Radioligand

A radioligand is a radioactive biochemical substance (in particular, a ligand) that is used for diagnosis or for research-oriented study of the receptor systems of the body.

In a neuroimaging application the radioligand is injected into the pertinent tissue, or infused into the bloodstream. It binds to its receptor. When the radioactive isotope in the ligand decays it can be measured by positron emission tomography (PET) or single photon emission computed tomography (SPECT). In [in vivo] systems it is often used to quantify the binding of a test molecule to the binding site of radio ligand. The higher the affinity of the molecule the more radio ligand is displaced from the binding site and the increasing radioactive decay can be measured by scintillography. This assay is commonly used to calculate binding constant of molecules to receptors.

The transport of the radioligand is described by receptor kinetics.

History
Radioligands are credited with making possible the study of biomolecular behaviour, a previously mysterious area of research that had evaded researchers. With this capacity radioligand techniques enabled researchers to identify receptor devices within cells.

Radioactive isotopes commonly used

 * Tritium, 3H
 * Carbon-14, 14C
 * Sulfur-35, 35S
 * Iodine-131, 131I
 * Fluorine-18, 18F
 * Technetium-99m, 99mTc

In PET the isotopes fluorine-18 and carbon-11 are often used in molecular neuroimaging.

List of radioligands
Radioligands may be constructed to bind selectively to a particular neuroreceptor or a particular neurotransmitter transporter. Examples of radioligands include:
 * 11C-WAY-100635 for the 5-HT1A receptor
 * N(1)-([11C]-methyl)-2-Br-LSD ([11C]-MBL) for 5-HT2 receptors
 * 18F-altanserin and 18F-setoperone for the 5-HT2A receptor
 * 11C-ketanserin and tritiated ketanserin
 * 11C-DASB for the serotonin transporter
 * 3H-WIN55,212-2 for cannabinoid receptors
 * [11C]flumazenil for GABAA receptors.
 * (+)PHNO for D2 dopamine receptors.
 * [11C]raclopride for D2 dopamine receptors.