Very high specific activity erbium $^{169}$Er production for potential receptor-targeted radiotherapy

Erbium $^{169}$Er is one of the most interesting radiolanthanides for new potential receptor-targeted β − therapy applications due to its low energy β − emissions, very low intensity ɣ rays and the possibility to use 68 Ga or 44 Sc as companion for diagnostic in a theranostics approach. Currently it can be produced in reactors through the neutron activation of highly enriched $^{168}$Er. The low specific activity of the produced carrier-added $^{169}$Er is limiting its use for receptor-targeted therapy. Nonetheless it is used for radiosynoviorthesis of small joints. The aim of this work is to develop a new large-scale production method for the supply of very high specific activity $^{169}$Er. Highly enriched $^{168}$Er target has been irradiated at ILL nuclear reactor and shipped to CERN-MEDICIS. There, the irradiated sample has been mass separated in order to isolate 169 Er from the high amount of remaining stable 168 Er. The proof of principle for a preclinical dose production has been demonstrated with a collection of ≈17 MBq. The specific activity obtained was ≈240 GBq/mg (≈200 times higher than the product obtained at End of Bombardment – EOB) and the overall separation efficiency was ≈0.2%. Several improvements for the future have been identified and are promising. One of them is the installation of the new laser laboratory at CERN-MEDICIS that will allow to improve the selective ionization of erbium atoms leading to an increase of the efficiency of the method. This method can provide the supply of high specific activity $^{168}$Er, first for preclinical studies, and opens also the potential for future large-scale supply.