radioligand

Radioligand is a ligand that has been labeled with a radioactive isotope, enabling the detection, localization, and quantification of biological targets in living systems or in vitro experiments. The radiolabel allows the ligand and its bound target to be visualized by imaging techniques or measured through radiometric methods.

Radioligands are used primarily for imaging or therapeutic purposes. Imaging radioligands are designed to bind selectively to receptors, transporters, or enzymes, and are detected with positron emission tomography (PET) or single-photon emission computed tomography (SPECT). Common imaging isotopes include carbon-11 and fluorine-18 for PET, and iodine-123 or technetium-99m for SPECT. Therapeutic radioligands use heavier beta- or alpha-emitting isotopes to deliver cytotoxic radiation to target cells, such as lutetium-177 or actinium-225, and are a component of radioligand therapy (RLT).

Applications include clinical imaging to diagnose and stage diseases, functional mapping in neurology and oncology, and research using in vitro radioligand binding assays to determine target affinity and density (Kd and Bmax). The theranostic approach uses the same ligand labeled with different isotopes for diagnosis and treatment, as in PSMA or somatostatin receptor-targeted ligands.

Production and handling require specialized radiochemistry facilities, rapid synthesis and quality control due to short half-lives, and strict regulatory oversight to limit radiation exposure and ensure patient safety. Availability is limited by isotope half-life, supply chains, and regulatory approvals.

Notable radioligands include 11C-raclopride for dopamine D2 receptors, 68Ga-DOTATATE for somatostatin receptor imaging in neuroendocrine tumors, 18F-FDOPA for dopaminergic function, 123I-MIBG for adrenergic innervation, and radioligands used in radioligand therapy such as 177Lu-PSMA-617 and 177Lu-DOTATATE.