COMPARATIVE RECEPTOR MAPPING OF SEROTONINERGIC 5-HT3 AND 5-HT4 BINDING-SITES

The clinical use of currently available drugs acting at the 5-HT4 rece ptor has been hampered by their lack of selectivity over 5-HT3 binding sites. For this reason, there is considerable interest in the medicin al chemistry of these serotonin receptor subtypes, and significant eff ort has been made towards the discovery of potent and selective ligand s. Computer-aided conformational analysis was used to characterize ser otoninergic 5-HT3 and 5-HT4 receptor recognition, On the basis of the generally accepted model of the 5-HT3 antagonist pharmacophore, we hav e performed a receptor mapping of this receptor binding site, followin g the active analog approach (AAA) defined by Marshall. The receptor e xcluded volume was calculated as the union of the van der Waals densit y maps of nine active ligands (pK(i) greater than or equal to 8.9), su perimposed in pharmacophoric conformations. Six inactive analogs (pK(i ) < 7.0) were subsequently used to define the essential volume, which in its turn can be used to define the regions of steric intolerance of the 5-HT3 receptor. Five active ligands (pK(i) greater than or equal to 9.3) at 5-HT4 receptors were used to construct an antagonist pharma cophore for this receptor, and to determine its excluded volume by sup erimposition of pharmacophoric conformations. The volume defined by th e superimposition of five inactive 5-HT4 receptor analogs that possess the pharmacophoric elements (pK(i) less than or equal to 6.6) did not exceed the excluded volume calculated for this receptor. In this case , the inactivity may be due to the lack of positive interaction of the amino moiety with a hypothetical hydrophobic pocket, which would inte ract with the voluminous substituents of the basic nitrogen of active ligands. The difference between the excluded volumes of both receptors has confirmed that the main difference is indeed in the basic moiety. Thus, the 5-HT3 receptor can only accommodate small substituents in t he position of the nitrogen atom, whereas the 5-HT4 receptor requires more voluminous groups. Also, the basic nitrogen is located at ca. 8.0 Angstrom from the aromatic moiety in the 5-HT4 antagonist pharmacopho re, whereas this distance is ca. 7.5 Angstrom in the 5-HT3 antagonist model. The comparative mapping of both serotoninergic receptors has al lowed us to confirm the three-component pharmacophore accepted for the 5-HT3 receptor, as well as to propose a steric model for the 5-HT4 re ceptor binding site. This study offers structural insights to aid the design of new selective ligands, and the resulting models have receive d some support from the synthesis of two new active and selective liga nds: 24 (K-i(5-HT3) = 3.7 nM; K-i(5-HT4) > 1000 nM) and 25 (K-i(4-HT4) = 13.7 nM; K-i(5-HT3) > 10 000 nM).