Study to assess the efficacy and safety of XC8, film-coated tablets, 10 mg in comparison with placebo in patients with dry non-productive cough against acute respiratory infection.

Inclusion Criteria:

1. Signing and dating of the IPP informed consent form by the patient.
2. Women and men between the ages of 18 and 65 years, inclusive, at the time of signing the informed consent form.
3. Clinical diagnosis of acute respiratory viral infection of the upper respiratory tract (ICD-10 code J00-J06) or acute bronchitis (J20, J21 according to ICD-10).
4. Onset of disease symptoms no more than 3 days prior to screening.
5. Presence of a patient with a dry unproductive cough.
6. Frequency of cough episodes ≥10 in the past 24 h before the screening visit and randomization.
7. Cough frequency rated by the patient as 3-4 on the daytime cough section and 2-4 on the nighttime cough section of the Daytime and Nighttime Cough Scales.
8. Patient rating of cough severity on the DRS ≥ 4 points.
9. No indication for therapy due to BHSA infection at the time of study inclusion: negative rapid test for Group A β-haemolytic streptococcus.
10. For women of preserved reproductive potential, a negative pregnancy test and agreement to use approved contraceptive methods for the duration of study participation, beginning at visit 0, and for 3 weeks after the end of the study; for men, agreement to use approved contraceptive methods throughout the study participation period and for 3 weeks after the end of the study.

Exclusion Criteria:

1. Known or suspected hypersensitivity to the active ingredient or any of the excipients of the study drug/placebo.
2. Known or suspected hypersensitivity to acetylcysteine.
3. Lactose intolerance, lactase deficiency, glucose-galactose malabsorption.
4. Trauma (including open wounds in the mouth and erosive-desquamous lesions of the oral mucosa) and/or oropharyngeal burns, scarlet fever, rubella, measles, mumps at the time of screening or within 3 months prior to screening.
5. Acute obstructive laryngitis or suspected obstructive laryngitis.
6. Presence of signs of laryngeal stenosis (stridor, shortness of breath).
7. Exacerbation of chronic bronchitis or chronic obstructive pulmonary disease (COPD).
8. Bronchial asthma, including a history.
9. History of tuberculosis.
10. History of pulmonary emphysema.
11. Acute or chronic pneumonia, or suspected pneumonia.
12. Condition after intubation.
13. Gastroesophageal reflux being the primary cause of cough (as judged by the investigating physician).
14. Allergic rhinitis as primary cause of cough (according to study physician).
15. Body temperature > 39.0 °C.
16. Presence of at least one of the epidemic signs: return from a foreign trip 14 days prior to the onset of symptoms; presence of close contact in the past 14 days with a person under COVID-19 surveillance who has subsequently become ill; having close contact in the past 14 days with a person who has a laboratory-confirmed diagnosis of COVID-19; having occupational contacts with an individual with a suspected or confirmed case of COVID-19.
17. Positive result of laboratory testing for SARS-CoV-2 RNA using nucleic acid amplification techniques or SARS-CoV-2 antigen using immunochromatographic analysis at the time of screening.
18. The need for systemic antibiotic therapy and/or other drugs/procedures from the list of prohibited therapies/procedures.
19. Use of analgesics or antipyretics within 12 hours prior to screening.
20. Use of glucocorticosteroids, β-adrenoblockers, ACE inhibitors, theophylline drugs, expectorants, cough suppressants, anesthetics, anti-allergic drugs (leukotriene receptor antagonists, H1-histamine receptor blockers, sodium cromoglycate preparations), antiviral drugs, immunosuppressants, systemic antibacterial drugs for 7 days before screening.
21. History of smoking more than 10 pack/years (pack-years: number of cigarettes smoked per day multiplied by the number of years of smoking divided by 20).
22. Aspartate aminotransferase (AST) and/or alanine aminotransferase (ALT) levels ≥ 2.5 × highest normal value, serum bilirubin levels ≥ 1.5 × highest normal value.
23. Glomerular filtration rate (GFR) ≤ 60 ml/min.
24. History of cancer of the respiratory system.
25. History of other malignancies, except for patients who have not had the disease in the past 5 years, patients with fully cured basal cell carcinoma of the skin, or fully cured carcinoma in situ.
26. Serious cardiovascular disease at the time of screening or within 12 months prior to screening, including: New York Heart Association Class III or IV chronic heart failure, severe arrhythmias requiring treatment with antiarrhythmic drugs, unstable angina pectoris, myocardial infarction, heart and coronary artery surgery, transient ischemic attack or stroke, uncontrolled hypertension with systolic blood pressure > 180 mm Hg Std. and diastolic blood pressure >110 mmHg, pulmonary embolism.
27. Other severe, decompensated or unstable somatic diseases (any disease or condition that threatens the patient's life or worsens the patient's prognosis, or makes it impossible for the patient to participate in a clinical trial).
28. Patient's unwillingness or inability to comply with protocol procedures (in the opinion of the research physician).
29. Pregnancy or breastfeeding (for women).
30. Alcoholism, drug dependence, substance abuse history and/or at the time of screening.
31. A history of schizophrenia, schizoaffective disorder, bipolar disorder or other psychiatric pathology.
32. Participation in another clinical trial within 3 months prior to inclusion in the study.
33. Other conditions that, in the opinion of the investigating physician, preclude inclusion of the patient in the study.

Withdrawal Criteria:

1. Identification of a probable or confirmed case of COVID-19
2. Ineffectiveness of therapy - persistence or increased frequency of cough attacks ≥ 1 by visit 3 (day 8-9) compared to visit 1 (day 1).
3. AEs requiring withdrawal of study drug/placebo.
4. The occurrence of any disease or condition during the study that, in the opinion of the study physician, worsens the patient's prognosis and also makes it impossible for the patient to continue participating in the clinical trial.
5. Erroneous inclusion of a patient who does not meet the inclusion criteria and/or meets the inclusion criteria.
6. Taking any of the drugs of prohibited therapy, performing a prohibited procedure.
7. The necessity of prescribing prohibited concomitant therapy/procedures.
8. Pregnancy of the patient.
9. Patient's desire to stop participating in the study.
10. Lack of adequate cooperation of the patient with the doctor-researcher during the study.
11. Other protocol violations that are significant in the opinion of the physician-researcher.
12. Other administrative reasons.