[(Pyridylmethyl)sulfinyl]benzimidazoles 1 (PSBs) are a class of highly potent antisecretory (H+,K+)-ATPase inhibitors which need to be activated by acid to form their active principle, the cyclic sulfenamide 4. Selective inhibitors of the (H+,K+)-ATPase in vivo give rise to the nonselective thiophile 4 solely at low pH, thus avoiding interaction with other thiol groups in the body. The propensity to undergo the acid-catalyzed transformation is dependent on the nucleophilic/electrophilic properties of the functional groups involved in the formation of 2 since this step is both rate-determining and pH-dependent. The aim of this study was to identify compounds with high (H+,K+)-ATPase inhibitory activity in stimulated gastric glands possessing acidic pH, but low reactivity (high chemical stability) at neutral pH as reflected by in vitro (Na+,K+)-ATPase inhibitory activity. The critical influence of substituents flanking the pyridine 4-methoxy substituent present in all derivatives was carefully studied. The introduction of a 3-methoxy group gave inhibitors possessing a combination of high potency, similar to omeprazole and lansoprazole, but increased stability. As a result of these studies, compound 1a (INN pantoprazole) was selected as a candidate drug and is currently undergoing phase III clinical studies.