Nepaprazole is a proton pump inhibitor of substituted 2-(2-pyridinylmethylsulfinyl)-1H-benzimidazole class evaluated as useful drug for the clinical treatment of peptic ulcer diseases. The effects of the TY-11345 (nepaprazole sodium salt) on gastric mucosal proton pump (H+/K(+)-ATPase) activity, gastric acid secretion and gastro-duodenal lesions were investigated in experimental animals. TY-11345 potently inhibited H+/K(+)-ATPase activity in isolated rabbit gastric mucosal microsomes and the inhibitory effect was enhanced under weak acid conditions. In Ghosh & Schild rats, intravenous injection of TY-11345 significantly inhibited gastric acid secretion stimulated by tetragastrin; the effect of TY-11345 was twice as potent as that of omeprazole. In pylorus ligated rats, TY-11345 inhibited basal gastric acid secretion by both the intraduodenal and oral routes with 9 and 5 times more greater potency than those of omeprazole, respectively. The antisecretory effect of TY-11345 persisted for more than 24 h in pylorus ligated rats. In experimental ulcer models, TY-11345 prevented the formation of water-immersion stress, ethanol or indomethacin-induced gastric lesions and mepirizole-induced duodenal lesions in rats. The antiulcer effects of TY-11345 were 3 to 15 times more potent than those of omeprazole. These results suggested that TY-11345 had potent antisecretory and antiulcer effects which are exerted by suppression of H+/K(+)-ATPase activity in gastric parietal cells. Nepaprazole sodium had been studied in phase II clinical trials for treatment of gastric ulser but this research has been discontinued.