Inclusion Criteria:
1. Signed informed consent to participate in the study.
2. Male and female chinese subjects aged 18 to 45 years inclusively at the time of signing the ICF.
3. The ability of the subject to follow the Protocol procedures, in the Investigator's opinion.
4. A diagnosis of "healthy", established according to standard clinical, laboratory, and instrumental methods of examination carried out at screening, according to the assessment of the Investigator, as well as historical data (respiratory system, cardiovascular system, nervous systems, gastrointestinal tract system, urinary systems and etc.).
5. Systolic blood pressure (SBP) ranging 90 to 139 mmHg, diastolic (DBP) ranging 50 to 89 mmHg, pulse ranging 50 to 100 bpm at screening.
6. Willingness of the subjects and their female partners of childbearing potential to use reliable contraceptive methods from the moment of signing the ICF, throughout the whole study participation. This requirement does not apply to participants who have undergone surgical sterilization.
7. Willingness of subjects with reproductive potential to refrain from donating sperm/ oocytes, starting from the moment of signing the ICF, throughout the whole study participation. Females of childbearing potential must have a negative serum pregnancy test, be non-lactating, and have no plans to become pregnant during the study.
8. Willingness to refrain from participating in any other clinical trials, starting from the moment of signing the ICF and throughout the whole study participation.
9. Willingness to refrain from vaccination with any vaccines during the period from the moment of signing the ICF throughout the whole study participation.
\-
Exclusion Criteria:
* 1\. Any medical or social condition that, in the opinion of the Investigator, prevents participation in this study.
2\. Any confirmed or suspected immunosuppressive or immunodeficiency condition. 3. History of malignant disease. 4. Any acute infectious and non-infectious diseases, including the period of convalescence, within 4 weeks from the moment of clinical recovery to signing the ICF, as well as during screening.
5\. Diagnosis of infectious mononucleosis (documented or reported by the subject) within 2 months before signing the ICF and during screening.
6\. Vaccination with live vaccines within 8 weeks and with any other vaccines within 4 weeks prior to signing the ICF and during screening.
7\. A history of allergies and signs of other significant adverse reactions after the administration of any medicinal products.
8\. Hypersensitivity to the components of BCD-261. 9. Body mass index (BMI) outside the normal range (18.0 to 28.0 kg/m2). 10. Results of standard laboratory and instrumental tests (ECG, chest radiogrpahs) outside the normal ranges adopted at the study site and considered clinically significant by the investigator.
11\. Positive results of screening tests for HIV (anti-HIV antibodies and HIV antigen), hepatitis B (HBsAg, total anti-HBc, anti-HBs ) and C (anti-HCV antibodies) and/or syphilis antibody.
12\. History of tuberculosis or current diagnosis of tuberculosis, or latent tuberculosis (positive IGRA).
13\. Repeated positive urine drug test, repeated positive alcohol test at screening.
14\. Difficulty with blood collection or intolerance to venipuncture (e.g., history of needle or blood phobia, skin disease at the puncture site).
15\. Administration and use of the prohibited drugs. 16. Smoking of more than 10 cigarettes a day. 17. Consumption of more than 10 units of alcohol per week (1 unit of alcohol is equivalent to ½ L of beer, 200 mL of wine or 20 mL of spirits) or a history of alcoholism, drug addiction or drug abuse.
18\. Surgical interventions performed less than within 90 days before the signing of the ICF or planned surgery during the study period.
19\. Donation of 450 mL or more of blood or plasma within 60 days prior to signing the ICF.
20\. Participation in any clinical study of medicinal products and devices within 90 days before signing the ICF or previous participation in the current study with the exception of subjects who withdrew before the administration of the investigational product
