Effect of Repeated Doses of YF476, Omeprazole and Placebo on Stomach Acidity in Healthy Volunteers

Inclusion Criteria: * Male or female aged 18-45 years. * No clinically relevant abnormal findings in the clinical history or physical examination at the screening assessment which could interfere with the objectives of the study or make the subject's participation hazardous. * No clinically relevant abnormal laboratory values at the screening evaluation (Attachment 2). * A normal ECG at the screening examination. * A body mass index (Quetelet index) in the range 19-30: Body Mass Index = weight \[kg\]\_ height \[m\]2 * Normal blood pressure and heart rate at the screening examination, i.e. BP 90-150mmHg systolic, 40-95mmHg diastolic; heart rate 40-100 beats/min in seated position. * Subjects must be of sufficient intelligence to understand the nature of the study and any hazards of their participation in it. They must be able to communicate satisfactorily with the Investigator and to participate in, and comply with the requirements of, the entire study. * Subjects must give their written consent to participate after reading the Information-for-Volunteers Leaflet and Consent Form, and after having the opportunity to discuss the study with the Investigator or his deputy. Exclusion Criteria: * Females who are pregnant or lactating, or who are sexually active and are not using a reliable method of contraception. * Clinically relevant abnormal history or physical findings at the screening assessment, which could interfere with the objectives of the study or the safety of the subject's participation. * Clinically relevant abnormalities of laboratory values or ECG at screening evaluation. * Presence of acute or chronic illness or history of chronic illness sufficient to invalidate subject's participation in the study or make it unnecessarily hazardous. * Impaired endocrine, thyroid, hepatic, respiratory or renal function, diabetes mellitus, coronary heart disease or history of any psychotic mental illness. * Participation in other clinical studies of a new chemical entity or a prescription medicine within the previous 3 months. * Presence or history of drug or alcohol abuse, or intake of more than 40 units of alcohol weekly. * Loss of more than 400mL blood during the 3 months before the study, e.g. as a blood donor. * Use of prescription medication during 30 days before the study. * Use of an over-the-counter medicine during 7 days before the study * Blood pressure or heart rate outside those values specified under inclusion criterion (f). * Possibility that the subject will not cooperate with the requirements of the protocol. * Evidence of drug abuse on urine testing at study entry. * Positive test for hepatitis B or C or HIV 1 \& 2. * High risk of hepatitis or HIV infection. * History of severe allergic disease.