(728c) Translating Stem Cells-Molecular Imaging of Stem Cell Transplantation In Porcine Myocardium Using Clinical MRI and PET-CT

Imaging cells after cardiac cellular therapy could be a promising approach to optimizing this new treatment strategy. Marrying the strengths of cardiovascular MRI (high spatial resolution) and PET (high sensitivity) enhances the ability to assess the cell location, viability, survival, and myocardial response to treatment. To build on our previous swine studies with adenovirally transduced cells, rat mesenchymal stem cells (rMSC)  were stably transduced with a second generation, self-inactivating lentivirus carrying a ubiquitin-driven triple fusion (TF) reporter gene (RG), comprised of green fluorescent protein (GFP), firefly luciferase (Fluc) and a mutant truncated herpes simplex virus type 1 thymidine kinase (HSV1-tsr39tk). In vitro studies indicated that sorting lentivirally transduced cells enhanced levels of TFRG levels, by sorting transduced cells that were 10² to 10³ above background fluorescence. Cells were injected in vivo and demonstrated a linear relationship between when implanted subcutaneously between average radiance (photons/s/cm² /sr) and cell number ( R²= 0.9682). The highest expressing cells demonstrated a 3.1 fold increase in average radiance (photons/s/cm² /sr) from the initial cells after sorting three times and demonstrated a semi-logarithmic relationship, as expected (R²= 0.9864). The PET reporter gene activity of MSC-TF was assessed of ³H-penciclovir. At 3 hours activity was significantly  higher than control C6 Rat glioma cells bearing the HSV-sr39ttk  gene.  For in vivo cell imaging studies, either all the MSC-TF cells, or 1/5 fraction of the total cells, were labeled with 30 µg/ml Iron bearing SPIO particles (35nm) using a protamine transfection technique, and injected into the hearts of swine after thoracotomy (n=4 swine). MRI was performed using the Fiesta or FGRE sequences allowed spatiotemporal distinction between cells and control particles, and allowed detection of cells after injection of 150-350 x10⁶ cells. Next, clinical PET-CT was performed dynamically up to five hours after injection of F18-FHBG (14 mCi), and nine hours after injection of the rMSC-TF.. A linear relationship was determined between the number of cells injected and the SUV (standard uptake value) and signal to background ratio compared to cell number injected  (n=3 swine).  After 5 hours post injection, Mean SUV (Standard uptake value) and Signal/Background ratio was 0.063 and 2.11 for 150 x 10⁶, 0.112 and 2.02 for 200 x 10⁶, 0.189 and 2.34 for 250 x 10⁶, and 0.227 and 2.82 for 300 x 10⁶. Our results indicate that MRI and PET can be married together to localize injected cells for translational cell imaging. Future work will aim to use swine TFRG-MSC in survival models of swine infarction. Studies of PET reporter genes in large animals should help translate stem cells to the clinic and establish the role for molecular imaging in monitoring stem cell therapies.