Ultrasmall Gold Nanoparticles Radiolabeled with Iodine-125 as Potential New Radiopharmaceutical

Abstract

The relatively high linear energy transfer of Auger electrons, which can cause clustered DNA damage and hence efficient cell death, makes Auger emitters excellent candidates for attacking metastasized tumors. Moreover, gammas or positrons are usually emitted along with the Auger electrons, providing the possibility of theragnostic applications. Despite the promising properties of Auger electrons, only a few radiopharmaceuticals employing Auger emitters have been developed so far. This is most likely explained by the short ranges of these electrons, requiring the delivery of the Auger emitters to crucial cell parts such as the cell nucleus. In this work, we combined the Auger emitter ¹²⁵I and ultrasmall gold nanoparticles to prepare a novel radiopharmaceutical. The ¹²⁵I labeled gold nanoparticles were shown to accumulate at the cell nucleus, leading to a high tumor-killing efficiency in both 2D and 3D tumor cell models. The results from this work indicate that ultrasmall nanoparticles, which passively accumulate at the cell nucleus, have the potential to be applied in targeted radionuclide therapy. Even better tumor-killing efficiency can be expected if tumor-targeting moieties are conjugated to the nanoparticles.

Keywords: Auger electron; Auger therapy; iodine-125; radionuclide therapy; ultrasmall gold nanoparticle.

Figure 1

Synthesis and characterization of PEG-AuNPs: (a) schematic illustration of the synthesis and radiolabeling of PEG-AuNPs with ¹²⁵I; (b) TEM image; (c) UV–Vis spectrum; (d) number weighted hydrodynamic diameter; and (e) ζ potential of PEG-AuNPs dispersed in PBS. Scale bar = 10 nm.

Figure 2

Colloidal stability of PEG-AuNPs: (a) the number weighted hydrodynamic diameter of PEG-AuNPs in PBS at 37 °C as a function of time, n = 3; (b) Normalized UV–Vis spectrum of PEG-AuNPs in 10% FBS at 37 °C and at different time points. P values: ns, p > 0.05.

Figure 3

(a) Radiolabeling efficiency of ¹²⁵I-PEG-AuNPs determined by iTLC and ultrafiltration experiments. ¹²⁵I:NP = 0.1, [¹²⁵I]NaI was neutralized before usage, n = 3; (b) radiochemical stability of ¹²⁵I-PEG-AuNPs in PBS or 10% FBS over 72 h at 37 °C, n = 3. P values: ns, p > 0.05.

Figure 4

In vitro behavior of ¹²⁵I-PEG-AuNPs: (a) viability of U87 cells treated with bare PEG-AuNPs at different concentrations, n = 5; (b) uptake of ¹²⁵I-PEG-AuNPs in U87 cell monolayers after 4 and 24 h incubation at 37 °C, data are shown as number of nanoparticles per single cell, n = 3; (c) uptake of ¹²⁵I-PEG-AuNPs in U87 cell spheroids after 4 and 24 h incubation at 37 °C, data are shown as number of nanoparticles per spheroid, n = 3; (d) subcellular distribution of ¹²⁵I-PEG-AuNPs in U87 cell monolayers after 4 and 24 h incubation, data are shown as the percentage of nanoparticles present in the cell nucleus from all internalized nanoparticles, n = 3. P values: ns, p > 0.05, *p ≤ 0.05, ****p ≤ 0.0001.

Figure 5

In vitro tumor-killing efficiency of ¹²⁵I-PEG-AuNPs or [¹²⁵I]NaI with different ¹²⁵I activity determined by (a) viability assay 24 h after removal of activity. The specific activity of the ¹²⁵I-PEG-AuNPs in each sample was 13.3 kBq/nM. The activity of [¹²⁵I]NaI was 740 kBq, n = 4; (b) DNA proliferation assay 24 h after removal of activity. The activity of [¹²⁵I]NaI is 740 kBq, n = 4; (c) colony formation assay. The specific activity of the ¹²⁵I-PEG-AuNPs in each sample was 26.3 kBq/nM. The activity of [¹²⁵I]NaI is 3.7 MBq, n = 3; (d) 3D spheroid growth inhibition assay. The spheroids were treated by ¹²⁵I-PEG-AuNPs with 37, 370, or 740 kBq of ¹²⁵I for 24 h. The specific activity of the ¹²⁵I-PEG-AuNPs in each sample was 13.3 kBq/nM. The size change of the spheroids was monitored for another 13 days after the removal of activity. Nontreated spheroids were used as control, n = 4. P values: ns, p > 0.05, *p ≤ 0.05, **p ≤ 0.01, ***p ≤ 0.001, ****p ≤ 0.0001.