Carboxylic mannan-coated iron oxide nanoparticles targeted to immune cells for lymph node-specific MRI in vivo

Carboxylic mannan (CM)-coated super paramagnetic iron oxide nanoparticles (CM-SPIONs) were prepared to target antigen-presenting cells (APCs), including macrophages, by the specific interaction between the mannose ligand tethered on CM-SPION and mannose receptors on APCs. Carboxylic mannan was synthesized by introducing the aldehyde group to mannan by oxidation, followed by the conversion of aldehyde groups to carboxyl groups. CM-SPION exhibited uniform-sized nanoparticles with a highly negative surface charge appropriate for longer blood circulation. It was demonstrated that CM-SPION could target macrophages bearing mannose receptors more specifically than polyvinyl alcohol (PVA) or dextran-coated SPION. The in vitro and in vivo toxicities of CM-SPION were evaluated, and the results showed that the LD50 of CM-SPION was much higher than that of mannan-SPION (80 mg Fe/kg vs. 44 mg Fe/kg in mice, respectively). The uptake of CM-SPION by peritoneal macrophages was also confirmed with Prussian blue staining and magnetic resonance (MR) phantom tube imaging. In the in vitro uptake study visualized by MR phantom tube imaging, the intracellular uptake of CM-SPION was much faster than those of dextran-coated SPION (Dex-SPION) and PVA-coated SPION (PVA-SPION) at the initial hours of incubation, and increased drastically up to 24 h post-incubation. The in vivo uptake of CM-SPION in lymph nodes (LNs) was tracked by MR imaging (MRI) after subcutaneous injection in a rat model. It was found that the injected CM-SPION predominantly accumulated in the popliteal LN, and the in vivo accumulation rate with CM-SPION in the LN was comparable to that of Dex-SPION, the positive control, as measured by a signal drop in MR intensity. Histological analysis with Prussian blue staining also confirmed the accumulation of SPION in the LN.