Chelator-Free Labeling of Layered Double Hydroxide Nanoparticles for in Vivo PET Imaging

Abstract

Layered double hydroxide (LDH) nanomaterial has emerged as a novel delivery agent for biomedical applications due to its unique structure and properties. However, in vivo positron emission tomography (PET) imaging with LDH nanoparticles has not been achieved. The aim of this study is to explore chelator-free labeling of LDH nanoparticles with radioisotopes for in vivo PET imaging. Bivalent cation (64)Cu(2+) and trivalent cation (44)Sc(3+) were found to readily label LDH nanoparticles with excellent labeling efficiency and stability, whereas tetravalent cation (89)Zr(4+) could not label LDH since it does not fit into the LDH crystal structure. PET imaging shows that prominent tumor uptake was achieved in 4T1 breast cancer with (64)Cu-LDH-BSA via passive targeting alone (7.7 ± 0.1%ID/g at 16 h post-injection; n = 3). These results support that LDH is a versatile platform that can be labeled with various bivalent and trivalent radiometals without comprising the native properties, highly desirable for PET image-guided drug delivery.

Figure 1. Schematic illustration and characterization of LDH nanoparticles.

(a) A schematic structure of ⁶⁴Cu-LDH-BSA. (b) TEM image of LDH nanoparticles. Scale bar, 100 nm. (c) LDH aggregated but LDH-BSA remained stable after incubating LDH and LDH-BSA (4.7 mg/mL) in PBS for 7 days. (d) The size distribution of LDH and LDH-BSA in both water and culture media measured by DLS. The size of LDH nanoparticles increased significantly in culture media, whereas the size of LDH-BSA is similar in both water and culture media.

Figure 2. Chelator-free labeling of LDH nanoparticles.

(a,c,e) Autoradiographic images of TLC plates of LDH, LDH-BSA and BSA after chelator-free labeling with ⁶⁴Cu, ⁴⁴Sc and ⁸⁹Zr for 60 min. (b,d,f) The labeling yield of LDH, LDH-BSA and BSA after chelator-free labeling with ⁶⁴Cu, ⁴⁴Sc and ⁸⁹Zr at different reaction times calculated from autoradiography images of TLC plates.

Figure 3. Labeling stability of LDH nanoparticles.

Labeling stability was observed with ⁶⁴Cu-LDH and ⁶⁴Cu-LDH-BSA in both PBS and complete mouse serum during 24 h incubation (n = 3).

Figure 4. In vivo PET imaging.

Serial coronal PET images at different time points post-injection of ⁶⁴Cu-LDH-BSA and ⁶⁴Cu-BSA were acquired in 4T1 tumor-bearing mice. Strong signal in tumor was observed in the mice injected with ⁶⁴Cu-LDH-BSA. Three mice were scanned per group (n = 3).

Figure 5. Quantitative analysis of the PET data.

(a) Time activity curves of the liver, 4T1 tumor, blood, and muscle upon intravenous injection of ⁶⁴Cu-LDH-BSA. (b) Time activity curves of the liver, 4T1 tumor, blood, and muscle upon intravenous injection of ⁶⁴Cu-BSA. (c) Comparison of tumor uptake at different time points post injection of ⁶⁴Cu-LDH-BSA and ⁶⁴Cu-BSA. The differences of the tumor uptake were statistically significant (P < 0.05) at all time points except 0.5 h. (d) Comparison of tumor/muscle ratio at different time points post injection of ⁶⁴Cu-LDH-BSA and ⁶⁴Cu-BSA. All data were back-decayed to the injection time. The differences of tumor/muscle ratio were statistically significant (P < 0.05) at all time points. All data represent 3 mice per group (n = 3).

Figure 6. Biodistribution of ⁶⁴Cu-LDH-BSA and ⁶⁴Cu-BSA in 4T1 tumor-bearing mice.

The tumor uptake of ⁶⁴Cu-LDH-BSA was significantly higher than that of ⁶⁴Cu-BSA at 24 h post injection (P < 0.05; n = 3).