Terbium-161 in nuclear medicine: Preclinical and clinical progress in comparison with lutetium-177

The use of new radiopharmaceuticals labeled with lutetium-177 represents a successful translation of experimental results into clinical practice. Recent experimental data suggests that terbium-161 might well follow the example of lutetium-177 regarding applicability in nuclear medicine. Similarly to lutetium-177, the terbium-161 emits beta particles and gamma-radiation, although terbium-161 emits short-ranged conversion and Auger electrons, creating an effect that may eliminate smaller tumor metastases more effectively than lutetium-177. Terbium-161 may exert a higher radiobiological effect in the target tissues in comparison with lutetium-177, a difference which makes possible a reduction in the doses of radioactivity administered. Further, due to the similar chemical properties of lutetium-177 and terbium-161, similar radiolabeling techniques can be used. The differences found in preclinical experiments on radiotoxicity of the counterparts seem to be minor. Despite intensive progress, the number of preclinical studies on ¹⁶¹Tb-labeled agents is still not comparable to studies on lutetium-177. Clinical trials with ¹⁶¹Tb-labeled radiopharmaceuticals focused on the treatment of prostate cancer and selected neuroendocrine tumors have already begun, although none of them have been completed yet.