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 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 constants of molecules to receptors.

The transport of the radioligand is described by receptor kinetics.


Radioligands are acredited for making possible the study of biomolecular behaviour, a previously mysterious area of research that had evaded researchers. [Cite book
last = Niehoff
first = Debra
authorlink = Debra Niehoff
title = The Language of Life: How cells communicate in life & disease
year = 2005
isbn = 0309089891
publisher = Joseph Henry Press
] 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

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 [Cite journal
author = Wong, Dean F.; Lever, John R.; Hartig, Paul R.; Dannals, Robert F.; Villemagne, Victor; Hoffman, Beth J.; Wilson, Alan A.; Ravert, Hayden T.; Links, Jonathan M.; et al.
title = Localization of serotonin 5-HT2 receptors in living human brain by positron emission tomography using N1-( [11C] -methyl)-2-bromo-LSD
journal = Synapse
year = 1987
volume = 1
issue = 5
pages = 393–398.
pmid = 2905532
doi = 10.1002/syn.890010502
* 18F-altanserin and 18F-setoperone for the 5-HT2A receptor [Cite journal
author = Karen H. Adams, Lars H. Pinborg, Claus Svarer, S. G. Hasselbalch, Søren Holm, Steven Haugbøl, K. Madsen, Vibe G. Frøkjær, L. Martiny Olaf B. Paulson, Gitte Moos Knudsen
title = A database of [18F] -altanserin binding to 5-HT2A receptors in normal volunteers: normative data and relationship to physiological and demographic variables
journal = NeuroImage
volume = 21
issue = 3
pages = 1105–1113
year = 2004
month = March
issn = 1053-8119
doi = 10.1016/j.neuroimage.2003.10.046
* 11C-ketanserin [Cite journal
author = J. C. Baron, Y. Samson, D. Comar, C. Crouzel, P. Deniker, Y. Agid
title = Etude in vivo des recepteurs serotoninergiques centraux chez l'homme par tomographie a positions. [In vivo study of central serotoninergic receptors in man using positron tomography]
language = French
year = 1985
journal = Revue Neurologique
volume = 141
issue = 8–9
pages = 537–545
pmid = 2935920
] and tritiated ketanserin
* 11C-DASB for the serotonin transportercite journal
author=Reimold M, Smolka MN, Zimmer A, "et al"
title=Reduced availability of serotonin transporters in obsessive-compulsive disorder correlates with symptom severity - a [11C] DASB PET study
journal=J Neural Transm
* 3H-WIN55,212-2 for cannabinoid receptorscite journal
author=Pertwee RG
title=Pharmacology of cannabinoid receptor ligands
journal=Curr. Med. Chem.
* [11C] flumazenil for GABAA receptors. [Cite journal
author = Alexander Hammers, Matthias J. Koepp, Mark P. Richardson, Rene Hurlemann, David J. Brooks & John S. Duncan
title = Grey and white matter flumazenil binding in neocortical epilepsy with normal MRI. A PET study of 44 patients
journal = Brain
volume = 126
issue = Pt 6
pages = 1300–1308
year = 2003
month = June
pmid = 12764053
url =

See also

* Binding potential
* Distribution volume
* Radioactivity in biology


Further reading

* Cite book
author = John Charles Matthews
title = Fundamentals of Receptor, Enzyme, and Transport Kinetics
publisher = CRC Press
year = 1993
isbn = 0849344263

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