Preparation of an Actinium-228 Generator

Advances in targeted α-therapies have increased the interest in actinium (Ac), whose chemistry is poorly defined due to scarcity and radiological hazards. Challenges associated with characterizing Ac3+ chemistry are magnified by its 5f06d0 electronic configuration, which precludes the use of many spectroscopic methods amenable to small amounts of material and low concentrations (like EPR, UV–vis, fluorescence). In terms of nuclear spectroscopy, many actinium isotopes (225Ac and 227Ac) are equally “unfriendly” because the actinium α-, β-, and γ-emissions are difficult to resolve from the actinium daughters. To address these issues, we developed a method for isolating an actinium isotope (228Ac) whose nuclear properties are well-suited for γ-spectroscopy. This four-step procedure isolates 228Ra from naturally occurring 232Th. The relatively long-lived 228Ra (t 1/2 = 5.75(3) years) radioisotope subsequently decays to 228Ac. Because the 228Ac decay rate [t 1/2 = 6.15(2) h] is fast, 228Ac rapidly regenerates after being harvested from the 228Ra parent. The resulting 228Ac generator provides frequent and long-term access (of many years) to the spectroscopically “friendly” 228Ac radionuclide. We have demonstrated that the 228Ac product can be routinely “milked” from this generator on a daily basis, in chemically pure form, with high specific activity and in excellent yield (∼95%). Hence, in the same way that developing synthesis routes to new starting materials has advanced coordination chemistry for many metals by broadening access, this 228Ac generator has the potential to broaden actinium access for the inorganic community, facilitating the characterization of actinium chemical behavior.