High affinity binding of azasetron hydrochloride to 5-hydroxytryptamine(3) receptors in the small intestine of rats

The binding affinity of azasetron hydrochloride (azasetron) for the 5-hydroxytryptamine(3) (5-HT3) receptor in a tissue preparation of rat small intestine was investigated by using [H-3]granisetron as a radioligand. Scatchard analysis of specific [H-3]granisetron binding revealed a single population of saturable binding sites in the tissue preparation. At this site, azasetron was concentration-dependently competitive with [3H]granisetron, and it inhibited the specific [H-3]granisetron binding with a K-i value of 0.33 nM. Azasetron has a high affinity for 5-HT3 receptor in the gastrointestinal organ, the very site of its antiemetic action against chemotherapy-induced emesis.