Altanserin
Altanserin is a compound that binds to the 5-HT receptor (5-Hydroxytryptamine (serotonin) 2A receptor). Labeled with the isotope fluorine-18 it is used as a radioligand in positron emission tomography (PET) studies of the brain, i.e., studies of the 5-HT neuroreceptors. Besides human neuroimaging studies altanserin has also been used in the study of rats. An alternative for PET imaging the 5-HT receptor is the [C]volinanserin (MDL-100,907) radioligand. F-altanserin and H-volinanserin have shown very comparable binding. Both altanserin and MDL 100,907 are 5-HT receptor antagonists. [F]-setoperone can also be used in PET. An  alternative SPECT radioligand is the [I]-5-I-R91150 receptor antagonist. A rapid chemical synthesis of fluorine-18 and H-2 dual-labeled altanserin has been described. Other ligands for other parts of the serotonin system used in PET studies are, e.g., DASB, ketanserin, and WAY-100635. Human brain mapping studies with altanserin. altanserin is probably not used in clinical routine. However, there have been performed several research-based neuroimaging studies with the compound in humans since the 1990s. Some of these studies have considered methodogical issues such as the reproducibility of the method or whether to use constant infusion or bolus-infusion delivery  of altanserin. Other studies have compared altanserin binding to subject variables such as age, personality trait and neuropsychiatric disorder. The altanserin PET scan shows high binding in neocortex. The cerebellum is often regarded as a region with no specific 5-HT binding and the brain region is used as a reference in some studies, even though an autoradiography study has found nonnegligible levels of 5-HT binding in the human cerebellum, and another type of study have observed strong immunoreaction against 5-HT receptor protein in rat Purkinje cells. In the table below is an overview of the results of  altanserin binding seen in human PET-studies. A consistent finding across altanserin studies has been that the binding decreases with age. This is in line with "in vitro" studies of the 5-HT receptor, as well as PET studies with other radioligands that binds to the receptor. The result for recovered bulimia-type anorexia nervosa is in line with a SPECT study of anorexia nervosa patients, that found a decrease in frontal, occipital and parietal cortices. The results of PET studies of the 5-HT in depression has been mixed. Altanserin binding has also been examine in twins, where one  study showed higher correlation between monozygotic twin pairs than between dizygotic twin pairs, giving evidence that the binding is "strongly genetically determined". Synthesis. Condensation of ethyl anthranilates with isothiocyanates provides entry to a closely related compound in which the carbonyl at the 2 position is replaced by a thione. The sequence starts with the alkylation of pyrrolidine nitrogen in (1) with 2-bromoethylamine. Reaction of the primary amine in the product (2) with thiophosgene leads to the isothiocyanate derivative (3). Reaction of that reactive intermediate with methyl anthranilate (4) leads initially to the transient addition product (5). This then undergoes  the customary internal ester exchange reaction to form the heterocyclic ring. There is thus obtained the serotonin antagonist "altanserin" (6).