19F MRI has favorable NMR properties in high sensitivity, and no endogenous background signal in animal bodies. Therefore, 19F MRI probes are attracting in vivo imaging tools for tracking specific cells and imaging enzyme activities in deep tissues. Several types of a perfluorocarbon (PFC)-encapsulated nanoparticle have developed. Silica nanoparticle-based nanoprobes enabled sensitive detection of 19F MRI signals in vivo. However, 19F MRI signal of silica nanoparticles remain in liver and spleen for a long time after intravenous injection in mice. Soft nanomaterials such as nanogels are attractive materials for DDS because they have elasticity and high penetration through deformation. We hypothesized soft polymer-coated nanoparticles can penetrate the tissue and undergo more rapid clearance from the liver, compared to silica nanoparticles. In this research, PFC-encapsulated polymer nanoparticles as a 19F MRI contrast agent were developed in order to suppress long-term accumulation in the body. This nanoparticle was covered with polymer layer instead of silica to enhance the elasticity. The polymer-coated nanoparticles were prepared by RAFT (Reversible Addition Fragmentation chain Transfer) polymerization on the surface of PFC-encapsulated micelles. RAFT polymerization is one of living radical polymerization and proceeds under various conditions such as in water or on lipid membrane. Accumulated amounts in mouse liver between polymer and silica nanoparticles were compared by multicolor 19F MRI. PFCE-encapsulated polymer nanoparticle and TPFBME-encapsulated silica nanoparticle were prepared and administrated into a mouse simultaneously. The 19F MR signal from both nanoparticles was observed from the liver after injection of nanoparticles. In contrast to the fact that the 19F MRI signal intensity from silica nanoparticles did not change over three weeks, the intensity of polymer nanoparticles decreased 20% within seven days and 8% in 2 weeks. The nanoparticle had desirable size distribution and high elasticity. The 19F MRI signal intensity of polymer nanoparticles declined after intravenous injection. It was concluded that the polymer nanoparticles underwent more rapid clearance than the silica nanoparticles, indicating the suppression of the long-term accumulation in the liver.
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