Novel potent 5-HT3 receptor ligands based on the pyrrolidone structure: synthesis, biological evaluation, and computational rationalization of the ligand-receptor interaction modalities

Novel conformationally constrained derivatives of classical 5-HT3 receptor antagonists were designed and synthesized with the aim of probing the central 5-HT3 receptor recognition site in a systematic way. The newly-synthesized compounds were tested for their potential ability to inhibit [H-3]granisetron specific binding to 5-HT3 receptor in rat cortical membranes. These studies revealed subnanomolar affinity in some of the compounds under study. The most potent ligand in this series was found to be quinuclidine derivative (S)-7i, which showed an affinity comparable with that of the reference ligand granisetron. The potential 5-HT3 agonist/antagonist activity of some selected compounds was assessed in vitro on the 5-HT3 receptor-dependent [C-14]guanidinium uptake in NG 108-15 cells. Both of the tropane derivatives tested in this functional assay (7a and 9a) showed antagonist properties, while the quinuclidine derivatives studied [the enantiomers of compounds 7i, 8g, and 9g, and compound (R)-8h] showed a full range of intrinsic efficacies. Therefore, the functional behavior of these 5-HT3 receptor ligands appears to be affected by the structural features of both the azabicyclo moiety and the heteroaromatic portion. In agreement with the data obtained on NG 108-15 cells, investigations on the 5-HT3 receptor-dependent Bezold-Jarisch reflex in urethane-anaesthetized rats confirmed the 5-HT3 receptor antagonist properties of compounds 7a and (S)-7i showing for these compounds ID50 values of 2.8 and 181 mug/kg, respectively. Finally, compounds 7a, (S)-7i and 9a (at the doses of 0.01, 1.0, and 0.01 mg/kg ip, respectively) prevented scopolamine-induced amnesia in the mouse passive avoidance test suggestive of a potential usefulness in cognitive disorders for these compounds. Qualitative and quantitative structure-affinity relationship studies were carried out by means of theoretical descriptors derived on a single structure and ad-hoc defined size and shape descriptors (indirect approach). The results showed to be useful in capturing information relevant to ligand-receptor interaction. Additional information derived by the analysis of the energy minimized 3-D structures of the ligand-receptor complexes (direct approach) suggested interesting mechanistic and methodological considerations on the binding mode multiplicity at the 5-HT3 receptors and on the degree of tolerance allowed in the alignment of molecules for the indirect approach, respectively.