radioligands

Radioligands are ligands that carry a radioactive isotope and are used to visualize and quantify biological targets in living organisms. By binding selectively to receptors, transporters, or enzymes, radioligands enable in vivo imaging of target distribution, receptor density, and drug occupancy. They are a subset of radiopharmaceuticals designed to retain their binding properties after radiolabeling.

Radiolabeling attaches a radioactive isotope to a chemical that preserves the pharmacological activity of the ligand. Common PET isotopes include carbon-11, nitrogen-13, oxygen-15, and fluorine-18; SPECT commonly uses technetium-99m and iodine-123. Labeling strategies vary from incorporating the isotope into the ligand’s core to attaching a short-lived radiolabeling prosthetic group, with the aim to maintain affinity, selectivity, and in vivo stability.

Radioligands are used primarily with PET or SPECT to study neural and peripheral targets. In neuroimaging, they map neurotransmitter receptors and transporters, assess receptor occupancy by drugs, and monitor disease-related changes. In other tissues they image receptor expression, enzyme activity, or transporter function, aiding research and clinical diagnosis.

Key properties include high affinity and selectivity for the target, appropriate lipophilicity to reach the site of action, favorable pharmacokinetics, and metabolic stability to minimize radiolabeled metabolites. Because the isotope imposes a short time window, study design must align with radioisotope half-lives. Radioligands require regulatory approval and careful handling to manage radiation exposure.