# Thorium-227

Source: https://nuclide.cc/isotopes/th-227/  
Nuclide record `th-227` (Isotope). Data CC BY-NC 4.0, attribute "Data from Nuclide (nuclide.cc)"; commercial use needs a licence.

## TL;DR

A radioactive metal that, once it decays, keeps generating more damaging radioactive particles inside the tumour for weeks. Because it lasts longer than most similar treatments, it can be attached to slow-moving carriers such as full-sized antibodies.

## Summary

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.

## Fields

- Kind: Isotope
- Status: phase-1
- Last checked: 2026-09-22
- Also known as: 227Th; Th-227
- Tags: therapy; alpha emitter; antibody-compatible; in vivo generator

## Sources

- Targeted thorium-227 conjugates as treatment options in oncology (Front Med, 2023): https://pmc.ncbi.nlm.nih.gov/articles/PMC9885765/
- Production and supply of alpha particle emitting radionuclides for TAT (JNM, 2021): https://jnm.snmjournals.org/content/jnumed/early/2021/07/22/jnumed.120.261016.full.pdf
- Development of radiopharmaceuticals for targeted alpha therapy (PMC, 2022): https://pmc.ncbi.nlm.nih.gov/articles/PMC9812962/

## Connected records

- isotopes: [Radium-223](https://nuclide.cc/isotopes/ra-223/)
- technologies: [Targeted alpha therapy](https://nuclide.cc/technologies/targeted-alpha-therapy/)
- targets: [HER2](https://nuclide.cc/targets/her2/), [Mesothelin (MSLN)](https://nuclide.cc/targets/mesothelin/), [PSMA](https://nuclide.cc/targets/psma/)

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