[18F]FDG labeling of neural stem cells for in vivo cell tracking with positron emission tomography: inhibition of tracer release by phloretin.

The introduction of neural stem cells into the brain has promising therapeutic potential for the treatment of neurodegenerative diseases. To monitor the cellular replacement therapy, that is, to determine stem cell migration, survival, and differentiation, in vivo tracking methods are needed. Ideally, these tracking methods are noninvasive. Noninvasive tracking methods that have been successfully used for the visualization of blood-derived progenitor cells include magnetic resonance imaging and radionuclide imaging using single-photon emission computed tomography (SPECT) and positron emission tomography (PET). The SPECT tracer In-111-oxine is suitable for stem cell labeling, but for studies in small animals, the higher sensitivity and facile quantification that can be obtained with PET are preferred. Here the potential of 2'-[18F]fluoro-2'-deoxy-D-glucose ([18F]-FDG), a PET tracer, for tracking of neural stem cell (NSCs) trafficking toward an inflammation site was investigated. [18F]-FDG turns out to be a poor radiopharmaceutical to label NSCs owing to the low labeling efficiency and substantial release of radioactivity from these cells. Efflux of [18F]-FDG from NSCs can be effectively reduced by phloretin in vitro, but inhibition of tracer release is insufficient in vivo for accurate monitoring of stem cell trafficking.

Size dependent biodistribution and SPECT imaging of (111)In-labeled polymersomes.

Polymersomes, self-assembled from the block copolymer polybutadiene-block-poly(ethylene glycol), were prepared with well-defined diameters between 90 and 250 nm. The presence of ~1% of diethylene triamine penta acetic acid on the polymersome periphery allowed to chelate radioactive (111)In onto the surface and determine the biodistribution in mice as a function of both the polymersome size and poly(ethylene glycol) corona thickness (i.e., PEG molecular weight). Doubling the PEG molecular weight from 1 kg/mol to 2 kg/mol did not change the blood circulation half-life significantly. However, the size of the different polymersome samples did have a drastic effect on the blood circulation times. It was found that polymersomes of 120 nm and larger become mostly cleared from the blood within 4 h, presumably due to recognition by the reticuloendothelial system. In contrast, smaller polymersomes of around 90 nm circulated much longer. After 24 h more than 30% of the injected dose was still present in the blood pool. This sharp transition in blood circulation kinetics due to size is much more abrupt than observed for liposomes and was additionally visualized by SPECT/CT imaging. These findings should be considered in the formulation and design of polymersomes for biomedical applications. Size, much more than for liposomes, will influence the pharmacokinetics, and therefore, long circulating preparations should be well below 100 nm.

In vivo biodistribution of prion- and GM1-targeted polymersomes following intravenous administration in mice.

Due to the aging of the population, the incidence of neurodegenerative diseases, such as Parkinson's and Alzheimer's, is expected to grow and, hence, the demand for adequate treatment modalities. However, the delivery of medicines into the brain for the treatment of brain-related diseases is hampered by the presence of a tight layer of endothelial cells that forms the blood-brain barrier (BBB). Furthermore, most conventional drugs lack stability and/or bioavailability. These obstacles can be overcome by the application of nanocarriers, in which the therapeutic entity has been incorporated, provided that they are effectively targeted to the brain endothelial cell layer. Drug nanocarriers decorated with targeting ligands that bind BBB receptors may accumulate efficiently at/in brain microvascular endothelium and hence represent a promising tool for brain drug delivery. Following the accumulation of drug nanocarriers at the brain vasculature, the drug needs to be transported across the brain endothelial cells into the brain. Transport across brain endothelial cells can occur via passive diffusion, transport proteins, and the vesicular transport pathways of receptor-mediated and adsorptive-mediated transcytosis. When a small lipophilic drug is released from its carrier at the brain vasculature, it may enter the brain via passive diffusion. On the other hand, the passage of intact nanocarriers, which is necessary for the delivery of larger and more hydrophilic drugs into brain, may occur via active transport by means of transcytosis. In previous work we identified GM1 ganglioside and prion protein as potential transcytotic receptors at the BBB. GM1 is a glycosphingolipid that is ubiquitously present on the endothelial surface and capable of acting as the transcytotic receptor for cholera toxin B. Likewise, prion protein has been shown to have transcytotic capacity at brain endothelial cells. Here we determine the transcytotic potential of polymersome nanocarriers functionalized with GM1- and prion-targeting peptides (G23, P50 and P9), that were identified by phage display, in an in vitro BBB model. In addition, the biodistribution of polymersomes functionalized with either the prion-targeting peptide P50 or the GM1-targeting peptide G23 is determined following intravenous injection in mice. We show that the prion-targeting peptides do not induce efficient transcytosis of polymersomes across the BBB in vitro nor induce accumulation of polymersomes in the brain in vivo. In contrast, the G23 peptide is shown to have transcytotic capacity in brain endothelial cells in vitro, as well as a brain-targeting potential in vivo, as reflected by the accumulation of G23-polymersomes in the brain in vivo at a level comparable to that of RI7217-polymersomes, which are targeted toward the transferrin receptor. Thus the G23 peptide seems to serve both of the requirements that are needed for efficient brain drug delivery of nanocarriers. An unexpected finding was the efficient accumulation of G23-polymersomes in lung. In conclusion, because of its combined brain-targeting and transcytotic capacity, the G23 peptide could be useful in the development of targeted nanocarriers for drug delivery into the brain, but appears especially attractive for specific drug delivery to the lung.

Imaging of cells and nanoparticles: implications for drug delivery to the brain.

A major challenge in the development of central nervous system drugs is to obtain therapeutic effective drug concentrations inside the brain. Many potentially effective drugs have never reached clinical application because of poor brain penetration. Currently, devices are being developed that may improve drug delivery into the brain. One approach involves the encapsulation of drugs into nanocarriers that are targeted to the brain, where the drug is released. Alternatively, living cells have been engineered to produce the pharmaceutical of interest at the target site. It is important to follow the fate of these drug delivery devices inside the body to verify their efficiency in reaching the brain. To this end, both ex-vivo approaches and in-vivo imaging techniques are used, including ex-vivo biodistribution, autoradiography, MRI, optical imaging, PET and SPECT. All these methods have their specific advantages and limitations. Consequently, selection of the tracking method should be based on the specific aims of the experiment. Here, we will discuss the methods that are currently applied for tracking brain drug delivery devices, including the most commonly used labels and labeling procedures for living cells and nanocarriers. Subsequently, we will discuss specific applications in tracking drug delivery devices.