Pumosetrag Hydrochloride (MKC-733; DDP-733) is an orally available 5-HT3 partial agonist developed for the treatment of irritable bowel syndrome and gastroesophageal reflux disease. IC50 & Target: 5-HT3^[1] In Vivo: Pumosetrag displays both regional and species specificities. Pumosetrag has a lower efficacy than 5-HT in the rat jejunum, ileum and distal colon; however, it has similar efficacy and potency to 5-HT in the rat proximal colon. The activity profile of Pumosetrag is different in the guinea pig intestine where it exhibits greater potency and efficacy than 5-HT in all regions. Pumosetrag shows little to no response in the regions of the mouse intestine. Responses to Pumosetrag in the rat and guinea pig tissues are inhibited by ondansetron, confirming its action on 5-HT(3) receptors^[1]. Pumosetrag delays liquid gastric emptying in association with relaxation of the proximal stomach, stimulates fasting antroduodenal migrating motor complex activity and accelerates small intestinal transit^[2].