Tuning oxidation states of terbium and europium towards high purity terbium-161 and samarium-153 for medical applications

Radionuclides have a variety of medical applications as radiopharmaceuticals. The most significant and frequent use is in diagnostic imaging to visualize infected, cancerous or damaged areas (e.g. after a heart attack) in the body. More recently, therapeutic radiopharmaceuticals have gained more interest, particularly in cancer treatment, as the ionizing radiation can target and destroy harmful tumor cells in a precise and selective manner. In recent years, various radionuclides from the lanthanide series have been utilized and are investigated in nuclear medicine. Radiolanthanides are highly promising for use in nuclear medicine due to their chemical similarities, allowing for coordination compounds of the same type to be used in a range of diagnostic and therapeutic applications. Terbium, for instance, has four radioisotopes with distinct decay properties, energies, and half-lives that can be utilized in non-invasive theranostic applications. To produce carrier-free terbium-161, highly enriched gadolinium-160 targets are neutron-irradiated in the Belgian Reactor 2 (BR2). Terbium-161 emits low-energy beta particles along with gamma photons, and possesses properties similar to lutetium-177. Moreover, the co-emission of Auger electrons makes terbium-161 even more attractive for beta and Auger electron therapy.

Another example, samarium-153, is a radiolanthanide that can be effectively used in medicine due to its favorable half-life and stable daughter isotope europium-153, which ensures no significant additional side effects in the human body. It is often coordinated by EDTMP, sold commercially as Quadramet® and Lexidronam®, used for treatment of painful bone metastases caused by various cancers. The beta emission of samarium-153 is utilized to destroy harmful cancer cells, and its gamma emission is also used in imaging to visualize infected areas in the human skeleton. Like many other medical radioisotopes, samarium-153 can be most efficiently produced in a nuclear research reactor by irradiating a highly enriched target with neutrons in a high thermal neutron flux, resulting in a product with high yield, purity, and specific activity.

In addition to their dominant trivalent oxidation state, some lanthanides can also exist in the divalent or tetravalent oxidation states. This change in valence state alters their chemical properties and make intragroup lanthanide separations easier. Although hydrated terbium(III) ions typically have a highly positive reduction potential, they can be electrochemically oxidized to their tetravalent state and stabilized in highly concentrated carbonate solutions. Terbium(III) can be oxidized to terbium(IV) in aqueous carbonate, nitrate, and periodate media. To examine the stability of terbium(III) complexes in aqueous electrolytes, spectroscopic and elemental analyses were conducted, with a parametric study focusing on pH, terbium concentration, salt concentration, and applied electrical potentials. The lowest applied potential value to oxidize terbium(III) was +0.9 V vs. Ag/AgCl in periodate medium, while nitrate and periodate media offered poor terbium(IV) stabilization compared to a carbonate medium. Therefore, an aqueous carbonate medium appears to be the most promising medium for developing a separation process based on terbium oxidation state change. Successful separation of terbium(IV) from trivalent lanthanides was achieved, performing ion exchange chromatography with the carbonate form of the Dowex-IX8 resin. Sufficient characterization and remarkable reproducibility of the methodology demonstrates the potential of this methodology as a new separation process for the purification of terbium(IV).

Finally, the divalent oxidation state of europium was investigated based on the electrochemical reduction of europium(III) to europium(II) in aqueous nitrate, chloride and perchlorate media. The kinetic parameters were studied using a rotating disk electrode. As a next step, separation of samarium from europium was achieved using a TEVA column in nitrate form. The feasibility of this separation method was tested using the samarium-153 and europium-152 radioisotopes in the same concentration ratio as the concentration ratio of these elements present in a dissolved irradiated enriched samarium-152 target.