{"entity":{"id":"idea-tb161-instead-of-lu177","kind":"idea","name":"Terbium-161 instead of lutetium-177 in the same ligands","aka":[],"tldr":"Terbium-161 is a close chemical cousin of lutetium-177 that, as well as giving off the same kind of radiation, also releases a shower of extremely short-range electrons. In theory that extra radiation could finish off single cancer cells and tiny clusters that a lutetium-177 treatment might miss, using the same targeting molecules already in use.","summary":"Terbium-161 decays with a half-life of about 6.95 days and emits beta particles similar in energy to lutetium-177's, plus roughly ten times more conversion and Auger electrons per decay. These electrons travel only nanometres to micrometres, so preclinical dosimetry modelling (Champion et al.; Müller et al.) predicts 2-3-fold higher absorbed dose to single cells and micrometastases than lutetium-177 at the same injected activity, while producing only modestly higher organ doses to kidney and salivary gland. Because terbium-161 is chemically a lanthanide like lutetium-177, the same DOTA-based chelators and labelling protocols can be reused for PSMA and somatostatin-receptor ligands.\n\nClinical translation is under way but not yet comparative. The first-in-human VIOLET trial at Peter MacCallum Cancer Centre (NCT05521412, 30 patients with metastatic castration-resistant prostate cancer) established that 161Tb-PSMA-I&T is feasible and safe up to 7.4 GBq, with organ doses similar to historical 177Lu-PSMA-617 data, and the study has since opened a 9.5 GBq expansion cohort. A German programme (REALITY, NCT04833517) is running an intra-individual head-to-head dosimetry comparison of 161Tb-PSMA-617 against 177Lu-PSMA-617 in the same patients. Neither trial has yet reported a comparative efficacy result, and global terbium-161 supply remains a research-scale bottleneck: the reactor-based no-carrier-added production route needed to avoid stable terbium contamination is far less mature than lutetium-177 supply chains.\n\nThe open question is whether the extra subcellular dose actually translates into better tumour control at a clinically tolerable normal-organ dose, rather than just a favourable dosimetry ratio in phantoms and mice.","asOf":"2026-09-22","links":[{"label":"Peter Mac / UCSF-UCLA PSMA Conference 2025: Michael Hofman on terbium-161 (VIOLET)","url":"https://www.urotoday.com/video-lectures/2025-ucsf-ucla-psma-conference/video/4752-beyond-lutetium-psma-exploring-terbium-161-for-treatment-resistant-prostate-cancer-presentation-michael-hofman.html"},{"label":"Terbium-161 in nuclear medicine: preclinical and clinical status (PubMed, 2025)","url":"https://pubmed.ncbi.nlm.nih.gov/39978166/"},{"label":"Opportunities and potential challenges of using terbium-161 for targeted radionuclide therapy in clinics (EJNMMI, 2023)","url":"https://link.springer.com/article/10.1007/s00259-023-06316-y"},{"label":"From lutetium to terbium: a new era in PSMA-targeted radioligand therapy (PMC, 2025)","url":"https://pmc.ncbi.nlm.nih.gov/articles/PMC12629902/"}],"tags":["prostate","isotope alternative","auger electrons"],"related":["lu177-radioligand-therapy","prrt","radioligand-dosimetry","psma-pet","lu-177","tb-161"],"indications":[],"sections":[],"technologies":[],"targets":[],"drugs":[],"companies":[],"institutions":[],"pathways":[],"terms":[],"trials":[],"people":[],"bottlenecks":[],"keyPapers":[],"journals":[],"dependsOn":[],"notes":[],"hypothesis":"Substituting terbium-161 for lutetium-177 in the same PSMA or somatostatin-receptor ligand will produce a higher objective response rate at an equivalent kidney and salivary-gland radiation dose, because terbium-161's additional Auger and conversion electrons deposit extra lethal energy in micrometastases that lutetium-177's longer-range beta particles largely miss.","rationale":"Preclinical dosimetry work (Müller et al., Grünberg et al., Champion et al.) consistently found terbium-161 delivers several-fold higher absorbed dose than lutetium-177 to single cells and small tumour clusters, and dual-isotope SPECT imaging confirmed the two radiolanthanides are pharmacokinetically interchangeable when attached to the same peptide. The first-in-human VIOLET trial (30 patients) showed 161Tb-PSMA-I&T is safe with organ doses comparable to 177Lu-PSMA-617, but was single-arm and not powered to detect an efficacy difference. No completed trial has yet reported a head-to-head efficacy comparison.","test":"A randomised phase 2 trial in metastatic castration-resistant prostate cancer comparing 161Tb-PSMA-I&T against 177Lu-PSMA-617 at kidney-dose-matched activities, powered on PSA50 response rate or PSMA-PET-defined progression-free survival, roughly 150-200 patients; the ongoing REALITY intra-individual dosimetry comparison (NCT04833517) is a smaller precursor step.","maturity":"early-clinical","actor":"research","cost":"medium","horizonYears":6},"route":"/ideas/idea-tb161-instead-of-lu177/","neighbours":{"technology":[{"id":"lu177-radioligand-therapy","kind":"technology","name":"Lutetium-177 radioligand therapy","route":"/technologies/lu177-radioligand-therapy/"},{"id":"prrt","kind":"technology","name":"Peptide receptor radionuclide therapy (PRRT)","route":"/technologies/prrt/"},{"id":"psma-pet","kind":"technology","name":"PSMA PET","route":"/technologies/psma-pet/"},{"id":"radioligand-dosimetry","kind":"technology","name":"Radioligand dosimetry","route":"/technologies/radioligand-dosimetry/"}],"isotope":[{"id":"lu-177","kind":"isotope","name":"Lutetium-177","route":"/isotopes/lu-177/"},{"id":"tb-161","kind":"isotope","name":"Terbium-161","route":"/isotopes/tb-161/"}]}}