Bismuth-213

Bismuth-213 is a daughter of actinium-225 with a half-life of only 45.6 minutes. It decays by branching: about 97.8% by beta emission to the ultra-short-lived pure alpha emitter polonium-213 (half-life about 4 microseconds), and a small fraction (roughly 2%) directly by alpha emission to thallium-209. Around 92-93% of the energy released per 213Bi decay is ultimately carried by alpha particles.

Because its half-life is far shorter than any other clinical radiotherapeutic isotope, 213Bi cannot be centrally manufactured and shipped; it is produced on demand from a 225Ac/213Bi radionuclide generator, typically eluted every two to three hours in clinical use, giving elution yields around 83-93%. The main generator design (an AG MP-50 cation-exchange resin loaded with 225Ac) was pioneered at the Joint Research Centre in Karlsruhe, Germany, which has supplied it free of charge for clinical trials in Europe and the US.

213Bi shares essentially all of 225Ac's supply constraints, since it is milked from the same scarce 225Ac stock, and using it consumes 225Ac less efficiently per alpha particle delivered than using 225Ac directly (only the final alpha of the decay chain is captured). It has been used in early-phase trials in leukaemia, glioma, melanoma, bladder cancer, lymphoma and neuroendocrine tumours, most with anti-CD33 or similar targeting constructs, but has not reached late-phase development.