A series of 52 conformationally constrained butyrophenones have been synthesized and pharmacologically tested as antagonists at 5-HT2A, 5-HT2B, and 5-HT2C serotonin receptors, useful for dissecting the role of each 5-HT2 subtype in pathophysiology. These compounds were also a consistent set for the identification of structural features relevant to receptor recognition and subtype discrimination. Six compounds were found highly active (pK i > 8.76) and selective at the 5-HT2A receptor vs 5-HT2B and/or 5-HT2C receptors. Piperidine fragments confer high affinity at the 5-HT2A receptor subtype, with benzofuranone- and thiotetralonepiperidine as the most selective derivatives over 5-HT2C and 5-HT2B receptors, respectively; K i  2A/2C and/or K B  2A/2B ratios greater than 100 were obtained. Compounds showing a more pronounced selectivity at 5-HT2A/5-HT2C than at 5-HT2A/5-HT2B bear 6-fluorobenzisoxazolyl- and p-fluorobenzoylpiperidine moieties containing one methylene bridging the basic piperidine to the alkanone moiety. An ethylene bridge between the alkanone and the amino moieties led to ligands with higher affinities for the 5-HT2B receptor. Significant selectivity at the 5-HT2B receptor vs 5-HT2C was observed with 1-1[(1-oxo-1,2,3,4-tetrahydro-3-naphthyl)methyl]-4-[3-(p-fluorobenzoyl)propyl]piperazine (more than 100-fold higher). Although piperidine fragments also confer higher affinity at 5-HT2C receptors, only piperazine-containing ligands were selective over 5-HT2A. Moderate selectivity was observed at 5-HT2C vs 5-HT2B (10-fold) with some compounds bearing a 4-[3-(6-fluorobenzisoxazolyl)]piperidine moiety in its structure. Molecular determinants for antagonists acting at 5-HT2A receptors were identified by 3D-QSAR (GRID-GOLPE) studies. Docking simulations at 5-HT2A and 5-HT2C receptors suggest a binding site for the studied type of antagonists (between transmembrane helices 2, 3, and 7) different to that of the natural agonist serotonin (between 3, 5, and 6).

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