Radiopharmaceuticals and theranostics

In short: Most targeted radiopharmaceuticals have four parts: a biological target, a vector that binds it, a chelator or prosthetic group, and a radionuclide.

Target

The target is the molecule the compound should find: a receptor such as SSTR2, an enzyme on the cell surface such as PSMA, a transporter such as the sodium/iodide symporter, or a process such as glucose metabolism or bone turnover.

Vector

The vector binds the target. It can be a small molecule, a peptide, an antibody or antibody fragment. Size drives pharmacokinetics: small molecules and peptides clear quickly; intact antibodies circulate for days.

Chelator and radionuclide

Radiometals are held by a chelator, a cage-like molecule such as DOTA, attached to the vector through a linker. Halogens such as fluorine-18 and iodine are usually attached by covalent chemistry instead. The radionuclide is chosen to match the vector’s timing and the purpose: imaging or therapy.

In short: Theranostics images a target with one radionuclide and treats it with another, using the same or a closely related molecule.

The idea

If a diagnostic scan shows that a target is present and where, a therapeutic version of the same molecule can carry radiation to the same places. Imaging first helps select who might benefit and can show where the therapy went afterwards.

Examples

• Radioiodine: iodine-123 or iodine-124 imaging and iodine-131 therapy share identical chemistry.
• Somatostatin receptors: gallium-68 or copper-64 labelled analogues for PET; lutetium-177 labelled analogues for therapy.
• PSMA: gallium-68 or fluorine-18 ligands for PET; lutetium-177 ligands for therapy; actinium-225 ligands under study.
• True isotope pairs such as copper-64 and copper-67 keep the chemistry identical and are being researched.

Not always a perfect match

When the imaging and therapy radionuclides differ chemically, the two molecules can behave slightly differently. Imaging uptake is a guide, not a guarantee of what therapy will deliver.

In short: A guided tour of the targets most often seen in nuclear medicine and theranostics research.

SSTR2

Somatostatin receptor subtype 2 is expressed by many well-differentiated neuroendocrine tumours. Somatostatin analogues such as DOTATATE and DOTATOC carry gallium-68, copper-64 or lutetium-177.

PSMA

Prostate-specific membrane antigen is a cell-surface enzyme encoded by the FOLH1 gene. Despite its name it is also expressed in other tissues, including salivary glands and kidneys, and in the blood vessels of some other tumours. Search engines may show "PSMA" and "FOLH1" as different entities; RadioGx shows both readings.

NIS and the norepinephrine transporter

The sodium/iodide symporter lets thyroid cells take up iodide, the basis of radioiodine imaging and therapy. The norepinephrine transporter takes up MIBG, used in some neuroendocrine tumours of the adrenal medulla and nervous system.

Research targets

Fibroblast activation protein in tumour stroma, CXCR4, HER2, GRPR and others are being explored. Many agents for these targets are investigational; status is always checked live in trial registries and regulator databases.

FDG is different

FDG reflects glucose metabolism rather than a single receptor. It is not part of a theranostic pair.

In short: Why actinium-225, lead-212, terbium-161 and astatine-211 attract attention, and the practical questions they raise.

What alpha emitters change

Alpha particles deposit much more energy per micrometre than beta particles and can cause complex DNA damage that cells struggle to repair. Their short range may spare surrounding tissue but also means the compound must reach the target cells themselves.

Decay chains and daughter recoil

Actinium-225 decays through several daughters, some of which are also alpha emitters. When an atom decays, its daughter can recoil out of the chelator and travel elsewhere. How much this matters in practice is an active research question.

Supply and production

Production routes for actinium-225 and lead-212 are scaling up but remain limited. Supply affects which trials can run. RadioGx does not make supply claims beyond what public sources report.

Other candidates

Terbium-161 emits beta particles plus low-energy conversion and Auger electrons; lead-212 acts as an in vivo generator of alpha-emitting daughters; astatine-211 is a cyclotron-produced alpha emitter with its own chemistry challenges.

In short: Labels, trial records and papers name the same molecule in different ways. Here is how to read them without turning them into advice.

Many names, one molecule

A product can have a brand name, a non-proprietary name, a laboratory code and informal names such as "Lu-PSMA". Trial registries often use codes; labels use official names. RadioGx groups known aliases and shows when a match is by name rather than by identifier.

What a label is and is not

A label describes the approved use in one jurisdiction, with warnings, adverse reactions and, for radiopharmaceuticals, radiation dosimetry tables. It is written for qualified professionals. Administered activities in a label are not instructions for any individual, and approved uses differ by country.

Investigational means investigational

A compound in a trial is not approved for that use, and a positive trial is not an approval. Always check the regulator.