(341f) Trimodal Magnetic-Plasmonic Core-Shell Nanosensors for Spatial and Functional Monitoring of Cell Therapy Via Ultrasound, Photoacoustic, and Magnetic Resonance Imaging

Stem cells (SCs) have shown significant potential in the restoration of degenerative, damaged, or aging tissues through remodeling and repair. Promising preclinical outcomes of SC therapy (SCT) have been reported for spinal cord damage, osteoporosis, sarcopenia, and Parkinson’s disease, yet many limitations hinder the clinical translation of SCT. A major intrinsic safety roadblock for clinical SCT is the lack of techniques to monitor the status of transplanted cells in vivo – such as location, viability, differentiation, etc. Current methods to determine the fate of transplanted cells are invasive end-point analyses, such as biopsies and histological staining, which cause damage to the regenerating tissue and do not reflect the real-time dynamics of the microenvironment. To improve the safety and efficacy of SCT, there is a critical need to track transplanted SCs non-invasively and monitor their fate in vivo.

Magnetic resonance (MR) cell tracking has been employed in clinical SC trials to track the location of transplanted SCs. While MRI performs well for spatial monitoring, it does not provide functional information about cell status or allow intraprocedural guidance. Alternatively, Ultrasound-guided photoacoustic (US/PA) imaging is capable of functionally monitoring cell status longitudinally, but the modality suffers from deep tissue imaging limitations. However, combined US/PA/MR imaging offers complementary advantages of functional monitoring of cell status along with deep tissue tracking capabilities. In this project iron-gold core-shell nanoparticles (IGNPs) acting as trimodal US/PA/MR contrast agents were designed. IGNPs modified with stimuli-responsive peptides will hydrophobically aggregate when exposed to a target biomarker, resulting in both PA signal changes due to plasmon coupling and MR contrast changes due to magnetic dipole-dipole interactions. Using this nanosensor (termed SRIGs), we can perform spatial and functional cell tracking through combined US/PA/MR imaging and exploit multimodal advantages, such as intraprocedural and functional monitoring through US/PA coupled with deep tissue pre- and post-procedural monitoring through MR imaging.

To create uniform (200 nm) IGNPs, silica-coated iron oxide nanospheres were synthesized and decorated with gold seeds on the surface followed by subsequent gold shell growth in the presence of poly(vinylpyrrolidone) (A). The surface of IGNPs were modified with a triblock peptide consisting of a cell penetrating sequence, an enzyme-cleavable sequence, and a hydrophobic aggregation-inducing sequence to cause hydrophobic aggregation in response to a target intracellular enzyme. Successful synthesis, morphology, and aggregation of SRIGs were confirmed by transmission electron microscopy (TEM) (B). For MRI, SRIGs were imaged in a tissue-mimicking gelatin phantom and images were acquired using a multi-slice multi-echo pulse sequence (7T Bruker PharmaScan; 38 mm coil). The relaxation time (TR) was held constant at 2000 ms. The echo time (TE) started at 7 ms and was ramped up in 7 ms increments for 32 time steps. The average T2 relaxation time was calculated using the built-in software and parametric fit algorithm. Next, aggregation-responsive PA signal amplification was tested in a tube phantom using naïve and aggregated SRIG solutions (20 MHz, 800 nm, Vevo/LAZR, VisualSonics Inc.). Samples were 3D imaged over 10.21 mm at 0.152 mm intervals and reconstructed in the coronal view using maximum intensity projections (VevoLAB 5.7.0).

SRIGs generated high negative contrast under T2-weighted MRI. The T2-weighted relaxation time and the MR signal intensity both showed qualitative differences in unaggregated vs. aggregated SRIGs (C). The T2 relaxation upon aggregation showed similar trends to unaggregated SRIGs, but the average relaxation time decreased, which reflects higher negative contrast (D-E). US/PA imaging of the SRIG solutions showed strong PA contrast, and a clear qualitative signal difference upon aggregation, which was quantified to show a ~45% change in PA signal at 800 nm (F-G). These results demonstrate the potential of SRIG as a trimodal contrast agent and sensor. Future work will involve modification of IGNPs with different peptide sequences to target particular biomarkers of interest, study cell-nanosensor interactions, and perform in vivo cell monitoring through combined US/PA/MR imaging.

In summary, our pilot studies showcased a US/PA/MRI contrast agent as a nanosensor that can detect a variety of target biomarkers by exploiting hydrophobic aggregation-driven plasmonic coupling and magnetic dipole-dipole signal changes. Following this, the nanosensor will be modified with a triblock peptide responsive to SC apoptosis. The ability of our sensor to detect SC location and viability will be validated in a mouse model of varying ischemic tissue damage under US/PA/MR imaging. Our approach will platform in vivo US/PA/MR guided spatial and functional tracking of nanosensor augmented MSC status and lead to optimized SC transplantation for safer and improved SCT for degenerative diseases through real-time therapeutic planning.