LaPO4 Nanoparticles Doped with Actinium-225 that Partially Sequester Daughter Radionuclides

Nanoscale materials have been envisioned as carriers for various therapeutic drugs, including radioisotopes. Inorganic nanoparticles (NPs) are particularly appealing vehicles for targeted radiotherapy because they can package several radioactive atoms into a single carrier and can potentially retain daughter radioisotopes produced by in vivo generators such as actinium-225 (Ac-225, t(1/2) = 10 cl). Decay of this radioisotope to stable bismuth-209 proceeds through a chain of short-lived daughters accompanied by the emission of four a-particles that release >27 MeV of energy. The challenge in realizing the enhanced cytotoxic potential of in vivo generators lies in retaining the daughter nuclei at the therapy site. When Ac-225 is attached to targeting agents via standard chelate conjugation methods, all of the daughter radionuclides are released after the initial a-decay occurs. In this work, Ac-225 was incorporated into lanthanum phosphate NPs to determine whether the radioisotope and its daughters would be retained within the dense mineral lattice. Further, the Ac-225-doped NPs were conjugated to the monoclonal antibody mAb 201B, which targets mouse lung endothelium through the vasculature, to ascertain the targeting efficacy and in vivo retention of radioisotopes. Standard biodistribution techniques and microSPECT/CT imaging of Ac-225 as well as the daughter radioisotopes showed that the NPs accumulated rapidly in mouse lung after intravenous injection. By showing that excess, competing, uncoupled antibodies or NPs coupled to control mAbs are deposited primarily in the liver and spleen, specific targeting of NP-mAb 201B conjugates was demonstrated. Biodistribution analysis showed that similar to 30% of the total injected dose of La(Ac-225)PO4 NPs accumulated in mouse lungs 1 h postinjection, yielding a value of % ID/g, >200. Furthermore, after 24 h, 80% of the Bi-213 daughter produced from Ac-225 decay was retained within the target organ and Bi-213 retention increased to similar to 87% at 120 h. In vitro analyses, conducted over a 1 month interval, demonstrated that similar to 50% of the daughters were retained within the La(Ac-225)PO4 NPs at any point over that time frame. Although most of the gamma-rays from radionuclides in the Ac-225 decay chain are too energetic to be captured efficiently by SPECT detectors, appropriate energy windows were found that provided dramatic microSPECT images of the NP distribution in vivo. We conclude that La(Ac-225)PO4-mAb 201B conjugates can be targeted efficiently to mouse lung while partially retaining daughter products and that targeting can be monitored by biodistribution techniques and microSPECT imaging.