# Molecular Imaging with \({}^{68}\)Ga Radio-Nanomaterials: Shedding Light on Nanoparticles
## 4 Biomedical Applications of \({}^{68}\)Ga-Labeled Nanoparticles
### Iron Oxide Nanoparticles (IONPs)
[101] radiolabeled PEG-coated iron oxide nanoparticles with \({}^{68}\)Ga using a chelator-free strategy previously described by [PERSON] et al. [70], incubating IONPs with GaCl\({}_{3}\) eluate for 30 min at room temperature. They obtained a 66.2% post labeling chemical purity that improved to 98.9% after magnetic separation nanoparticle purification (using a MACS column). In vivo PET imaging in mice revealed fast blood clearance and major liver and spleen nanoparticle accumulation. This biodistribution profile is typical for numerous nanoparticulate-based tracers, as they are mainly eliminated by the reticuloendothelial (RES) organs. RES is part of the immune system of the body, being a network of cells and tissues, especially in the blood, general connective tissue, spleen, liver, lungs, bone marrow, and lymph nodes. These varied cell types which acted in host defense by phagocytosis of foreign invaders such as microbes are grouped collectively into the RES. Nanoradiotracers smaller than \(\sim\)5 nm will be able to leak through renal  
Figure 4: In vivo MPI/CT image of a human breast tumor in a murine model, using SPIO nanoparticles specifically designed for MPI [94]. SPIO nanoparticles can be observed to accumulate in tumor having a contrast peak at 24 h post injection. Adapted from [PERSON] et al. [78]. glomeruli and excreted via urinary system [102]. However, if nanoradiotracer has a larger size, they will interact with immune blood components, as for instance opsonins. This eases nanoradiotracer recognition by Kupffer cells, which are a macrophage type resident in the liver. These will capture nanoradiotracer via endocytosis and process nanoparticles in their lysosomes, in the case of iron oxide nanoparticles, to release free iron that will become part of the physiological iron reservoirs in the form of ferritins and hemosiderins [103; 104]. These will have no impact in homeostasis when nanoradiotracer is administered at suitable concentrations. The spleen is also a host organ for macrophages that phagocyte the nanoradiotracer. This explains biodistribution trend followed by a great number of untargeted nanoradiotracers. [PERSON] et al. [98] entrapped maghemite nanoparticles in a polymeric matrix composed by PLGA and PEG and radiolabeled them with \({}^{68}\)Ga using NODAGA as a chelator. NODAGA was incorporated to the surface of the nanoparticles via peptide bond formation with an amine coming from NODAGA-ethylenediamine and a polyCOOH-containing organic shell on nanoparticle surface. Radiolabeling yield obtained with \({}^{68}\)Ga was 84%. In vivo PET imaging and time-activity curves for different organs pointed towards a slow blood clearance pattern and high accumulation in liver and spleen. They also observed significant accumulation in the lungs, suggesting nanoparticle clustering or aggregation. [PERSON] et al. [105] obtained a dual PET/MRI nanoradiotracer composed by iron oxide nanoparticles conjugated to DPD, a tetradentate ligand (with two phosphonates and two carboxylate groups) that apart from acting as surfactant, serves as \({}^{68}\)Ga chelator. They obtained a 70% radiolabeling yield and a >91% chemical purity. In vivo PET imaging and ex vivo biodistribution studies in Swiss mice showed high accumulation in liver and spleen. [PERSON] et al. [106] described a new class of silica-coated iron oxide nanorods radiolabeled with \({}^{68}\)Ga in a chelator-free manner via surface interactions. Radiolabeled construct stability was assessed incubating it with human serum at physiological conditions. After 3 h, 95% of \({}^{68}\)Ga remained attached to nanoconstructs, denoting complex was highly stable. PET and MR imaging revealed liver accumulation of the probe. Lymph nodes play a key role in cancer cell metastasis; hence the development of probes capable of targeting and visualizing these structures, has become the main focus of several study groups. [PERSON] et al. [70] developed chelator-free \({}^{68}\)Ga-labeled SPIONs.