In Vitro and In Vivo Radiotoxicity and Biodistribution of Thallium-201 Delivered to Cancer Cells by Prussian Blue Nanoparticles

Thallium-201 (t(1/2) = 73 h) emits around 37 Auger and other secondary electrons per decay and is highly radiotoxic when internalized into cancer cells. However, the lack of effective chelators hinders its application in molecular radiotherapy. This study evaluates Prussian blue nanoparticles, coated with citric acid (Tl-201-caPBNPs) or chitosan (Tl-201-chPBNPs), as a Tl-201 delivery vehicle compared with unbound Tl-201(+). Cellular uptake and efflux kinetics and radiotoxicity using clonogenic and gamma H2AX DNA damage assays were evaluated in vitro for both nanoparticle types. Subcellular localization was also assessed using electron microscopy with energy-dispersive X-ray spectroscopy. Biodistribution of Tl-201-chPBNPs was evaluated in vivo in mice bearing subcutaneous A549 tumor xenografts, using single photon computed tomography imaging and ex vivo tissue counting. Compared with unbound Tl-201(+), Tl-201-chPBNPs showed higher cellular uptake, while Tl-201-caPBNP uptake was reduced. Both showed delayed efflux of Tl-201 from cancer cells. PBNPs prelocalized within cells enhanced the capture and retention of Tl-201(+) ions. Both types of PBNPs accumulated in cytoplasmic vesicular compartments and were not visible in the nuclei. Furthermore, Tl-201-radiolabeled chPBNPs but not Tl-201-caPBNPs showed significantly greater radiotoxicity than unbound Tl-201(+) per Becquerel of radiotoxicity provided in media, resulting from their higher uptake and delayed efflux. However, when corrected for the greater activity accumulated in cells and delayed efflux, the radiotoxicity of Tl-201-chPBNPs was lower than that of unbound Tl-201(+), possibly due to differences in subcellular localization. These findings highlight the potential of chPBNPs for enhancing the uptake and retention of Tl-201 in cancer cells and development of targeted radionuclide therapy.