Comparison of different classes of radionuclides for potential use in radioimmunotherapy

Currently, beta-emitting radionuclides are used almost exclusively in the clinic and in clinical radioimmunotherapy studies. The main advantage of beta-emitters is the relatively long path length in biological tissue (in the mm range), which is sufficient to irradiate cancer cells that do not have bound radiolabelled antibody (cross-fire effect). This alleviates problems with inadequate uptake and heterogeneous distribution of radiolabelled antibodies in tumours. Hence, beta-emitters provide a relatively uniform radiation dose to the tumour and it is generally accepted that this class of radionuclides is more appropriate for radioimmunotherapy of solid tumours and large tumour burdens (> 0.5 cm). However, the shorter-range alpha-emitters (50-100 mm) and the ultra-short range Auger electron-emitting radionuclides (the majority of electrons traverse a few nm), have been shown to be more efficient than beta-emitters at inducing lethal lesions in single cells. It has been suggested that these classes of radionuclides may have the potential to provide a more favourable therapeutic index than beta-emitters for radioimmunotherapy of single tumour cells in the circulation, micrometastases and in certain cases, minimal residual disease. The aim of this article is to discuss the different classes of radionuclides with potential for clinical use in radioimmunotherapy.