Comparative study of the pharmacodynamic parameters and pharmacodynamic equivalence (bioequivalence) of drug Antareit 800 mg/10 ml oral suspension and drug Riopan 800 mg chewable tablets in healthy volunteers.

Inclusion Criteria:

1. Voluntary and handwritten informed consent form signed by a healthy volunteer to participate in the study prior to any of the study procedures;
2. Healthy male and female caucasian volunteers aged 18 to 45 years (inclusive);
3. Verified diagnosis "healthy" (without abnormal findings in the protocol-defined clinical, laboratory, and instrumental test data);
4. pH according to hourly pH-metry in the screening period, carried out at least 3 hours after the last meal, is completely in the range from 1 to 3 inclusive throughout the entire astronomical hour of measurement;
5. Blood pressure (BP) levels: 100 to 139 mm Hg, inclusive (systolic, SBP), 60 to 89 mm Hg, inclusive (diastolic, DBP);
6. Heart rate (HR) of 60 to 90 bpm, inclusive;
7. Body mass index (BMI) of 18.5 kg/m2 ≤ BMI ≤ 30 kg/m2, where the body weight range is ≥ 55 kg for men and ≥ 45 kg for women;
8. Non-smoking healthy volunteers (who have never smoked or gave up smoking more than 6 months prior to the Screening);
9. Consent to use adequate methods of contraception throughout the study and for 30 days after completion; for women of preserved reproductive potential, a negative urine pregnancy test result;
10. The volunteers must have adequate behavior and coherent speech.

Exclusion Criteria:

1. A history of allergy;
2. History of drug intolerance to the active and/or excipients included in the study drugs;
3. History of drug intolerance of or hypersensitivity/allergic reactions to lidocaine, xylocaine or other topical anesthetics which will be used at the trial site for anesthesia during esophagogastroduodenoscopy (EGD) and insertion of the probe for the pH measurements;
4. Chronic diseases of the circulatory, lymphatic, respiratory, nervous, endocrine, gastrointestinal, musculoskeletal, integumentary, immune, urogenital, and hematopoietic systems;
5. Esophageal, gastric, and/or duodenal diseases based on EGC performed at screening and based on the medical history; a history of esophageal, gastric, and/or duodenal surgery;
6. Diseases/conditions which, in the opinion of the investigator, may affect the pH measurement results;
7. Acute infectious diseases less than 4 weeks prior to screening;
8. Use of antacids (including sodium bicarbonate solution)/H2 antagonists 24 h prior to the Screening Visit;
9. Use of proton pump inhibitors 72 h prior to the Screening Visit;
10. Use of (including use of a single dose) steroids and/or other ulcerogenic drug (e.g., nonsteriodal anti-inflammatory drugs [NSAID]) less than 4 weeks prior to the Screening Visit;
11. Regular use of any medicinal products, including prescription only and OTC (Over-the-counter) drugs and dietary supplements within 2 weeks prior to the Screening Visit;
12. Blood or plasma donation less than 3 months prior to the Screening Visit;
13. Use of hormonal contraceptives (in women) less than 2 months prior to the Screening Visit;
14. Pregnancy or breastfeeding; a positive pregnancy test for women of childbearing potential;
15. Participation in another clinical trial less than 3 months prior to screening or simultaneously with this study;
16. Use of more than 10 units of alcohol (1 unit of alcohol is equivalent to 330 ml of beer, 150 ml of wine or 40 ml of spirits) a week within a month prior to the enrollment in the study or history data of alcohol/drug dependence or drug abuse.
17. Positive antibody blood test for HIV-1 and HIV-2, Treponema pallidum antigen, hepatitis B surface antigen (HBsAg), or hepatitis C virus antigen;
18. Positive urine test for narcotic substances or strong drugs;
19. Positive test for alcohol vapor in exhaled breath or a positive alcohol saliva test;
20. Deviated septum/nasal obstruction preventing the placement of the pH probe;
21. Medical history of chronic constipation;
22. Medical history of severe maxillofacial injuries;
23. Expected admission to hospital during this study for any reason, except for hospitalization provided for by this protocol;
24. Impossibility or failure to comply with the protocol, undergo protocol-defined procedures, or adhere to the diet or activity regime;
25. Positive SARS-CoV-2 (severe acute respiratory syndrome related coronavirus) (COVID-19) rapid test;
26. Other circumstances which in the opinion of the investigator prevent a volunteer from being included in the study or may result in premature drop-out from the study.

Withdrawal criteria:

1. The volunteer's refusal to further participate in the study;
2. Failure of the volunteer to comply with the rules of participation in the study (skipping study procedures, independent use of drugs prohibited in the study, violation of dietary and lifestyle restrictions, etc.);
3. Taking prohibited therapy;
4. Occurrence of causes/occurrence during the study of situations that threaten the safety of the volunteer, including severe AEs;
5. Volunteers selected to participate in the study in violation of the inclusion/non-inclusion criteria;
6. Violation of the rules for conducting pH measurements or the occurrence of conditions that required early termination of pH measurements (vomiting, nosebleeds, etc.);
7. Missing a dose of the study drug/comparator drug during any period of the study for any reason;
8. Positive urine test for narcotics and powerful drugs;
9. Positive breath alcohol vapor test or alcohol in saliva;
10. Positive pregnancy test in women;
11. Positive test for COVID-19;
12. The occurrence in the course of the study of other reasons that prevent the study according to the protocol.