Complementary early-phase magnetic particle imaging and late-phase positron emission tomography reporter imaging of mesenchymal stem cells in vivo.

Stem cell-based therapies have demonstrated significant potential in clinical applications for many debilitating diseases. The ability to non-invasively and dynamically track the location and viability of stem cells post administration could provide important information on individual patient response and/or side effects. Multi-modal cell tracking provides complementary information that can offset the limitations of a single imaging modality to yield a more comprehensive picture of cell fate. In this study, mesenchymal stem cells (MSCs) were engineered to express human sodium iodide symporter (NIS), a clinically relevant positron emission tomography (PET) reporter gene, as well as labeled with superparamagnetic iron oxide nanoparticles (SPIOs) to allow for detection with magnetic particle imaging (MPI). MSCs were additionally engineered with a preclinical bioluminescence imaging (BLI) reporter gene for comparison of BLI cell viability data to both MPI and PET data over time. MSCs were implanted into the hind limbs of immunocompromised mice and imaging with MPI, BLI and PET was performed over a 30-day period. MPI showed sensitive detection that steadily declined over the 30-day period, while BLI showed initial decreases followed by later rapid increases in signal. The PET signal of MSCs was significantly higher than the background at later timepoints. Early-phase imaging (day 0-9 post MSC injections) showed correlation between MPI and BLI data (R2 = 0.671), while PET and BLI showed strong correlation for late-phase (day 10-30 post MSC injections) imaging timepoints (R2 = 0.9817). We report the first use of combined MPI and PET for cell tracking and show the complementary benefits of MPI for sensitive detection of MSCs early after implantation and PET for longer-term measurements of cell viability.

Dual Magnetic Particle Imaging and Akaluc Bioluminescence Imaging for Tracking Cancer Cell Metastasis.

Magnetic particle imaging (MPI) provides hotspot tracking and direct quantification of superparamagnetic iron oxide nanoparticle (SPIO)-labelled cells. Bioluminescence imaging (BLI) with the luciferase reporter gene Akaluc can provide complementary information on cell viability. Thus, we explored combining these technologies to provide a more holistic view of cancer cell fate in mice. Akaluc-expressing 4T1Br5 cells were labelled with the SPIO Synomag-D and injected into the mammary fat pads (MFP) of four nude mice. BLI was performed on days 0, 6 and 13, and MPI was performed on days 1, 8 and 14. Ex vivo histology and fluorescence microscopy of MFP and a potential metastatic site was conducted. The BLI signal in the MFP increased significantly from day 0 to day 13 (p < 0.05), mirroring tumor growth. The MPI signal significantly decreased from day 1 to day 14 (p < 0.05) due to SPIO dilution in proliferating cells. Both modalities detected secondary metastases; however, they were visualized in different anatomical regions. Akaluc BLI complemented MPI cell tracking, allowing for longitudinal measures of cell viability and sensitive detection of distant metastases at different locations. We predict this multimodal imaging approach will help to evaluate novel therapeutics and give a better understanding of metastatic mechanisms.

Cellular MRI Reveals Altered Brain Arrest of Genetically Engineered Metastatic Breast Cancer Cells.

Purpose: The combined use of anatomical magnetic resonance imaging (MRI), cellular MRI, and bioluminescence imaging (BLI) allows for sensitive and improved monitoring of brain metastasis in preclinical cancer models. By using these complementary technologies, we can acquire measurements of viable single cell arrest in the brain after systemic administration, the clearance and/or retention of these cells thereafter, the growth into overt tumours, and quantification of tumour volume and relative cancer cell viability over time. While BLI is very useful in measuring cell viability, some considerations have been reported using cells engineered with luciferase such as increased tumour volume variation, changes in pattern of metastatic disease, and inhibition of in vivo tumour growth. Procedures: Here, we apply cellular and anatomical MRI to evaluate in vivo growth differences between iron oxide labeled naïve (4T1BR5) and luciferase-expressing (4T1BR5-FLuc-GFP) murine brain-seeking breast cancer cells. Balb/C mice received an intracardiac injection of 20,000 cells and were imaged with MRI on days 0 and 14. Mice that received 4T1BR5-FLuc-GFP cells were also imaged with BLI on days 0 and 14. Results: The number of signal voids in the brain (representing iron-labeled cancer cells) on day 0 was significantly higher in mice receiving 4T1BR5 cells compared to mice receiving 4T1BR5-FLuc-GFP cells (p < 0.0001). Mice that received 4T1BR5 cells also had significantly higher total brain tumour burden and number of brain metastases than mice that received 4T1BR5-FLuc-GFP cells (p < 0.0001). Conclusions: By employing highly sensitive cellular MRI tools, we demonstrate that engineered cells did not form tumours as well as their naïve counterparts, which appear to primarily be due to a reduction in cell arrest. These results indicate that engineering cancer cells with reporter genes may alter their tropism towards particular organs and highlight another important consideration for research groups that use reporter gene imaging to track metastatic cancer cell fate in vivo.

A multimodality imaging model to track viable breast cancer cells from single arrest to metastasis in the mouse brain.

Cellular MRI involves sensitive visualization of iron-labeled cells in vivo but cannot differentiate between dead and viable cells. Bioluminescence imaging (BLI) measures cellular viability, and thus we explored combining these tools to provide a more holistic view of metastatic cancer cell fate in mice. Human breast carcinoma cells stably expressing Firefly luciferase were loaded with iron particles, injected into the left ventricle, and BLI and MRI were performed on days 0, 8, 21 and 28. The number of brain MR signal voids (i.e., iron-loaded cells) on day 0 significantly correlated with BLI signal. Both BLI and MRI signals decreased from day 0 to day 8, indicating a loss of viable cells rather than a loss of iron label. Total brain MR tumour volume on day 28 also correlated with BLI signal. Overall, BLI complemented our sensitive cellular MRI technologies well, allowing us for the first time to screen animals for successful injections, and, in addition to MR measures of cell arrest and tumor burden, provided longitudinal measures of cancer cell viability in individual animals. We predict this novel multimodality molecular imaging framework will be useful for evaluating the efficacy of emerging anti-cancer drugs at different stages of the metastatic cascade.

Cellular magnetic resonance imaging of monocyte-derived dendritic cell migration from healthy donors and cancer patients as assessed in a scid mouse model.

BACKGROUND AIMS. The use of dendritic cells (DC) as an adjuvant in cell-based immunotherapeutic cancer vaccines is a growing field of interest. A reliable and non-invasive method to track the fate of autologous DC following their administration to patients is required in order to confirm that clinically sufficient numbers are reaching the lymph node (LN). We demonstrate that an immunocompromised mouse model can be used to conduct translational studies employing cellular magnetic resonance imaging (MRI). Such studies can provide clinically relevant information regarding the migration potential of clinical-grade DC used in cancer immunotherapies. METHODS. Human monocyte-derived dendritic cells (mo-DC) were generated from negatively selected monocytes obtained from either healthy donors or cancer patients. DC were labeled with superparamagnetic iron oxide (SPIO) nanoparticles in order to track them in vivo in a CB17scid mouse model using cellular MRI. SPIO did not have any adverse effects on DC phenotype or function, independent of donor type. Cellular MRI readily detected migration of SPIO-loaded DC in CB17scid mice. No differences in migration were observed between DC obtained from healthy donors and those obtained from donors undergoing autologous stem cell transplant for cancer therapy. CONCLUSIONS. Cellular MRI provided semi-quantitative image data that corresponded with data obtained by digital morphometry, validating cellular MRI's potential to assess DC migration in DC-based cancer immunotherapy clinical trials.

Imaging iron-loaded mouse glioma tumors with bSSFP at 3 T.

The poor prognosis for patients with high-grade glioma is partly due to the invasion of tumor cells into surrounding brain tissue. The goal of the present work was to develop a mouse model of glioma that included the potential to track cell invasion using MRI by labeling GL261 cells with iron oxide contrast agents prior to intracranial injection. Two types of agents were compared with several labeling schemes to balance between labeling with sufficient iron to curb the dilution effect of cell division while avoiding overwhelming signal loss that could prevent adequate visualization of tumor boundaries. The balanced steady-state free precession (bSSFP) pulse sequence was evaluated for its suitability for imaging glioma tumors and compared to T(2)-weighted two-dimensional fast spin echo (FSE) and T(1)-weighted spoiled gradient recalled echo (SPGR) at 3 T in terms of signal-to-noise ratio and contrast-to-noise ratio efficiencies. Ultimately, a three-dimensional bSSFP protocol consisting of a set of two images with complementary contrasts was developed, allowing excellent tumor visualization with minimal iron contrast when using pulse repetition time = 6 ms and alpha = 40 degrees, and extremely high sensitivity to iron when using pulse repetition time = 22 ms and alpha = 20 degrees. Quantitative histologic analysis validated that the strong signal loss seen in balanced steady state free precession pulse sequence images of iron-loaded tumors correlated well with the presence of iron.

Semiquantitation of mouse dendritic cell migration in vivo using cellular MRI.

Despite recent therapeutic advances, including the introduction of novel cytostatic drugs and therapeutic antibodies, many cancer patients will experience recurrent or metastatic disease. Current treatment options, particularly for those patients with metastatic breast, prostate, or skin cancers, are complex and have limited curative potential. Recent clinical trials, however, have shown that cell-based therapeutic vaccines may be used to generate broad-based, antitumor immune responses. Dendritic cells (DC) have proved to be the most efficacious cellular component for therapeutic vaccines, serving as both the adjuvant and antigen delivery vehicle. At present it is not possible to noninvasively determine the fate of DC-based vaccines after their administration to human subjects. In this study, we demonstrate that in vitro-generated mouse DC can be readily labeled with superparamagnetic iron oxide nanoparticles, Feridex, without altering cell morphology, or their phenotypic and functional maturation. Feridex-labeling enables the detection of DC in vivo after their migration to draining lymph nodes using a 1.5 T clinical magnetic resonance scanner. In addition, we report a semiquantitative approach for analysis of magnetic resonance images and show that the Feridex-induced signal void volume, and fractional signal loss, correlates with the delivery and migration of small numbers of in vitro-generated DC. These findings, together with ongoing preclinical studies, are key to gaining information critical for improving the efficacy of therapeutic vaccines for the treatment cancer, and potentially, chronic infectious diseases.

High-resolution MRI and micro-CT in an ex vivo rabbit anterior cruciate ligament transection model of osteoarthritis.

OBJECTIVE: The aim of this study was to investigate the potential of using non-invasive, multi-modality imaging techniques to quantify disease progression in a rabbit model of experimentally induced osteoarthritis (OA). METHODS: High-resolution 4-T magnetic resonance imaging (MRI) and micro-computed tomography (micro-CT) techniques were implemented and validated in an ex vivo rabbit anterior cruciate ligament transection (ACLT) model of OA. A three-dimensional (3-D) rigid body registration technique was executed and evaluated to allow combined MR-CT analysis in co-registered image volumes of the knee. RESULTS: The 3-D MRI and micro-CT data formats made it possible to quantify cartilage damage, joint-space, and osseous changes in the rabbit ACLT model of OA. Spoiled gradient-recalled echo and fast-spin echo (FSE) sequences were jointly used to evaluate femorotibial cartilage and determine the sensitivity (78.3%) and specificity (95.3%) of 4-T MRI to detect clinically significant cartilage lesions. Overall precision error of the micro-CT technique for analysis of joint-space, volumetric bone mineral density (vBMD), and bone volume fraction (BV/TV) was 1.8%, 1.2%, and 2.0%, respectively. Co-registration of the 3-D data sets was achieved to within 0.36 mm for completed intermodality registrations, 0.22 mm for extrapolated intramodality registrations, and 0.50mm for extrapolated intermodality registrations. CONCLUSIONS: These results indicate that high-resolution 4-T MRI and micro-CT can be used to accurately quantify cartilage damage and calcified tissue changes in the rabbit ACLT model of OA. In addition, image volumes can be successfully co-registered to facilitate a comprehensive multi-modality examination of localized changes in both soft tissue and bone within the rabbit femorotibial joint.