A simple method for stem cell labeling with fluorine 18

M. de Rosales R. Direct Cell Radiolabeling for in Vivo Cell Tracking with PET and SPECT Imaging

The arrival of cell-based therapies is a revolution in medicine. However, its safe clinical application in a rational manner depends on reliable, clinically applicable methods for determining the fate and trafficking of therapeutic cells in vivo using medical imaging techniques─known as in vivo cell tracking. Radionuclide imaging using single photon emission computed tomography (SPECT) or positron emission tomography (PET) has several advantages over other imaging modalities for cell tracking because of its high sensitivity (requiring low amounts of probe per cell for imaging) and whole-body quantitative imaging capability using clinically available scanners. For cell tracking with radionuclides, ex vivo direct cell radiolabeling, that is, radiolabeling cells before their administration, is the simplest and most robust method, allowing labeling of any cell type without the need for genetic modification. This Review covers the development and application of direct cell radiolabeling probes utilizing a variety of chemical approaches: organic and inorganic/coordination (radio)chemistry, nanomaterials, and biochemistry. We describe the key early developments and the most recent advances in the field, identifying advantages and disadvantages of the different approaches and informing future development and choice of methods for clinical and preclinical application.

Single-Cell Imaging Using Radioluminescence Microscopy Reveals Unexpected Binding Target for [18F]HFB

PURPOSE

Cell-based therapies are showing great promise for a variety of diseases, but remain hindered by the limited information available regarding the biological fate, migration routes and differentiation patterns of infused cells in trials. Previous studies have demonstrated the feasibility of using positron emission tomography (PET) to track single cells utilising an approach known as positron emission particle tracking (PEPT). The radiolabel hexadecyl-4-[¹⁸F]fluorobenzoate ([¹⁸F]HFB) was identified as a promising candidate for PEPT, due to its efficient and long-lasting labelling capabilities. The purpose of this work was to characterise the labelling efficiency of [¹⁸F]HFB in vitro at the single-cell level prior to in vivo studies.

PROCEDURES

The binding efficiency of [¹⁸F]HFB to MDA-MB-231 and Jurkat cells was verified in vitro using bulk gamma counting. The measurements were subsequently repeated in single cells using a new method known as radioluminescence microscopy (RLM) and binding of the radiolabel to the single cells was correlated with various fluorescent dyes.

RESULTS

Similar to previous reports, bulk cell labelling was significantly higher with [¹⁸F]HFB (18.75 ± 2.47 dpm/cell, n = 6) than 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG) (7.59 ± 0.73 dpm/cell, n = 7; p ≤ 0.01). However, single-cell imaging using RLM revealed that [¹⁸F]HFB accumulation in live cells (8.35 ± 1.48 cpm/cell, n = 9) was not significantly higher than background levels (4.83 ± 0.52 cpm/cell, n = 12; p > 0.05) and was 1.7-fold lower than [¹⁸F]FDG uptake in the same cell line (14.09 ± 1.90 cpm/cell, n = 13; p < 0.01). Instead, [¹⁸F]HFB was found to bind significantly to fragmented membranes associated with dead cell nuclei, suggesting an alternative binding target for [¹⁸F]HFB.

CONCLUSION

This study demonstrates that bulk analysis alone does not always accurately portray the labelling efficiency, therefore highlighting the need for more routine screening of radiolabels using RLM to identify heterogeneity at the single-cell level.

Three-dimensional spatiotemporal tracking of fluorine-18 radiolabeled yeast cells via positron emission particle tracking

A method for Positron Emission Particle Tracking (PEPT) based on optical feature point identification techniques is demonstrated for use in low activity tracking experiments. A population of yeast cells of approximately 125,000 members is activated to roughly 55 Bq/cell by 18F uptake. An in vitro particle tracking experiment is performed with nearly 20 of these cells after decay to 32 Bq/cell. These cells are successfully identified and tracked simultaneously in this experiment. This work extends the applicability of PEPT as a cell tracking method by allowing a number of cells to be tracked together, and demonstrating tracking for very low activity tracers.

Evaluation of a BGO-Based PET System for Single-Cell Tracking Performance by Simulation and Phantom Studies

A recent method based on positron emission was reported for tracking moving point sources using the Inveon PET system. However, the effect of scanner background noise was not further explored. Here, we evaluate tracking with the Genisys4, a bismuth germanate-based PET system, which has no significant intrinsic background and may be better suited to tracking lower and/or faster activity sources. Position-dependent sensitivity of the Genisys4 was simulated in Geant4 Application for Tomographic Emission (GATE) using a static ¹⁸F point source. Trajectories of helically moving point sources with varying activity and rotation speed were reconstructed from list-mode data as described previously. Simulations showed that the Inveon’s ability to track sources within 2 mm of localization error is limited to objects with a velocity-to-activity ratio < 0.13 mm/decay, compared to < 0.29 mm/decay for the Genisys4. Tracking with the Genisys4 was then validated using a physical phantom of helically moving [¹⁸F] fluorodeoxyglucose-in-oil droplets (< 0.24 mm diameter, 139-296 Bq), yielding < 1 mm localization error under the tested conditions, with good agreement between simulated sensitivity and measured activity (Pearson correlation R = .64, P << .05 in a representative example). We have investigated the tracking performance with the Genisys4, and results suggest the feasibility of tracking low activity, point source-like objects with this system.

Increasing the sensitivity for stem cell monitoring in system-function based magnetic particle imaging

The use of superparamagnetic iron oxide nanoparticles (SPIONs) has provided new possibilities in biophysics and biomedical imaging technologies. The magnetization dynamics of SPIONs, which can be influenced by the environment, are of central interest. In this work, different biological SPION environments are used to investigate three different calibration methods for stem cell monitoring in magnetic particle imaging. It is shown that calibrating using SPIONs immobilized via agarose gel or intracellular uptake results in superior stem cell image quality compared to mobile SPIONs in saline. This superior image quality enables more sensitive localization and identification of a significantly smaller number of magnetically labeled stem cells. The results are important for cell tracking and monitoring of future SPION based therapies such as hyperthermia based cancer therapies, targeted drug delivery, or tissue regeneration approaches where it is crucial to image a sufficiently small number of SPIONs interacting with biological matter.

Stem Cell Monitoring with a Direct or Indirect Labeling Method

The molecular imaging techniques allow monitoring of the transplanted cells in the same individuals over time, from early localization to the survival, migration, and differentiation. Generally, there are two methods of stem cell labeling: direct and indirect labeling methods. The direct labeling method introduces a labeling agent into the cell, which is stably incorporated or attached to the cells prior to transplantation. Direct labeling of cells with radionuclides is a simple method with relatively fewer adverse events related to genetic responses. However, it can only allow short-term distribution of transplanted cells because of the decreasing imaging signal with radiodecay, according to the physical half-lives, or the signal becomes more diffuse with cell division and dispersion. The indirect labeling method is based on the expression of a reporter gene transduced into the cell before transplantation, which is then visualized upon the injection of an appropriate probe or substrate. In this review, various imaging strategies to monitor the survival and behavior change of transplanted stem cells are covered. Taking these new approaches together, the direct and indirect labeling methods may provide new insights on the roles of in vivo stem cell monitoring, from bench to bedside.

Simple Methods for Tracking Stem Cells with (64)Cu-Labeled DOTA-hexadecyl-benzoate

The purpose of this study was to evaluate (64)Cu-labeled hexadecyl-1,4,7,10-tetraazacyclododecane-tetraacetic acid-benzoate ((64)Cu-DOTA-HB) (1) as positron emission tomography (PET) radiotracer for stem cell imaging. Hexadecyl-DOTA-benzoate (DOTA-HB) (2) was efficiently labeled with (64)Cu (>99%), and cell labeling efficiency with adipose-derived stem cells (ADSCs) was over 50%. Labeling with 1 did not compromise cell viability. In the PET imaging, intramuscularly transplanted 1-labeled ADSCs were monitored for 18 h in normal rat heart. These results indicate that 1 can be utilized as a promising radiotracer for monitoring of transplanted stem cells.

PET imaging of a collagen matrix reveals its effective injection and targeted retention in a mouse model of myocardial infarction

Injectable biomaterials have shown promise for cardiac regeneration therapy. However, little is known regarding their retention and distribution upon application in vivo. Matrix imaging would be useful for evaluating these important properties. Herein, hexadecyl-4-[(18)F]fluorobenzoate ((18)F-HFB) and Qdot labeling was used to evaluate collagen matrix delivery in a mouse model of myocardial infarction (MI). At 1 wk post-MI, mice received myocardial injections of (18)F-HFB- or Qdot-labeled matrix to assess its early retention and distribution (at 10 min and 2h) by positron emission tomography (PET), or fluorescence imaging, respectively. PET imaging showed that the bolus of matrix at 10 min redistributed evenly within the ischemic territory by 2h. Ex vivo biodistribution revealed myocardial matrix retention of ∼ 65%, which correlated with PET results, but may be an underestimate since (18)F-HFB matrix labeling efficiency was ∼ 82%. For covalently linked Qdots, labeling efficiency was ∼ 96%. Ex vivo Qdot quantification showed that ∼ 84% of the injected matrix was retained in the myocardium. Serial non-invasive PET imaging and validation by fluorescence imaging confirmed the effectiveness of the collagen matrix to be retained and redistributed within the infarcted myocardium. This study identifies matrix-targeted imaging as a promising modality for assessing the biodistribution of injectable biomaterials for application in the heart.

Mesenchymal stem cell tracking in the intervertebral disc

Low back pain is a common clinical problem, which leads to significant social, economic and public health costs. Intervertebral disc (IVD) degeneration is accepted as a common cause of low back pain. Initially, this is characterized by a loss of proteoglycans from the nucleus pulposus resulting in loss of tissue hydration and hydrostatic pressure. Conservative management, including analgesia and physiotherapy often fails and surgical treatment, such as spinal fusion, is required. Stem cells offer an exciting possible regenerative approach to IVD disease. Preclinical research has demonstrated promising biochemical, histological and radiological results in restoring degenerate IVDs. Cell tracking provides an opportunity to develop an in-depth understanding of stem cell survival, differentiation and migration, enabling optimization of stem cell treatment. Magnetic Resonance Imaging (MRI) is a non-invasive, non-ionizing imaging modality with high spatial resolution, ideally suited for stem cell tracking. Furthermore, novel MRI sequences have the potential to quantitatively assess IVD disease, providing an improved method to review response to biological treatment. Superparamagnetic iron oxide nanoparticles have been extensively researched for the purpose of cell tracking. These particles are biocompatible, non-toxic and act as excellent MRI contrast agents. This review will explore recent advances and issues in stem cell tracking and molecular imaging in relation to the IVD.

Longitudinal monitoring adipose-derived stem cell survival by PET imaging hexadecyl-4-¹²⁴I-iodobenzoate in rat myocardial infarction model

This study aims to monitor how the change of cell survival of transplanted adipose-derived stem cells (ADSCs) responds to myocardial infarction (MI) via the hexadecyl-4-(124)I-iodobenzoate ((124)I-HIB) mediated direct labeling method in vivo. Stem cells have shown the potential to improve cardiac function after MI. However, monitoring of the fate of transplanted stem cells at target sites is still unclear. Rat ADSCs were labeled with (124)I-HIB, and radiolabeled ADSCs were transplanted into the myocardium of normal and MI model. In the group of (124)I-HIB-labeled ADSC transplantation, in vivo imaging was performed using small-animal positron emission tomography (PET)/computed tomography (CT) for 9 days. Twenty-one days post-transplantation, histopathological analysis and apoptosis assay were performed. ADSC viability and differentiation were not affected by (124)I-HIB labeling. In vivo tracking of the (124)I-HIB-labeled ADSCs was possible for 9 and 3 days in normal and MI model, respectively. Apoptosis of transplanted cells increased in the MI model compared than that in normal model. We developed a direct labeling agent, (124)I-HIB, and first tried to longitudinally monitor transplanted stem cell to MI. This approach may provide new insights on the roles of stem cell monitoring in living bodies for stem cell therapy from pre-clinical studies to clinical trials.

Imaging modalities for the in vivo surveillance of mesenchymal stromal cells

Bone marrow stromal cells exist as mesenchymal stromal cells (MSCs) and have the capacity to differentiate into multiple tissue types when subjected to appropriate culture conditions. This property of MSCs creates therapeutic opportunities in regenerative medicine for the treatment of damage to neural, cardiac and musculoskeletal tissues or acute kidney injury. The prerequisite for successful cell therapy is delivery of cells to the target tissue. Assessment of therapeutic outcomes utilize traditional methods to examine cell function of MSC populations involving routine biochemical or histological analysis for cell proliferation, protein synthesis and gene expression. However, these methods do not provide sufficient spatial and temporal information. In vivo surveillance of MSC migration to the site of interest can be performed through a variety of imaging modalities such as the use of radiolabelling, fluc protein expression bioluminescence imaging and paramagnetic nanoparticle magnetic resonance imaging. This review will outline the current methods of in vivo surveillance of exogenously administered MSCs in regenerative medicine while addressing potential technological developments. Furthermore, nanoparticles and microparticles for cellular labelling have shown that migration of MSCs can be spatially and temporally monitored. In vivo surveillance therefore permits time-stratified assessment in animal models without disruption of the target organ. In vivo tracking of MSCs is non-invasive, repeatable and non-toxic. Despite the excitement that nanoparticles for tracking MSCs offer, delivery methods are difficult because of the challenges with imaging three-dimensional systems. The current advances and growth in MSC research, is likely to provide a wealth of evidence overcoming these issues.

Single amino acid chelate complexes of the M(CO)3 (+) core for correlating fluorescence and radioimaging studies (M = (99m) Tc or Re)

Single amino acid chelates (SAACs) and SAAC-like bifunctional ligands can be exploited in the design of a variety of bioconjugates for facile metallation with the M(CO)3 (+) unit with M = (99m) Tc or Re. When the donor groups of the ligand are quinolone, thiazole or other similarly conjugated heterocycles, the rhenium complexes are fluorescent, affording complementary and isostructural fluorescent probes to the radioactive (99m) Tc analogues. The versatility of the approach has been demonstrated by the preparation of bioconjugates incorporating peptides, biotin, folic acid, thymidine and vitamin B12 . In addition, the unusual photophysical properties observed for rhenium of the [bisthiazole-diamino butane-Re(CO)3 (+) ] derivative [BTBA-Re(CO)3 ](+) are discussed.

Superparamagnetic iron oxide nanoparticles as MRI contrast agents for non-invasive stem cell labeling and tracking

Stem cells hold great promise for the treatment of multiple human diseases and disorders. Tracking and monitoring of stem cells in vivo after transplantation can supply important information for determining the efficacy of stem cell therapy. Magnetic resonance imaging (MRI) combined with contrast agents is believed to be the most effective and safest non-invasive technique for stem cell tracking in living bodies. Commercial superparamagnetic iron oxide nanoparticles (SPIONs) in the aid of transfection agents (TAs) have been applied to labeling stem cells. However, owing to the potential toxicity of TAs, more attentions have been paid to develop novel SPIONs with specific surface coating or functional moieties which facilitate effective cell internalization in the absence of TAs. This review aims to summarize the recent progress in the design and preparation of SPIONs as cellular MRI probes, to discuss their applications and current problems facing in stem cell labeling and tracking, and to offer perspectives and solutions for the future development of SPIONs in this field.

18F-FDG Cell Labeling May Underestimate Transplanted Cell Homing: More Accurate, Efficient, and Stable Cell Labeling with Hexadecyl-4-[18F]Fluorobenzoate for in Vivo Tracking of Transplanted Human Progenitor Cells by Positron Emission Tomography

Cell therapy is expected to restore perfusion and improve function in the ischemic/infarcted myocardium; however, the biological mechanisms and local effects of transplanted cells remain unclear. To assess cell fate in vivo, hexadecyl-4-[18F]fluorobenzoate (18F-HFB) cell labeling was evaluated for tracking human circulating progenitor cells (CPCs) with positron emission tomography (PET) and was compared to the commonly used 2-[18F]fluoro-2-deoxy-d-glucose (18F-FDG) labeling method in a rat myocardial infarction model. CPCs were labeled with 18F-HFB or 18F-FDG ex vivo under the same conditions. 18F-HFB cell-labeling efficiency (23.4 ± 7.5%) and stability (4 h, 88.4 ± 6.0%) were superior to 18F-FDG (7.6 ± 4.1% and 26.6 ± 6.1%, respectively; p < 0.05). Neither labeling approach significantly altered cell viability, phenotype or migration potential up to 24 h postlabeling. Two weeks after left anterior descending coronary artery ligation, rats received echo-guided intramyocardial injection in the infarct border zone with 18F-HFB-CPCs, 18F-FDG-CPCs, 18F-HFB, or 18F-FDG. Dynamic PET imaging of both 18F-HFB-CPCs and 18F-FDG-CPCs demonstrated that only 16–37% of the initial injection dose (ID) was retained in the injection site at 10 min postdelivery, and remaining activity fell significantly over the first 4 h posttransplantation. The 18F-HFB-CPC signal in the target area at 2 h (23.7 ± 14.7% ID/g) and 4 h (17.6 ± 13.3% ID/g) postinjection was greater than that of 18F-FDG-CPCs (5.4 ± 2.3% ID/g and 2.6 ± 0.7% ID/g, respectively; p < 0.05). Tissue biodistribution confirmed the higher radioactivity in the border zone of 18F-HFB-CPC rats. Immunostaining of heart tissue sections revealed no significant difference in cell retention between two labeled cell transplantation groups. Good correlation with biodistribution results was observed in the 18F-HFB-CPC rats ( r = 0.81, p < 0.05). Compared to 18F-FDG, labeling human CPCs with 18F-HFB provides a more efficient, stable, and accurate way to quantify the distribution of transplanted cells. 18F-HFB cell labeling with PET imaging offers a better modality to enhance our understanding of early retention, homing, and engraftment with cardiac cell therapy.

In vivo cell tracking via ¹⁸F-fluorodeoxyglucose labeling: a review of the preclinical and clinical applications in cell-based diagnosis and therapy

The rising interest in using functional cells for diagnosis and treatment has created an urgent need for in vivo cell-tracking techniques. Certain advanced techniques, such as those involving reporter genes or nanoparticles, are still awaiting confirmation of their safety and feasibility in human patients. Tracking cells by labeling them with (18)F-fluorodeoxyglucose, a tracer clinically used in positron emission tomography (PET), may be one way to rapidly translate some of these principles from bench to bedside. The preliminary results are exciting, although further development, optimization, and validation are required. Here, several applications of the technique are surveyed: finding inflammatory foci, targeting cancer immunotherapies, tracking transplanted islet cells, and monitoring cardiac stem cells. Advantages, limitations, and prospects of the technique are discussed. These early experiences only highlight the existing need to improve cell-labeling techniques using PET tracers. This method may finally lead to the development of effective and convenient methods for clinical cell-tracking techniques involving PET/computed tomography.

Controllable labelling of stem cells with a novel superparamagnetic iron oxide-loaded cationic nanovesicle for MR imaging

OBJECTIVE

To investigate the feasibility of highly efficient and controllable stem cell labelling for cellular MRI.

METHODS

A new class of cationic, superparamagnetic iron oxide nanoparticle (SPION)-loaded nanovesicles was synthesised to label rat bone marrow mesenchymal stem cells without secondary transfection agents. The optimal labelling conditions and controllability were assessed, and the effect of labelling on cell viability, proliferation activity and multilineage differentiation was determined. In 18 rats, focal ischaemic cerebral injury was induced and the rats randomly injected with 1 × 10(6) cells labelled with 0-, 8- or 20-mV nanovesicles (n = 6 each). In vivo MRI was performed to follow grafted cells in contralateral striata, and results were correlated with histology.

RESULTS

Optimal cell labelling conditions involved a concentration of 3.15 μg Fe/mL nanovesicles with 20-mV positive charge and 1-h incubation time. Labelling efficiency showed linear change with an increase in the electric potentials of nanovesicles. Labelling did not affect cell viability, proliferation activity or multilineage differentiation capacity. The distribution and migration of labelled cells could be detected by MRI. Histology confirmed that grafted cells retained the label and remained viable.

CONCLUSION

Stem cells can be effectively and safely labelled with cationic, SPION-loaded nanovesicles in a controllable way for cellular MRI.

KEY POINTS

• Stem cells can be effectively labelled with cationic, SPION-loaded nanovesicles. • Labelling did not affect cell viability, proliferation or differentiation. • Cellular uptake of SPION could be controlled using cationic nanovesicles. • Labelled cells could migrate along the corpus callosum towards cerebral infarction. • The grafted, labelled cells retained the label and remained viable.

Emerging therapeutic approaches for multipotent mesenchymal stromal cells

PURPOSE OF REVIEW

Multipotent mesenchymal stromal cells (MSCs) are rare cells resident in bone marrow and other organs capable of differentiating into mesodermal lineage tissues. MSCs possess immunomodulatory properties and have extensive capacity for ex-vivo expansion. Early clinical studies demonstrated safety and feasibility of infusing autologous MSCs and suggested a role in enhancing engraftment after hematopoietic cell transplant (HCT). Subsequent pilot studies using allogeneic MSCs showed safety but presented contradictory results regarding efficacy in treating graft-versus-host disease (GVHD).

RECENT FINDINGS

Larger, phase II allogeneic MSC infusion studies, including cells obtained from haploidentical and third-party donors, showed efficacy in GVHD treatment; however, recent randomized, placebo-controlled studies failed to corroborate these results. New investigations include MSC infusions in umbilical cord blood transplantation, MSC therapy for tissue regeneration/repair, harvest and use of MSCs from adipose tissue and cell-tracking/imaging studies using radionuclides, gene and fluorescent dye-labeled MSCs.

SUMMARY

MSCs remain the subject of intense investigation in HCT because of their differentiation potential and immunomodulatory properties. Whereas infusions of autologous, allogeneic and third-party donor MSCs are well tolerated, further research is needed to clarify the optimal methods for harvesting and expansion, optimal timing of administration and efficacy in the setting of HCT.

Transcriptional profiling of human mesenchymal stem cells transduced with reporter genes for imaging

Mesenchymal stem cells (MSCs) can differentiate into osteogenic, adipogenic, chondrogenic, myocardial, or neural lineages when exposed to specific stimuli, making them attractive for tissue repair and regeneration. We have used reporter gene-based imaging technology to track MSC transplantation or implantation in vivo. However, the effects of lentiviral transduction with the fluc-mrfp-ttk triple-fusion vector on the transcriptional profiles of MSCs remain unknown. In this study, gene expression differences between wild-type and transduced hMSCs were evaluated using an oligonucleotide human microarray. Significance Analysis of Microarray identified differential genes with high accuracy; RT-PCR validated the microarray results. Annotation analysis showed that transduced hMSCs upregulated cell differentiation and antiapoptosis genes while downregulating cell cycle, proliferation genes. Despite transcriptional changes associated with bone and cartilage remodeling, their random pattern indicates no systematic change of crucial genes that are associated with osteogenic, adipogenic, or chondrogenic differentiation. This correlates with the experimental results that lentiviral transduction did not cause the transduced MSCs to lose their basic stem cell identity as demonstrated by osteogenic, chondrogenic, and adipogenic differentiation assays with both transduced and wild-type MSCs, although a certain degree of alterations occurred. Histological analysis demonstrated osteogenic differentiation in MSC-loaded ceramic cubes in vivo. In conclusion, transduction of reporter genes into MSCs preserved the basic properties of stem cells while enabling noninvasive imaging in living animals to study the biodistribution and other biological activities of the cells.

Cellular MRI and its role in stem cell therapy

Hematopoietic, stromal and organ-specific stem cells are under evaluation for therapeutic efficacy in cell-based therapies of cardiac, neurological and other disorders. It is critically important to track the location of directly transplanted or infused cells that can serve as gene carrier/delivery vehicles for the treatment of disease processes and be able to noninvasively monitor the temporal and spatial homing of these cells to target tissues. Moreover, it is also necessary to determine their engraftment efficiency and functional capability following transplantation. There are various in vivo imaging modalities used to track the movement and incorporation of administered cells. Tagging stem cells with different contrast agents can make these cells probes for different imaging modalities. Recent reports have shown that stem cells labeled with iron oxides can be used as cellular MRI probes demonstrating the cell trafficking to target tissues. In this review, we will discuss the status and future prospect of stem cell tracking by cellular MRI for cell-based therapy.

Imaging Stem Cell Implant for Cellular-Based Therapies

Stem cell–based cellular therapy represents a promising outlook for regenerative medicine. Imaging techniques provide a means for noninvasive, repeated, and quantitative tracking of stem cell implant or transplant. From initial deposition to the survival, migration and differentiation of the transplant/implanted stem cells, imaging allows monitoring of the infused cells in the same live object over time. The current review briefly summarizes and compares existing imaging methods for cell labeling and imaging in animal models. Several studies performed by our group using different imaging techniques are described, with further discussion on the issues with these current imaging approaches and potential directions for future development in stem cell imaging.

Magnetically-labeled sensitized splenocytes to identify glioma by MRI: a preliminary study

This study investigated the feasibility of imaging the migration and incorporation of magnetically-labeled sensitized splenocytes in an experimental 9L glioma brain tumor model. Splenocytes collected from tumor-bearing (sensitized splenocytes) or control (nonsensitized splenocytes) host rats were analyzed to determine the population of different cells, labeled with ferumoxides-protamine sulfate (FePro) and injected intravenously to recipient rats (N=4, for each group) bearing intracranial 9L tumors. Day 3 postinjection of splenocytes multiecho T2*-weighted and three-dimensional (3D) gradient echo MRI were obtained using a 7 Tesla MR system. R2* (1/T2*) maps were created from the T2*-weighted images. Signal intensities (SIs) and R2* values in the tumors and contralateral brain were determined by hand drawn regions of interest (ROIs). Brain sections were stained for the evidence of administered cells. Both 3D and T2*-weighted MRI showed low signal intensity areas in and around the tumors in rats that received labeled sensitized splenocytes. Prussian blue (PB), CD45- and CD8-positive cells were present in areas at the corresponding sites of low signal intensities seen on MRI. Rats that received labeled nonsensitized splenocytes did not show low signal intensity areas or PB positive cells in or around the implanted tumors. In conclusion, the immunogenic reaction can be exploited to delineate recurrent glioma using MRI following systemically delivered magnetically labeled sensitized splenocytes or T-cells.

Long-circulating liposomes radiolabeled with [18F]fluorodipalmitin ([18F]FDP)

Synthesis of a radiolabeled diglyceride, 3-[(18)F]fluoro-1,2-dipalmitoylglycerol [[(18)F]fluorodipalmitin ([(18)F]FDP)], and its potential as a reagent for radiolabeling long-circulating liposomes were investigated. The incorporation of (18)F into the lipid molecule was accomplished by nucleophilic substitution of the p-toluenesulfonyl moiety with a decay-corrected yield of 43+/-10% (n=12). Radiolabeled, long-circulating polyethylene-glycol-coated liposomes were prepared using a mixture of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, cholesterol, 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethyleneglycol)-2000] ammonium salt (61:30:9) and [(18)F]FDP with a decay-corrected yield of 70+/-8% (n=4). PET imaging and biodistribution studies were performed with free [(18)F]FDP and liposome-incorporated [(18)F]FDP. Freely injected [(18)F]FDP had the highest uptake in the liver, spleen and lungs. Liposomal [(18)F]FDP remained in blood circulation at near-constant levels for at least 90 min, with a peak concentration near 2.5%ID/cc. Since [(18)F]FDP was incorporated into the phospholipid bilayer, it could potentially be used for radiolabeling a variety of lipid-based drug carriers.

Molecular imaging promotes progress in orthopedic research

Modern orthopedic research is directed towards the understanding of molecular mechanisms that determine development, maintenance and health of musculoskeletal tissues. In recent years, many genetic and proteomic discoveries have been made which necessitate investigation under physiological conditions in intact, living tissues. Molecular imaging can meet this demand and is, in fact, the only strategy currently available for noninvasive, quantitative, real-time biology studies in living subjects. In this review, techniques of molecular imaging are summarized, and applications to bone and joint biology are presented. The imaging modality most frequently used in the past was optical imaging, particularly bioluminescence and near-infrared fluorescence imaging. Alternate technologies including nuclear and magnetic resonance imaging were also employed. Orthopedic researchers have applied molecular imaging to murine models including transgenic mice to monitor gene expression, protein degradation, cell migration and cell death. Within the bone compartment, osteoblasts and their stem cells have been investigated, and the organic and mineral bone phases have been assessed. These studies addressed malignancy and injury as well as repair, including fracture healing and cell/gene therapy for skeletal defects. In the joints, molecular imaging has focused on the inflammatory and tissue destructive processes that cause arthritis. As described in this review, the feasibility of applying molecular imaging to numerous areas of orthopedic research has been demonstrated and will likely result in an increase in research dedicated to this powerful strategy. Molecular imaging holds great promise in the future for preclinical orthopedic research as well as next-generation clinical musculoskeletal diagnostics.