Ketanserin and Spiperone as Templates for Novel Serotonin 5-HT2A Antagonists

The structures of ketanserin (1) and spiperone (2) were examined in detail to determine the role of various substituent groups on 5-HT2A receptor affinity and selectivity. It was found that the presence of the quinazoline ring of ketanserin detracts from selectivity and that various ring-opened analogs displayed ketanserin-like affinity and up to 30-fold enhanced selectivity. The triazaspirodecanone portion of spiperone is a major determinant of its 5-HT2A affinity and selectivity. The conformational rigidity imposed by the ring, as well as the nature of the N1-substituent, are important factors in controlling binding at 5-HT2A, 5-HT2C, 5-HT1A, and dopamine D2 receptors. Replacement of the N1-phenyl ring of spiperone with a methyl group (KML-010 48) resulted in a compound that binds at 5-HT2A receptors with slightly lower affinity than spiperone, but that lacked affinity (Ki >10,000 nM) for 5-HT2C and 5-HT1A receptors and binds with 400-fold reduced affinity at D2 receptors.