Actinium225

Actinium-225 is a radioactive isotope of the element actinium (atomic number 89) with a mass number of 225. It has a half-life of about 9.92 days and decays predominantly by alpha emission to francium-221, with a decay chain that includes several short-lived daughter nuclides such as 221Fr, 217At, 213Bi, and others. The alpha emissions give actinium-225 a high linear energy transfer, making it an attractive source of radiation for targeted therapies when delivered to tumors.

Production and availability of 225Ac are limited and technically demanding. The principal production route uses irradiation of a thorium-232 target with high-energy protons (the 232Th(p,3n)225Ac reaction) in cyclotrons or similar facilities. Additional production pathways involve generators or indirect production from related actinide decay chains. Due to its scarcity and the complexity of production, 225Ac supply is concentrated in specialized laboratories and radiopharmacy networks.

Medical use and challenges: Actinium-225 is used in targeted alpha therapy, where its short-range, high-LET alpha particles can destroy cancer cells while minimizing damage to surrounding tissue when the isotope is precisely delivered. In radiopharmaceuticals, actinium-225 is bound to antibodies or peptides via chelators such as DOTA to form imaging or therapeutic agents; a prominent area is research into 225Ac-labeled compounds for prostate cancer and other malignancies. A major challenge is the recoil of daughter nuclides during decay, which can lead to radiochemical instability and potential off-target radiation, requiring robust chelation and containment strategies.

Chemistry and safety: Actinium commonly forms a +3 oxidation state, and 225Ac is administered as a salt or within a chelated complex. Handling requires specialized facilities, shielding, and strict regulatory controls due to radiological hazards.