H(+),K(+)-ATPase is a key enzyme in the process of gastric acid secretion, and proton pump inhibitors (PPIs) have been accepted as one of the most effective treatments for peptic ulcer and gastroesophageal reflux disease. To discover a novel class of PPIs, the authors screened a low-molecular-weight compound library and identified two prospective acid blockers that were pyrrole derivatives. Both compounds inhibited H(+),K(+)-ATPase in a reversible and potassium-competitive manner. These compounds led to the development of TAK-438 (1-[5-(2-fluorophenyl)-1-(pyridin-3-ylsulfonyl)-1H-pyrrol-3-yl]-N-methylmethanamine monofumarate), which is currently undergoing clinical trials as a novel potassium-competitive acid blocker for the treatment of acid-related diseases.