196

Development and in vitro evaluation of Silica-based iron oxide particles as intracellular contrast agent for cellular imaging with MRI

Carolin Langer¹, Nathanael Raschzok¹, Peter Behringer¹, Nils Billecke¹, Susanne Rohn¹, Kerstin Nehls¹, Christian Schmidt², Lars Stelter³, Ulf Teichgräber^3,4, Igor M. Sauer¹

¹General, Visceral, and Transplant Surgery, Charité Universitätsmedizin Berlin, Berlin; ²Microparticles GmbH, Berlin; ³Department of Radiology, Charité Universitätsmedizin Berlin, Berlin; ⁴Department of Radiology, Universitätsklinikum Jena, Jena, Germany

Objectives: Magnet Resonance Imaging (MRI) can be used for non-invasive monitoring during and following cell transplantation. Cellular imaging with MRI requires labeling of cells with intracellular contrast agents. Micrometer-sized iron oxide particles have a stronger effect on the magnetic field compared to nanometer-sized particles, but clinical applicable micron-sized particles not available. The aim of this study was to develop a new class of micron-sized particles for cellular imaging based on clinical applicable materials.

Methods: We developed silica-based particles (1,18µm) with different surface modifications: positively charged Poly-L-Lysin, neutral Streptavidin, and negatively COOH-groups. Huh7-cells, primary rat hepatocytes, and primary human hepatocytes were used for in vitro evaluation. Uptake mechanisms were investigated by inhibiting endocytosis using hypothermia (4°C.) or culture medium supplemented with NaN₃. Phantom studies were performed using 3.0 MRI and a clinical whole body coil. Transaminase leakage was measured to investigate possible adverse effects of particles on labeled cells.

Results: Labeling with Poly-L-Lysin particles produced iron load of approximately 35pg iron/cell. COOH-coupled particles lead to about 22pg iron/cell and Streptavidin-particles achieved 6pg iron/cell. Treatment with endocytosis inhibitors caused no significant differences in particle uptake, indicating passive particle uptake. Cells labeled with Poly-L-Lysin-particles were detectable from a cluster of at least 5000 cells, whereas detection threshold for COOH- and Streptavidin-labeled cells were 10.000 and 25.000 cells, respectively. Effects on transaminase leakage were similarly low for the three particles.

Conclusions: We showed that the new Silica-based particles enabled in vitro detection of cell cluster under conditions of clinical MRI without negative effects on labeled cells. The different surface modifications offer the opportunity of creation of multifunctional theranostic agents enabling diagnostic imaging, drug delivery and therapeutic monitoring.