Chemical properties

Tb-161 is a radiolanthanide, most often in trivalent state and chemically very similar to lutetium. The ionic radius of Tb³⁺ is 92 pm, i.e. just 7% larger than Lu³⁺, which explains that macrocyclic chelators suitable for Lu-177 can be directly employed for Tb-161 too, in particular DOTA, but also others . Also Tb-161 radiolabelling of heat-sensitive molecules has been reported .

Nuclear properties

Tb-161 decays by beta-minus emission with a half-life of 6.953(2) days to stable Dy-161. Its half-life is marginally (<5%) larger than that of Lu-177. In addition to beta-minus emission it shows ample emission of low energy conversion and Auger electrons , thus emitting in total about 2.27 electrons (with energies above 3 keV) per decay.

Moreover Tb-161 emits gamma- and X-rays, notably 43.1(5) % around 48(+/-10%) keV and 10.3(2) % at 74.6 keV . Both energies make Tb-161 suitable for SPECT imaging.

The mean electron energy emitted per decay is 197 keV, the mean photon energy per decay is 35 keV or 37 keV considering the newly measured photon intensities [, complemented by Evaluated Nuclear Structure Data File (ENSDF) for photons below 45 keV].

Production

Tb-161 is produced indirectly by thermal neutron irradiation of enriched Gd-160 oxide targets in the research reactors of the PRISMAP network: RHF at ILL, BR2 at SCK CEN and MARIA at NCBJ, or the SINQ spallation neutron source at PSI. The short-lived Gd-161 produced upon neutron capture of Gd-160 decays quickly (T_1/2=3.66 min) to Tb-161 via beta-minus decay. A radiochemical Tb/Gd separation via cation exchange and extraction chromatography leads to n.c.a. Tb-161 . The effective specific activity of such produced n.c.a. Tb-161 is comparable to that of commercially available n.c.a. Lu-177 (EndolucinBeta^®) .

Distribution

Radiochemical separations are performed at the PRISMAP facilities PSI Radiochemistry Laboratory (PSI, Villigen, Switzerland), SCK CEN (Mol, Belgium) or POLATOM (NCBJ, Otwock-Swierk, Poland). Activity will be shipped from one of these locations to the users in form of TbCl₃ solution, ready for labelling of e.g. DOTA-compounds.

Examples of use

• Tb-161 is part of the terbium radionuclide quadruplet .
• Due to the strong similarities of chemical and nuclear properties of Tb-161 and Lu-177, Tb-161 is considered a logic evolution from Lu-177, increasing the local dose deposition in therapeutic applications with respect to the latter.
• Dosimetric calculations show that at short distances (below about 0.1 mm) the dose deposition per decay is more than doubled with respect to Lu-177 .
• The therapeutic benefit of Tb-161 over Lu-177 has been demonstrated preclinically with different compounds .
• The preclinical SPECT imaging capabilities of Tb-161 have been demonstrated in different settings .
• Tb-161 and Lu-177 emit photons of sufficiently distinct energies to enable simultaneous SPECT imaging with both radionuclides .
• A clinical SPECT/CT protocol has been proposed for imaging with Tb-161 .
• A first-in-human application of Tb-161-DOTATOC has been reported including clinical SPECT/CT imaging up to 113 h post injection .
• Tb-161 is also suited as source for Mößbauer spectroscopy , a highly sensitive technique to characterize the chemical state and chemical environment of terbium bound in solids or frozen ex vivo samples.