The low-energy β- and electron emitter 161Tb as an alternative to 177Lu for targeted radionuclide therapy

Introduction: The low-energy β- emitter 161Tb is very similar to 177Lu with respect to half-life, beta energy and chemical properties. However, 161Tb also emits a significant amount of conversion and Auger electrons. Greater therapeutic effect can therefore be expected in comparison to 177Lu. It also emits low-energy photons that are useful for gamma camera imaging. Methods: The 160Gd(n,γ)161Gd→161Tb production route was used to produce 161Tb by neutron irradiation of massive 160Gd targets (up to 40 mg) in nuclear reactors. A semiautomated procedure based on cation exchange chromatography was developed and applied to isolate no carrier added (n.c.a.) 161Tb from the bulk of the 160Gd target and from its stable decay product 161Dy. 161Tb was used for radiolabeling DOTA-Tyr3-octreotate; the radiolabeling profile was compared to the commercially available n.c.a. 177Lu. A 161Tb Derenzo phantom was imaged using a small-animal single-photon emission computed tomography camera. Results: Up to 15 GBq of 161Tb was produced by long-term irradiation of Gd targets. Using a cation exchange resin, we obtained 80%-90% of the available 161Tb with high specific activity, radionuclide and chemical purity and in quantities sufficient for therapeutic applications. The 161Tb obtained was of the quality required to prepare 161Tb-DOTA-Tyr3-octreotate. Conclusions: We were able to produce 161Tb in n.c.a. form by irradiating highly enriched 160Gd targets; it can be obtained in the quantity and quality required for the preparation of 161Tb-labeled therapeutic agents.