232Th-Spallation-Produced 225Ac with Reduced 227Ac Content

Recent clinical results have demonstrated remarkable treatment responses of late-stage cancer patients when treated with alpha-emitting radionuclides such as actinium-225 (²²⁵Ac). The resulting intense global effort to produce greater quantities of ²²⁵Ac has triggered a number of emerging technologies to produce this rare, yet important, radionuclide. Accelerator-based methods for increasing global ²²⁵Ac production capacity have focused on the high energy (>100 MeV) proton irradiation of thorium, despite the coproduction of the undesirable ²²⁷Ac byproduct at 0.1-0.3% of the ²²⁵Ac activity. We at TRIUMF have developed a process for the production of a ²²⁵Ra/²²⁵Ac generator from irradiated thorium that results in an ²²⁵Ac product with reduced ²²⁷Ac content. ²²⁵Ac was separated from irradiated thorium and coproduced radioactive spallation and fission products using a thorium peroxide precipitation method followed by cation exchange and extraction chromatography. Stable and radioactive tracer studies demonstrated the ability of this method to separate Ac from most other elements, providing a directly produced Ac product with measured ²²⁷Ac content of (0.15 ± 0.04)%. A second, indirectly produced Ac product with ²²⁷Ac content of <7.5 × 10^-5% is obtained by repeating the final extraction chromatography step with the ²²⁵Ra-containing fraction. The ²²⁵Ra-derived ²²⁵Ac showed similar or improved quality compared to the initial, directly produced ²²⁵Ac product in terms of chemical purity and radiolabeling capability, the latter of which was comparable with other ²²⁵Ac sources reported in the literature.