(415f) Photoacoustic Image-Guided Magnetic Delivery and Retention of Dual Nanoengineered Stem Cells

Over the past two decades, stem cell therapy (SCT) has garnered significant attention as a promising strategy for tissue repair/regeneration and functional restoration. Despite encouraging outcomes in numerous preclinical trials for treating spinal cord injuries, osteoporosis, and cardiac pathologies, translating SCT to clinical settings remains hampered by significant challenges. A major limitation lies in the poor delivery efficiency and inadequate long-term retention of stem cells at the target site. Minimally invasive delivery routes, such as intravenous (IV) administration, are preferred but often result in suboptimal engraftment and delocalization of the transplanted cells from the injury site over time. Additionally, locally administered stem cells exhibited poor long-term retention at injury sites. Regardless of cell type or delivery route, acute retention of less than 10% is generally reported. Therefore, novel approaches are required to deliver transplanted stem cells to their intended target regions and facilitate their sustained engagement with the damaged tissue.

Magnetic targeting is a common technique used to address the challenge of poor stem cell retention. This technique is based on manufacturing of magnetically-targeted stem cells followed by in vivo targeting with the aid of magnetic fields. While other researchers have employed magnetic guidance strategies with encouraging outcomes, their evaluation has primarily relied on end-point analysis methods, such as histology, which fail to reflect the real-time dynamics of the tissue region. Magnetic resonance imaging (MRI) has been a commonly explored modality for tracking transplanted cells, but it does not allow for image-guided delivery or real-time monitoring of their spatial distribution. Additionally, the strong magnetic fields inherent to MRI can interfere with maintaining magnetically-targeted stem cells at the treatment site. Hence, there is a critical need for a non-invasive imaging approach that allows for real-time monitoring of magnetically-guided stem cell delivery and distribution within the target tissue region, without disrupting the magnetic targeting process itself.

Our approach decouples the targeting mechanism from the imaging modality by employing an external magnetic field to guide magnetically-labeled stem cells and photoacoustic imaging to monitor the delivery/distribution. Silica-coated iron oxide nanospheres (IONPs) facilitate magnetic cell retention and delivery, while silica-coated gold nanorods (AuNRs) enable ultrasound/photoacoustic (US/PA) image-guidance and monitoring (A). US imaging provides excellent anatomical images of tissue with high spatial resolution, while complementary PA imaging can convey functional information with high contrast, resolution, imaging depth, and sensitivity. Successful synthesis and morphology of the dual nanoparticles were confirmed through transmission electron microscopy (TEM), revealing well-defined nanoparticles with distinct silica shells (B). UV-vis-NIR spectroscopy and dynamic light scattering were employed to assess optical absorbance and surface charge. Photostability of AuNRs was evaluated under pulsed laser irradiation, while magnetic retention capability of IONPs under simulated venous flow conditions was assessed.

Following synthesis and characterization, human adipose tissue-derived mesenchymal stem cells (MSCs) were labeled with the dual-nanoparticle system. In vitro photoacoustic (PA) imaging (20 MHz, 680–970 nm, Vevo/LAZR, Visual Sonics Inc.) using a tissue-mimicking phantom was used to characterize the PA signals from dual-labeled MSCs. The photoacoustic spectrum of labeled MSCs exhibited spectral characteristics consistent with AuNRs, and US/PA imaging of cells at decreasing concentrations revealed a detection limit of 10-20 cells/μL. (C, E). Based on cytotoxicity studies and demonstrated US/PA sensitivity, the optimal labeling concentration was determined. Magnetic homing capability was demonstrated using a cell-laden tissue-mimicking phantom and an external magnet (1 cm distance, 5 min exposure), resulting in a 2.4-fold increase in PA signal intensity on the side nearest the magnetic field, indicating enhanced cell trafficking and localization (D, F). The control condition showed no signal difference, confirming the specificity of magnetic homing facilitated by the dual-nanoparticle system. (D, F).

To demonstrate photoacoustic image-guided stem cell delivery in vivo, mice received intravenous injections of dual-labeled MSCs via the tail vein (100 μL, 500 cells/μL) for US/PA imaging (40 MHz, 680–970 nm, Vevo/LAZR, Visual Sonics Inc.). A magnet was placed over the femoral artery to demonstrate magnetic trapping at the hind limb. Multiwavelength photoacoustic datasets were acquired before injection, immediately following 30 minutes of exposure to visualize magnetic trapping of dual-labeled MSCs, and 30 minutes after removing the magnet to visualize dual-labeled MSC clearance. Control groups consist of mice that received dual-labeled MSCs with no magnet, and mice that received no cell injection. In the magnetic retention group, a higher concentration of labeled stem cells localized in the hindlimb region, demonstrating successful magnetic targeting and delivery (G). In contrast, control mice exhibited dispersed stem cells in vasculature with no preferential localization (G).

Our findings demonstrate the effective magnetic guidance and retention of dual-labeled MSCs to target regions, facilitated by an external magnetic field and monitored non-invasively in real-time through photoacoustic imaging. Unlike previous approaches that either perform magnetic delivery without visualization or real-time guidance or employ MRI-assisted visualization without strategies for guidance or retention, our multifaceted approach synergistically permits for photoacoustic-guided magnetic delivery of stem cells allowing us to localize cells at site-specific regions within the body. The imaging feedback mechanism enables dynamic adjustments to the administration injection location, optimizing stem cell delivery efficacy. This innovative approach promises to advance clinical stem cell therapy by enhancing cell retention and enabling precise, noninvasive tracking within the body.