Alpha- and beta-emitting radiolanthanides in targeted cancer therapy: the potential role of terbium-149.

The efficacy of systemic cancer therapy rests on the ability of a toxin to be preferentially located in cancer cells, so that cancer cell kill is maximized and normal tissue spared. This requires that the lifetime of the toxin be less than the lifetime of the carrier in the body, effectively ruling out chemical toxins, as they remain effective until excreted from the body. The requirement of localization of dose to the cancer cell makes radioactive beta-emitting radionuclides unattractive. Alpha-emitting radionuclides are much more appropriate toxins, as their efficacy depends on the high energy and short range of the alpha particles, and terbium-149 is shown to be the most efficacious of these. However, the merit of various alpha- and beta-emitting radionuclides depends on the stage and type of cancer. Recommendations are made with respect to the matching of the target cancer with required properties of the radiolabel and carrier.