RS 42358-197, a novel and potent 5-HT3 receptor antagonist, in vitro and in vivo.

The pharmacological activity of RS 42358-197, a novel 5-HT3 receptor antagonist has been evaluated in vitro and in vivo. In functional experiments in vitro, RS 42358-197 behaved as a competitive antagonist against 5-HT-induced contractions in the guinea pig ileum (low-potency phase), yielding a pA2 estimate of 8.1. RS 42358-197 was devoid of any agonistic or antagonistic activity at 5-HT1-like receptors (contraction of canine saphenous vein), 5-HT2 receptors (contraction of rabbit aorta) or 5-HT4 receptors (contraction of guinea pig ileum, high-potency phase). RS 42358-197 failed to affect the concentration-effect curve to substance P in guinea pig ileum. In anesthetized rats. RS 42358-197, administered by the intravenous, intraduodenal or transdermal route, dose-dependently inhibited the Bezold-Jarisch reflex induced by 2-methyl 5-HT (ID50:0.05 micrograms/kg; i.v., 5.7 micrograms/kg; i.d., and 11.6 micrograms/chamber, respectively). In this regard, when administered intraduodenally, RS 42358-197 was more potent and exhibited a longer duration of action than either ondansetron or granisetron. In dogs, RS 42358-197, administered either intravenously or orally, dose-dependently inhibited the emesis induced by cisplatin, actinomycin and cyclophosphamide, but not that induced by apomorphine. When tested at maximally effective doses against cisplatin-induced emesis in dogs, RS 42358-197 had a longer duration of antiemetic activity (> 6 h) than ondansetron (2 h). RS 42358-197, administered orally, also afforded protection against cisplatin-induced emesis in ferrets. At doses that showed marked anti-emetic activity in dogs (10-100 micrograms/kg; i.v. and 100-1000 micrograms/kg; i.d.), RS 42358-197 did not produce any hemodynamic changes.(ABSTRACT TRUNCATED AT 250 WORDS)