Antagonist properties of the novel antipsychotic, S16924, at cloned, humanserotonin 5-HT2C receptors: a parallel phosphatidylinositol and calcium accumulation comparison with clozapine and haloperidol

The novel benzopyranopyrrolidine and potential antipsychotic, S16924 ((+)-2 -{1-[2-(2,3-dihydro-benzo[1,4] dioxin-5-yloxy)-ethyl]-pyrrolidin-3yl}-1-(4- fluoro-phenyl)-ethanone), displays marked affinity for serotonin (5-HT)(1A) , 5-HT2A and dopamine D-2 receptors. Herein, we show that it also possesses high affinity for the cloned, INI isoform of h5-HT2C receptors (pK(i)=8.28 ) stably expressed in CHO cells. Similarly, clozapine (8.04) was a potent l igand, whereas haloperidol (<6.0) showed low affinity. As demonstrated by f ura2-detection, S16924 concentration-dependently abolished (pK(b)=7.93) the 5-HT-induced elevation in intracellular levels of Ca2+ ([Ca2+](i)) in a CH O cell line stably expressing the INI isoform of 5-HT2C receptors. Further, as determined by depletion of membrane-bound levels of pre-labelled [H-3]p hosphatidylinositols ([H-3]PI), S16924 concentration-dependently, surmounta bly and competitively blocked the activation of phospholipase C by 5-HT. Th is action was expressed with a pA(2) of 7.89 according to Schild analysis. Clozapine likewise inhibited 5-HT-induced alterations in [Ca2+](i) and [H-3 ]PI levels with pK(b)s of 7.43 and 7.84, respectively, whereas haloperidol was inactive (<5.0 in each case). Applied alone, S16924, clozapine and halo peridol modified levels of neither [Ca2+](i) nor [H-3]PI. In conclusion, in analogy to clozapine, and in contrast to haloperidol, S16924 behaves as a potent and competitive antagonist at h5-HT2C receptors, the blockade of whi ch may contribute to its distinctive functional profile of activity.