Au@¹⁰⁹Pd core-shell nanoparticle conjugated to trastuzumab for the therapy of HER2+cancers: studies on the applicability of ¹⁰⁹Pd/^109mAg in vivo generator in combined ^- auger electron therapy

Background: In radionuclide therapy, to enhance therapeutic efficacy, an intriguing alternative is to ensure the simultaneous implementation of low- and high-LET radiation emitted from a one radionuclide. In the present study, we introduce the concept of utilizing Pd-109 (T-1/2 = 13.7 h) in the form of a Pd-109/Ag-109m in vivo generator. In this system, Pd-109 emits beta particles of medium energy, while Ag-109m releases a cascade of conversion and Auger electrons. Pd-109 was utilized in the form of 15 nm gold nanoparticles, which were coated with a monolayer of Pd-109. In this system, the Pd-109 atoms are on the surface of the nanoparticle, while the Ag-109m atoms generated in the decay reaction possess the capability for unhindered emission of Auger electrons.Results: Pd-109, obtained through neutron irradiation of natural palladium, was deposited onto 15-nm gold nanoparticles, exceeding a efficiency rate of 95%. In contrast to previously published data on in vivo generators based on chelators, where the daughter radionuclide diffuses away from the molecules, daughter radionuclide Ag-109m remains on the surface of gold nanoparticles after the decay of Pd-109. To obtain a radiobioconjugate with an affinity for HER2 receptors, polyethylene glycol chains and the monoclonal antibody trastuzumab were attached to the Au@Pd nanoparticles. The synthesized bioconjugate contained an average of 9.5 trastuzumab molecules per one nanoparticle. In vitro cell studies indicated specific binding of the Au@Pd-109-PEG-trastuzumab radiobioconjugate to the HER2 receptor on SKOV-3 cells, resulting in 90% internalization. Confocal images illustrated the accumulation of Au@Pd-109-PEG-trastuzumab in the perinuclear area surrounding the cell nucleus. Despite the lack of nuclear localization, which is necessary to achieve an effective cytotoxic effect of Auger electrons, a substantial cytotoxic effect, significantly greater than that of pure beta(-) and pure Auger electron emitters was observed. We hypothesize that in the studied system, the cytotoxic effect of the Auger electrons could have also occurred through the damage to the cell's nuclear membrane by Auger electrons emitted from nanoparticles accumulated in the perinuclear area.Conclusion: The obtained results show that trastuzumab-functionalized Pd-109-labeled nanoparticles can be suitable for the application in combined beta(-)-Auger electron targeted radionuclide therapy. Due to both components decay (beta(-) and conversion/Auger electrons), the Pd-109/Ag-109m in vivo generator presents unique potential in this field. Despite the lack of nuclear localization, which is highly required for efficient Auger electron therapy, an adequate cytotoxic effect was attained.