Current concepts and future directions in radioimmunotherapy

Radioimmunotherapy relies on the principles of immunotherapy, but expands the cytotoxic effects of the antibody by complexing it with a radiation-emitting particle. If we consider radioimmunotherapy as a step beyond immunotherapy of cancer, the step was prompted by the (relative) failure of the latter. The conventional way to explain the failure is a lack of intrinsic killing effect and a lack of penetration into poorly vascularized tumor masses. The addition of a radioactive label (usually a β-emitter) to the antibody would improve both. Radiation is lethal and the type of radiation used (beta rays) has a sufficient range to overcome the lack of antibody penetration. At present, the most successful (and FDA approved) radioimmunotherapy agents for lymphomas are anti-CD20 monoclonal antibodies. Rituximab (Rituxan(®)) is a chimeric antibody, used as a non-radioactive antibody and to pre-load the patient when Zevalin(®) is used. Zevalin(®) is the Yttrium-90 ((90)Y) or Indium-111 ((111)In) labeled form of Ibritumomab Tiuxetan. Bexxar(®) is the Iodine-131 ((131)I) labeled form of Tositumomab. Ibritumomab Tiuxetan and Tositumomab are murine anti-CD20 monoclonal antibodies, not chimeric antibodies. Promising research is being done to utilize radioimmunotherapy earlier in the treatment algorithm for lymphoma, including as initial, consolidation, and salvage therapies. However, despite more than 8 years since initial regulatory approval, radioimmunotherapy still has not achieved widespread use due to a combination of medical, scientific, logistic, and financial barriers. Other experimental uses for radioimmunotherapy include other solid tumors to treat infections. Optimization can potentially be done with pre-targeting and bi-specific antibodies. Alpha particle and Auger electron emitters show promise as future radioimmunotherapy agents but are mostly still in pre-clinical stages.