Study to Assess Efficacy and Safety of Treamid for Patients With Reduced Exercise Tolerance After COVID-19

Inclusion Criteria: 1. Signed patient explanation sheet and informed consent for participation in the study. 2. Men and women at the age from 18 through 75 years old. 3. History of acute coronavirus infection: * The patient has a history of confirmed diagnosis of COVID-19 pneumonia, which resolved more than 2 weeks before screening; * History of COVID-19 is confirmed by positive qualitative analysis of SARS-CoV-2 by polymerase chain reaction (PCR) or detection of SARS-CoV-2 antigens; * Negative rapid test for SARS-CoV-2 antigen at screening and randomization; * Onset of first symptoms of COVID-19 from 42 to 120 days prior to randomization. 4. Severity of mMRC dyspnea ≥2 at screening and randomization. 5. Severity of exercise tolerance impairment at screening and randomization based on the 6-minute Walk Test: * reduction of the distance traveled below the normal limit (\< 80% of predicted) at the level of shortness of breath ≥2 units according to the Borg scale; * severity of shortness of breath \> 2 units according to the Borg scale, regardless of the presence of violations in the distance traveled. 6. The presence of foci of lung tissue seals such as "frosted glass" and/or interlobular septum seals, and/or areas of reticular changes in lung tissue, and/or the presence of areas (linear, focal) of lung tissue seals characteristic of COVID-19 confirmed by chest CT at screening. The percentage of lung tissue damage is ≥10%, but \< 75% of the total lung volume. 7. The pulmonary diffusivity parameter is less than 80%, but more than 40% of predicted at the time of screening. 8. The patient's consent to use adequate contraception methods during the entire study and within 3 weeks after its completion. The adequate contraception methods include the use of the following: * oral or transdermal contraceptives; * condom or diaphragm (barrier method) with spermicide; * intrauterine device. Exclusion Criteria: 1. Hypersensitivity to Treamid (active pharmaceutical substance XC268BG) and/or other components of the study drug. 2. History of invasive mechanical ventilation and high-flow oxygenation in the last 45 days or need for oxygen support at screening and randomization. 3. Chronic respiratory diseases diagnosed before COVID-19, including idiopathic pulmonary fibrosis and other interstitial lung diseases, moderate to severe bronchial asthma, moderate to severe chronic obstructive pulmonary disease, tuberculosis (including suspected tuberculosis based on CT examination at screening) or pulmonary hypertension. 4. Severe anemia (Haemoglobin \< 70 g/L at screening). 5. Inability to perform a CT procedure (for example, a gypsum bandage or metal structures in the study area). 6. Severe cardiovascular disease at present or within 6 months prior to screening, including: Class III or IV chronic heart failure (New York Heart Association classification), clinically significant ventricular arrhythmias (ventricular tachycardia, ventricular fibrillation), unstable angina, myocardial infarction, heart and coronary vessel surgery, significant heart valve disease, uncontrolled hypertension with systolic blood pressure \> 180 mmHg and diastolic blood pressure \> 110 mmHg, pulmonary embolism arteries or deep vein thrombosis. 7. Chronic kidney disease or other significant kidney disease with glomerular filtration rate (GFR) \< 60 mL/min (Cockcroft-Gault formula) at screening. Hemodialysis requirement at patient screening. 8. Stage II chronic liver failure (decompensated) and above; History of liver cirrhosis; alanine aminotransferase (ALT) and/or aspartate aminotransferase (AST) elevations of 3 or more times the upper limit of normal (ULN) at screening; increase in total bilirubin 2 or more times from ULN at screening (exception - Gilbert's syndrome in the history). 9. Severe diseases of the central nervous system, including a history of seizures or conditions that may lead to their development; stroke or transient ischemic attack within 6 months prior to screening; traumatic brain injury or loss of consciousness within 6 months prior to screening; brain tumor. 10. Malignancies requiring chemotherapy treatment within 5 years prior to screening or currently suspected cancer. 11. Participation in other clinical trials within 1.5 months prior to screening. 12. Requirement for or administration of the following drugs for 1 month prior to screening: erythropoietin, cytostatics, colchicine, cyclosporin A, interferon- γ -1b, bosentan, macitentan, etanercept, sildenafil, imatinib, warfarin, ambrisentan, nintedanib, pirfenidone, hyaluronidase asoxime 1 month prior to screening; glucocorticosteroids and N-acetylcysteine (requirement or 7 days prior to screening) 13. Lactase deficiency, lactose intolerance, glucose-galactose malabsorption. 14. Signs of a marked uncontrolled concomitant disease, such as disorders of the nervous system, kidney, liver, endocrine system and gastrointestinal tract, which, in the opinion of the Investigator, could prevent the patient from participating in the study. 15. Positive test for human immunodeficiency virus (HIV), hepatitis B and/or C. 16. Alcohol or drug addiction, history of mental illness. 17. Severe, decompensated or unstable somatic diseases (any diseases or conditions that threaten the patient's life or impair the patient's prognosis, and make it impossible for him to participate in a clinical trial). 18. Patient's unwillingness or inability to comply with Protocol procedures (in the opinion of the study physician). 19. Pregnancy or breastfeeding period. 20. Other conditions that, in the opinion of the study physician, prevent the patient from entering the study.