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Imaging is crucial for stem cell therapy to monitor the location(s), numbers, and state of the implanted cells. Real-time imaging in particular can ensure proper cell delivery for best engraftment. However, established imaging tools such as MRI are limited by their temporal resolution for guidance during delivery. In contrast, photoacoustic imaging is ideally suited for real time, image-guided therapy. Here, we use silica-coated gold nanorods as photoacoustic contrast agents and deploy them to image and quantitate mesenchymal stem cells during implant into the muscle tissue of live mice. Silica-coated gold nanorods (SiGNRs) were created with standard methods and loaded into mesenchymal stem cells (MSCs) without transfection agents. There was no significant (p<0.05) toxicity or changes to cell proliferation after incubating MSCs with 0.05 nM SiGNRs for 3 hours. A panel of cytokines should only minor upregulation of inflammatory markers including interleukin-6. We used electron microscopy to illustrate vacuole-bound SiGNRs inside the cells. This cell staining increased photoacoustic signal 175% relative to MSCs without contrast agent—the silica coat itself increased signal 55% relative to uncoated GNRs. Using inductively coupled plasma spectroscopy, we found that there were 100,000 SiGNRs per MSC. This value was 5-fold higher than a MSC population stained with GNRs in the absence of silica coat. After labeling, cells were washed and injected into murine muscle tissue to simulate a muscular dystrophy patient. Mice (N=5) treated with these SiGNRlabeled MSCs exhibited no adverse events and implants up to 5 mm deep were easily visualized. The in vivo detection limit was 90,000 cells in a 100 uL bolus in mouse thigh muscle. Here, the B-mode signal is useful for orienting the treatment area and visualizing the delivery catheter while the photoacoustic mode offers cell-specific content. The photoacoustic signal was validated with histology a long-term fluorescent tracking dye after MSC transplant.
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