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To evaluate safety, tolerability, pharmacokinetics, and pharmacodynamics of TAK-438 (vonoprazan, a potassium-competitive acid blocker) in healthy male subjects.In two phase I, randomized, double-blind, placebo-controlled, single rising-dose studies, healthy male subjects (JapanN=84; UKN=63) received a single TAK-438 dose (1–120 mg in Japan and 1–40 mg in the UK). Assessments included safety, tolerability, pharmacokinetics, and pharmacodynamics (intragastric pH).Plasma concentration–time profiles of TAK-438 at all dose levels showed rapid absorption (medianTmax up to 2 h). Estimated mean elimination half-life was up to 9 h. Exposure was slightly greater than dose proportional. No clear difference in TAK-438 pharmacokinetics was observed between Japanese and non-Japanese subjects. Acid suppression was dose dependent and similar in both studies. The 24-h intragastric pH ≥4 holding time ratio with 40 mg TAK-438 was 92% in Japan and 87% in the UK. TAK-438 was well tolerated, with no adverse events reported in Japanese subjects; 10 of 63 UK subjects experienced 12 treatment-emergent adverse events (non-serious). Increases in serum gastrin and pepsinogen I and II concentrations were observed at doses ≥10 mg, but there were no changes in alanine aminotransferase concentrations.Single oral doses of TAK-438 20–120 mg caused rapid, profound, and 24-h suppression of gastric acid secretion in healthy male subjects, regardless of geographical region, and TAK-438 was well tolerated at all doses studied, making it a potential alternative to proton pump inhibitors for the treatment of acid-related disorders.