225Ac

225Ac, or actinium-225, is a radioactive isotope of the element actinium. It has a half-life of 9.92 days and decays primarily via alpha emission through a short decay chain to the stable nuclide lead-209. Each decay releases multiple alpha particles, giving 225Ac a high linear energy transfer suited to damaging malignant cells when delivered selectively to tumors.

Production and availability: 225Ac is produced mainly from the decay of thorium-229 within the 229Th decay series. Other production routes include irradiation of radium-226 targets and subsequent chemical separation. Global supply is limited and concentrated in a few facilities, leading to high costs and competitive demand for research and clinical use.

Medical use and research: In targeted alpha therapy, 225Ac is chelated to targeting vectors such as antibodies or peptides and administered to irradiate cancer cells while sparing surrounding tissue. Experimental therapies include 225Ac-labeled PSMA-617 for metastatic prostate cancer and 225Ac-labeled somatostatin analogs for neuroendocrine tumors. The method allows high-dose delivery over a short range, reducing systemic exposure.

Challenges and safety: The decay chain includes short-lived daughter nuclides that can recoil and migrate from the targeting conjugate, complicating dosimetry and toxicity management. Handling requires specialized facilities, shielding, and radiopharmaceutical chemistry expertise. Availability, production scale, regulatory approval, and long-term safety data continue to be active areas of research.

See also: targeted alpha therapy; radiopharmaceuticals; actinium-225 therapy.