Chelator (bifunctional chelating agent)

A chelator (or bifunctional chelating agent, BFC/BFCA) is the part of a metal-based radiopharmaceutical that binds the radiometal ion in a stable coordination complex while a second reactive group lets it be covalently attached to a targeting molecule, such as a peptide, antibody or small molecule. The three-part architecture, radiometal plus chelator plus targeting vector (often via a linker), is the standard design of essentially all radiometal radiopharmaceuticals, from diagnostic 68Ga and 64Cu tracers to therapeutic 177Lu, 90Y and 225Ac agents.

DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) is the dominant macrocyclic chelator in the field, used with 68Ga, 111In, 86Y/90Y, 44Sc/47Sc and 177Lu, among others, because its rigid ring gives very high thermodynamic and kinetic stability once the metal is locked in; the trade-off is that labelling usually needs heating (60-95°C for 30-90 minutes), which limits its use with heat-sensitive biomolecules such as intact antibodies. Acyclic chelators (such as DTPA) and other macrocycles (such as NOTA, which forms a stable gallium-68 complex at room temperature) trade some of that ultimate stability for faster, milder labelling conditions.

A poorly matched chelator-metal pair, or incomplete complexation, shows up as radiochemical impurity: unbound or loosely bound radiometal that redistributes in the body (free gallium-68 to bone, free lutetium-177 to bone and liver) rather than following the targeting vector, degrading image quality and increasing off-target dose. Chelator choice is therefore one of the first decisions in designing any new radiometal agent and is documented in the drug's CMC (chemistry, manufacturing and controls) package.