Actinium Bismuth and Other Alpha Emitters

225Ac has the advantages of ready availability and the lack of radon isotope or high-energy gamma emitters in the decay scheme. 225Ac also has a sufficiently long half-life (10 days) that results in optimal pharmacokinetics and is conveniently produced from Thorium 229(Th) (T1/2 = 7340 years) generators and has the potential for continuous supply. It has been estimated that for each gram of

229 225

Th, five Ci of Ac can be produced per year benefiting 4000 cancer patients per year (43). Current emphasis is on using chelators to label the antibody, but biodistribution problems, including clearance from the body, pose major challenges.

213 229

Bi (T1/2 = 45.6 minutes), the last alpha emitter in the decay scheme of Th, is produced from 225Ac generators (42). The eluted product can then be labeled to antibody yielding to high specific activities using established methods. Its gamma emission (440 keV) makes imaging and biodistribution studies possible.

Use of radioisotopes in treating medical conditions has always been an attractive option and takes advantage of the targeted delivery of radiation to the tissues. Beginning with P-32 and I-131, a number of useful isotopes have been added to the line up, while many have been retired. In spite of the initial negative set back on its popularity that was seen following the introduction of modern cytotoxic chemotherapy, advances in radiochemistry, and chelating techniques, coupled with a better understanding of radiation-absorbed dose estimation have made them attractive options for therapy again. A great deal of progress has been made in the synthesis, purification, and labeling of therapeutic radiopharma-ceuticals. A majority of these agents are beta emitters but alpha emitters, used as nanogenerators have generated greater excitement and promise to be a powerful new tool. Recent introduction of two radiolabeled monoclonal antibody products for the treatment of non-Hodgkins lymphoma (NHL) in the United States promises to provide a strong impetus for the growth of therapeutic nuclear oncology.

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