111In oxine labelled mesenchymal stem cell SPECT after intravenous administration in myocardial infarction

Determining the minimum number of detectable cardiac-transplanted 111In-tropolone-labelled bone-marrow-derived mesenchymal stem cells by SPECT

In this work, we determined the minimum number of detectable 111In-tropolone-labelled bone-marrow-derived stem cells from the maximum activity per cell which did not affect viability, proliferation and differentiation, and the minimum detectable activity (MDA) of 111In by SPECT. Canine bone marrow mesenchymal cells were isolated, cultured and expanded. A number of samples, each containing 5x10(6) cells, were labelled with 111In-tropolone from 0.1 to 18 MBq, and cell viability was measured afterwards for each sample for 2 weeks. To determine the MDA, the anthropomorphic torso phantom (DataSpectrum Corporation, Hillsborough, NC) was used. A point source of 202 kBq 111In was placed on the surface of the heart compartment, and the phantom and all compartments were then filled with water. Three 111In SPECT scans (duration: 16, 32 and 64 min; parameters: 128x128 matrix with 128 projections over 360 degrees) were acquired every three days until the 111In radioactivity decayed to undetectable quantities. 111In SPECT images were reconstructed using OSEM with and without background, scatter or attenuation corrections. Contrast-to-noise ratio (CNR) in the reconstructed image was calculated, and MDA was set equal to the 111In activity corresponding to a CNR of 4. The cells had 100% viability when incubated with no more than 0.9 MBq of 111In (80% labelling efficiency), which corresponded to 0.14 Bq per cell. Background correction improved the detection limits for 111In-tropolone-labelled cells. The MDAs for 16, 32 and 64 min scans with background correction were observed to be 1.4 kBq, 700 Bq and 400 Bq, which implies that, in the case where the location of the transplantation is known and fixed, as few as 10,000, 5000 and 2900 cells respectively can be detected.

Dynamic imaging of allogeneic mesenchymal stem cells trafficking to myocardial infarction

BACKGROUND

Recent results from animal studies suggest that stem cells may be able to home to sites of myocardial injury to assist in tissue regeneration. However, the histological interpretation of postmortem tissue, on which many of these studies are based, has recently been widely debated.

METHODS AND RESULTS

With the use of the high sensitivity of a combined single-photon emission CT (SPECT)/CT scanner, the in vivo trafficking of allogeneic mesenchymal stem cells (MSCs) colabeled with a radiotracer and MR contrast agent to acute myocardial infarction was dynamically determined. Redistribution of the labeled MSCs after intravenous injection from initial localization in the lungs to nontarget organs such as the liver, kidney, and spleen was observed within 24 to 48 hours after injection. Focal and diffuse uptake of MSCs in the infarcted myocardium was already visible in SPECT/CT images in the first 24 hours after injection and persisted until 7 days after injection and was validated by tissue counts of radioactivity. In contrast, MRI was unable to demonstrate targeted cardiac localization of MSCs in part because of the lower sensitivity of MRI.

CONCLUSIONS

Noninvasive radionuclide imaging is well suited to dynamically track the biodistribution and trafficking of mesenchymal stem cells to both target and nontarget organs.

Cellular MR Imaging

Cellular MR imaging is a young field that aims to visualize targeted cells in living organisms. In order to provide a different signal intensity of the targeted cell, they are either labeled with MR contrast agents in vivo or prelabeled in vitro. Either (ultrasmall) superparamagnetic iron oxide [(U)SPIO] particles or (polymeric) paramagnetic chelates can be used for this purpose. For in vivo cellular labeling, Gd3+- and Mn2+- chelates have mainly been used for targeted hepatobiliary imaging, and (U)SPIO-based cellular imaging has been focused on imaging of macrophage activity. Several of these magneto-pharmaceuticals have been FDA-approved or are in late-phase clinical trials. As for prelabeling of cells in vitro, a challenge has been to induce a sufficient uptake of contrast agents into nonphagocytic cells, without affecting normal cellular function. It appears that this issue has now largely been resolved, leading to an active research on monitoring the cellular biodistribution in vivo following transplantation or transfusion of these cells, including cell migration and trafficking. New applications of cellular MR imaging will be directed, for instance, towards our understanding of hematopoietic (immune) cell trafficking and of novel guided (stem) cell-based therapies aimed to be translated to the clinic in the future.

In vivo tracking of stem cells for clinical trials in cardiovascular disease

Various stem cells hold promise for the treatment of human cardiovascular disease. Regardless of stem cell origin, future clinical trials will require that the location and number of such cells be tracked in vivo, over long periods of time. The problem of tracking small numbers of cells in the body is a difficult one, and an optimal solution does not yet exist. We review the many contrast agents and detectors that have been proposed for stem cell tracking during clinical trials, define the characteristics of an ideal imaging technology, and suggest future directions for research.