The objectives of this study are:

• To evaluate the safety and tolerability of single and multiple oral doses of CJ-12420 in healthy male subjects.
• To evaluate the single- and multiple-dose PK of orally administered CJ-12420 in healthy male subjects.
• To evaluate the single- and multiple-dose PD of orally administered CJ-12420 in healthy male subjects.
• To compare the multiple-dose pharmacodynamics (PD) of orally administered CJ-12420 and esomeprazole in healthy male subjects.

Inclusion Criteria:

• Male volunteers in the age between 20 and 45 years old (inclusive)
• Body mass index (BMI) in the range of 19 to 28 kg/m2 and weighing at least 50 kg
• Medically healthy with no clinically significant vital signs (sitting position blood pressure, pulse rate) (90 mmHg ≤ systolic blood pressure ≤ 140 mmHg, 50 mmHg ≤ diastolic blood pressure ≤ 95 mmHg, 45 beats per minute ≤ pulse rate ≤ 95 beats per minute)
• Non-smokers or non-users of nicotine-containing products for at least 3 months

Exclusion Criteria:

• History of clinically significant gastrointestinal, renal, hepatic, neurologic, hematologic, endocrine, oncologic, pulmonary, immunologic, psychiatric, musculoskeletal or cardiovascular disease or any other condition, which, in the opinion of the Investigator, would jeopardize the safety of the subject or impact the validity of the study results
• History of allergy or sensitivity to any drug, including any prior serious adverse reaction to PPIs (e.g. omeprazole, rabeprazole, lansoprazole)
• History of symptomatic GERD, erosive esophagitis, duodenal ulcer, gastric ulcer, Barrett's esophagus or Zollinger-Ellison syndrome, previous positive H. pylori