CP2289, a new 5-HT3 receptor ligand: agonistic activities on gastroenteric motility

5-Fluorotryptamine is a partial agonist at 5-HT3 receptors, and reveals that size and electronegativity at the 5 position of tryptamine are critical for efficient receptor function

Antagonists, but not agonists, of the 5-HT3 receptor are useful therapeutic agents, and it is possible that partial agonists may also be potentially useful in the clinic. Here we show that 5-fluorotryptamine (5-FT) is a partial agonist at both 5-HT3A and 5-HT3AB receptors with an Rmax (Imax/Imax 5-HT) of 0.64 and 0.45 respectively. It is about 10 fold less potent than 5-HT: EC50=16 and 27 microM, and Ki for displacement of [3H]granisetron binding=0.8 and 1.8 microM for 5-HT3A and 5-HT3AB receptors respectively. We have also explored the potencies and efficacies of tryptamine and a range of 5-substituted tryptamine derivatives. At 5-HT3A receptors tryptamine is a weak (Rmax=0.15), low affinity (EC50=113 microM; Ki=4.8 microM) partial agonist, while 5-chlorotryptamine has a similar affinity to 5-FT (EC50=8.1 microM; Ki=2.7 microM) but is a very weak partial agonist (Rmax=0. 0037). These, and data from 5-methyltryptamine and 5-methoxytryptamine, reveal the importance of size and electronegativity at this location for efficient channel opening.

Regulatory molecules for the 5-HT3 receptor ion channel gating system

Substituted benzoxazole derivatives which possess a nitrogen-containing heterocycle at C2 are selective partial agonists of the 5-HT(3) receptor. Alteration of substituents on the benzoxazole nucleus affords both agonist-like and antagonist-like compounds, and uniquely modifies the function of the 5-HT(3) receptor ion channel gating system. SAR and corroborative computational docking study for these partial agonists successfully explained structure and function of the 5-HT(3) receptor.

Comparison between partial agonist (ME3412) and antagonist (alosetron) of 5-hydroxytryptamine 3 receptor on gastrointestinal function

Therapeutic use of 5-hydroxytryptamine 3 (5-HT(3)) receptor antagonists for diarrhoea-predominant irritable bowel syndrome may be accompanied by constipation. We hypothesized that ME3412, 5-chloro-2-(1,4-diazacycloheptan-1-yl)-7-methylbenzoxazole, a novel partial agonist of the 5-HT(3) receptor, would minimize constipation without reducing antidiarrhoeal activity. Receptor binding studies showed that ME3412 is highly selective for the human 5-HT(3) receptor (K(i) = 1.51 nmol L(-1)). A 5-HT(3) receptor agonist, 2-methyl-5-HT, caused contractile response in the isolated guinea-pig ileum and accelerated secretion in the guinea-pig colonic mucosal preparation. ME3412 and 5-HT(3) receptor antagonist, alosetron, antagonized the 2-methyl-5-HT-induced responses with similar potency in insurmountable and surmountable manner, respectively. ME3412 caused weak agonism in isolated ileum strips and also in the colonic mucosa with intrinsic activity of 0.09 and 0.59, respectively. In conscious dogs, alosetron (3 microg kg(-1) i.v.) suppressed the migrating motor complex (MMC), whereas a relatively high dose (300 microg kg(-1)) of ME3412 was required for inhibition of MMC. ME3412 and alosetron suppressed 5-HT induced-diarrhoea in mice. In contrast, ME3412 did not significantly affect colonic propulsion compared with alosetron. These results imply that the partial agonist may relieve diarrhoea with low risk of inducing constipation.

Synthesis and pharmacological characterization of a new benzoxazole derivative as a potent 5-HT3 receptor agonist

N-(2-Benzoxazol-2-yl-ethyl)-guanidine hydrochloride (10) was synthesized and pharmacologically tested. This compound showed high affinity for the 5-HT(3) receptor (K(i)=0.77 nM) and potently triggered the von Bezold-Jarisch reflex (BJR) in rats with an ED(50)=0.52 microg/kg iv and intrinsic activity next to 1 (i.a.=0.94). This stimulant effect was abolished by pretreatment with the 5-HT(3) receptor antagonist granisetron and was subject to a rapid and pronounced tachyphylaxis, due to desensitization of the peripheric cardiac 5-HT(3) receptor. Consequently, 10 acts as an in vivo 5-HT(3) antagonist inhibiting the BJR responses evoked by submaximal doses of 5-HT with an ID(50)=5.8 microg/kg iv.