Thorium-227

Thorium-227 is an alpha emitter with an 18.7-day half-life -- long enough, unlike most alpha emitters, to be conjugated to slow-clearing targeting molecules such as full antibodies rather than only fast peptides. It decays through a five-alpha cascade (via 223Ra, 219Rn, 215Po, 211Pb, 211Bi, 211Po) to stable 207Tl/207Pb, releasing a total of about 32.4 MeV of alpha energy and roughly 0.96 MeV of beta energy per decay chain, making it an in vivo generator of its own daughter, radium-223.

It is chelated using octadentate 3,2-hydroxypyridinone (3,2-HOPO) ligands rather than the DOTA-family macrocycles used for most other alpha and beta emitters, and has been developed by Bayer as 'targeted thorium conjugates' (TTCs), including antibody-based constructs such as BAY 2315497 (anti-PSMA) and BAY2701439 (anti-HER2), both tested in early-phase trials.

227Th is obtained from the same generator/supply chain as radium-223: it is the immediate alpha-decay daughter of actinium-227 (itself made by neutron irradiation of radium-226 in a reactor, 226Ra(n,γ)227Ra → β− → 227Ac), separated from the growing 227Ac/227Th/223Ra generator column. Because production piggybacks on the well-established 223Ra supply chain (ORNL, PNNL, Rosatom, Bayer), 227Th is considered available at the research and early-clinical scale, though its co-produced daughter 223Ra must be removed and accounted for as a radioactive impurity in any 227Th-labelled drug product.