5-I-R91150 (or R93274) is a compound that acts as a potent and selective

antagonist An antagonist is a character in a story who is presented as the chief foe of the protagonist. Etymology The English word antagonist comes from the Greek ἀνταγωνιστής – ''antagonistēs'', "opponent, competitor, villain, enemy, riv ...

of 5-HT_2A receptors. Its main application is as its iodine-123 radiolabeled form, in which it can be used in SPECT scanning in human neuroimaging studies, to examine the distribution of the 5-HT_2A receptor subtype in the brain, e.g. with respect to sex and age and in adults with Asperger syndrome or

Alzheimer's disease Alzheimer's disease (AD) is a neurodegeneration, neurodegenerative disease that usually starts slowly and progressively worsens. It is the cause of 60–70% of cases of dementia. The most common early symptom is difficulty in short-term me ...

. An alternative 5-HT_2A receptor ligand also used in neuroimaging is altanserin.

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