Shinobu Akuzawa

Effects of serotonin 5-HT3 receptor antagonists on stress-induced colonic hyperalgesia and diarrhoea in rats: a comparative study with opioid receptor agonists, a muscarinic receptor antagonist and a synthetic polymer.

Abstractu2002 In this study, we examined the effects of serotonin (5‐HT)3 receptor antagonists (5‐HT3RAs) including ramosetron, alosetron, and cilansetron on colonic nociceptive threshold in rats. Furthermore, we established a restraint stress‐induced colonic hyperalgesia model in rats, and compared the inhibitory effects of 5‐HT3RAs on restraint stress‐induced colonic hyperalgesia and diarrhoea with those of loperamide, trimebutine, tiquizium and polycarbophil. The colonic nociceptive threshold was measured as the balloon pressure at the time the rat showed a nociceptive response during colonic distension by an intrarectally inserted balloon. Oral administration of ramosetron (3–30u2003μgu2003kg−1), alosetron (30–300u2003μgu2003kg−1), or cilansetron (30–300u2003μgu2003kg−1) increased the colonic nociceptive threshold in a dose‐dependent manner in non‐stressed rats. Restraint stress for 1u2003h significantly decreased the colonic nociceptive threshold, but ramosetron (0.3–3u2003μgu2003kg−1), alosetron (3–30u2003μgu2003kg−1), cilansetron (3–30u2003μgu2003kg−1) and trimebutine (100–1000u2003mgu2003kg−1) significantly inhibited the decrease in the threshold. Loperamide (3–30u2003mgu2003kg−1), tiquizium (100–1000u2003mgu2003kg−1) and polycarbophil (1000u2003mgu2003kg−1) did not affect the restraint stress‐induced decrease in the colonic nociceptive threshold. All drugs tested in this study showed dose‐dependent inhibition of restraint stress‐induced diarrhoea in rats. These results indicate that, unlike existing antidiarrhoeal and spasmolytic agents, 5‐HT3RAs have inhibitory effects on colonic nociception, and prevented restraint stress‐induced both diarrhoea and hyperalgesia at almost the same doses in rats. This suggests that the 5‐HT3RAs may be useful in ameliorating both colonic hyperalgesia and diarrhoea in patients with irritable bowel syndrome.

Effects of serotonin 5-HT3 receptor antagonists on CRF-induced abnormal colonic water transport and defecation in rats

The effects of corticotropin releasing factor (CRF) and serotonin (5-HT)(3) receptor antagonists on intestinal water transport are not well understood. Hence, we established a CRF-induced abnormal water transport model in rat colon, and evaluated the effects of 5-HT(3) receptor antagonists including ramosetron, alosetron, and cilansetron, and the antidiarrheal agent loperamide, in this model. In addition, the effects of 5-HT(3) receptor antagonists and loperamide on abnormal defecation induced by CRF in rats were examined. Colonic water transport was measured in colonic loops in conscious rats. Centrally administered CRF (3-30 microg/kg) markedly decreased colonic fluid loss, whereas oral administration of ramosetron (3, 30 microg/kg), alosetron (300 microg/kg), cilansetron (300 microg/kg), or loperamide (3 mg/kg) significantly inhibited it. Ramosetron (1-10 microg/kg), alosetron (10-100 microg/kg), cilansetron (10-100 microg/kg), or loperamide (0.3-3 mg/kg) also showed dose-dependent inhibition of CRF-induced defecation in rats. These results suggest that 5-HT(3) receptors are involved in both abnormal colonic water transport and defecation induced by CRF, and that the inhibitory effects of 5-HT(3) receptor antagonists on CRF-induced abnormal defecation partly result from their ameliorating action on colonic water transport.

Synthesis and structure-activity relationships of new carbonyl guanidine derivatives as novel dual 5-HT2B and 5-HT7 receptor antagonists.

We previously reported that the novel dual 5-HT₂B and 5-HT7 receptor antagonist N-(9-hydroxy-9H-fluorene-2-carbonyl)guanidine (4) exerted a suppressing effect on 5-HT-induced dural protein extravasation in guinea pigs. To develop a synthetic strategy, we performed docking studies of lead compound 4 bound to 5-HT₂B and 5-HT₇ receptors, and observed that the carbonyl guanidine group forms a tight interaction network with an active center Asp (D135:5-HT2B, D162:5-HT₇), Tyr (Y370:5-HT₂B, Y374:5-HT₇) and aromatic residue (W131:5-HT2B, F158:5-HT₇). Based on molecular modeling results, we optimized the substituents at the 5- to 8-position and 9-position of the fluorene ring and identified N-(diaminomethylene)-9-hydroxy-9-methyl-9H-fluorene-2-carboxamide (24a) exhibits potent affinity for 5-HT₂B (Ki=4.3 nM) and 5-HT7 receptor (Ki=4.3 nM) with high selectivity over 5-HT₂A, 5-HT₂C, α₁, D₂ and M₁ receptors. Compound 24a reversed the hypothermic effect of 5-carboxamidotryptamine (5-CT) in mice and also showed a suppressing effect on 5-HT-induced dural protein extravasation in guinea pigs when orally administered at 30 mg/kg. Compound 24a is therefore a promising candidate for a novel class of anti-migraine agent without any adverse effects.

Synthesis and pharmacological evaluation of optically pure, novel carbonyl guanidine derivatives as dual 5-HT2B and 5-HT7 receptor antagonists

A series of 9-disubstituted N-(9H-fluorene-2-carbonyl)guanidine derivatives have been discovered as potent and orally active dual 5-HT(2B) and 5-HT(7) receptor antagonists. Upon screening several compounds, N-(diaminomethylene)-4,5-dihydro-3H-spiro[fluorene-9,2-furan]-2-carboxamide (17) exhibited potent affinity for both 5-HT(2B) (Ki = 5.1 nM) and 5-HT(7) (K(i) = 1.7 nM) receptors with high selectivity over 5-HT(2A), 5-HT(2C), α(1), D(2) and M(1) receptors. Optical resolution of the intermediate carboxylic acid 16 via the formation of diastereomeric salts using chiral alkaloids gave the optically pure compounds (R)-17 and (S)-17. Both enantiomers suppressed 5-HT-induced dural protein extravasation in guinea pigs in a dose-dependent manner and the amount of leaked protein was suppressed to near normal levels when orally administrated at 10 mg/kg. (R)-17 and (S)-17 were therefore selected as candidates for human clinical trials.