Gastric antisecretory activity of arbaprostil as affected by gastric pH

Arbaprostil [15(R), 15-methyl prostaglandin E₂] is inactive. In the presence of acid it is converted to the active 15(S) configuration. The degree of acidity needed for the conversion is not known. We inferred the degree of conversionin vivo in man at different gastric pHs by administering arbaprostil to normal volunteers whose gastric pH was maintained constant at various levels by intragastric titration. A pH-dependent inhibition of gastric acid secretion after stimulation with a peptone meal was observed, with 100 g inhibiting 98% of acid production at pH 2, but only 15% at pH 6. Significant gastric acid inhibition occurred with the gastric pH at, or less than, 5.