage distribution and density in the brain. This will enhance our understanding of the contribution of periphery-derived macrophages in bioscaffold degradation and its role in brain tissue regeneration.

Cargo-influences on the biodistribution of hollow mesoporous silica nanoparticles as studied by quantitative 19F-magnetic resonance imaging

HYPOTHESIS

Biodistribution is a key issue when it comes to medical applications of nanomaterials. Hollow mesoporous silica nanoparticles (HMSNs) loaded with fluorine compounds can be applied as positive magnetic resonance imaging (MRI) contrast agents (CAs). These CAs exhibit an unusual biodistribution which is influenced by the cargo and which could be linked to their serum protein adsorption behaviour.

EXPERIMENTS

HMSNs were post-synthetically loaded with perfluoro-15-crown-5-ether (PFCE). The ¹⁹F signal was quantified with MRI in a murine model. Furthermore protein adsorption tests were performed in full serum.

FINDINGS

Quantitative analysis of the ¹⁹F-signal revealed that the particles were exclusively accumulating in the liver 24h post-injection, and no accumulation in other reticuloendothelial system (RES) organs like spleen or lung was observed. The protein corona around non-loaded and loaded particles was therefore analysed, and more proteins adsorbed on PFCE-loaded particles as compared to the bare particles, and importantly, the amount of apolipoproteins A-1 and A-2, was clearly elevated for the PFCE-loaded particles. The results underline that the type of cargo may have major influences on the biodistribution of mesoporous silica drug vectors.

(19)F spin-lattice relaxation of perfluoropolyethers: Dependence on temperature and magnetic field strength (7.0-14.1T)

Fluorine ((19)F) MRI of perfluorocarbon-labeled cells has become a powerful technique to track the migration and accumulation of cells in living organisms. It is common to label cells for (19)F MRI with nanoemulsions of perfluoropolyethers that contain a large number of chemically equivalent fluorine atoms. Understanding the mechanisms of (19)F nuclear relaxation, and in particular the spin-lattice relaxation of these molecules, is critical to improving experimental sensitivity. To date, the temperature and magnetic field strength dependence of spin-lattice relaxation rate constant (R1) for perfluoropolyethers has not been described in detail. In this study, we evaluated the R1 of linear perfluoropolyether (PFPE) and cyclic perfluoro-15-crown-5 ether (PCE) at three magnetic field strengths (7.0, 9.4, and 14.1T) and at temperatures ranging from 256-323K. Our results show that R1 of perfluoropolyethers is dominated by dipole-dipole interactions and chemical shift anisotropy. R1 increased with magnetic field strength for both PCE and PFPE. In the temperature range studied, PCE was in the fast motion regime (ωτc<1) at all field strengths, but for PFPE, R1 passed through a maximum, from which the rotational correlation time was estimated. The importance of these measurements for the rational design of new (19)F MRI agents and methods is discussed.

Injectable hyaluronic acid hydrogel for 19F magnetic resonance imaging

We report on a 19F labeled injectable hyaluronic acid (HA) hydrogel that can be monitored by both 1H and 19F MR imaging. The HA based hydrogel formed via carbazone reaction can be obtained within a minute by simple mixing of HA-carbazate and HA-aldehyde derivatized polymers. 19F contrast agent was linked to with carbazate and thiol dually functionalized HA via orthogonal Michael addition reaction which afforded cross-linkable and 19F labeled HA. The 19F labeling of HA polymer did not affect the mechanical properties of the formed hydrogel. As a result, the shape of a hydrogel sample could be imaged very well by both 1H MRI and high resolution 19F MRI. This hydrogel has high potential in clinical applications since it is injectable, biocompatible, and can be tracked in a minimally invasive manner. The present approach can be applied in preparation of injectable 19F labeled hydrogel biomaterials from other natural biomacromolecules.