Inclusion Criteria:
1. Written informed consent
2. Men and women aged 18 to 60 years inclusive
3. Body weight ≥45 kg and body mass index ≤32 kg/m²
4. Diagnosis of RA according to the 2010 ACR/EULAR classification criteria, for at least 3 months before Screening
5. Stable dose of MTX for at least 4 weeks before Day 1
6. Ability to comply with the study protocol for the duration of the study, including inpatient confinement for about 3 days
7. Adequate venous access for blood collection
8. Women must not be pregnant or breastfeeding and
1. be of non-childbearing potential (postmenopausal \[no menses for 12 months without an alternative medical cause\] or surgically sterilized \[documented hysterectomy, bilateral salpingectomy, or bilateral oophorectomy\]) or
2. if of childbearing potential, must use a highly effective contraceptive method for the duration of the study and have a negative pregnancy test at Screening (blood test) Note: Acceptable highly effective methods of contraception include the intrauterine device; intrauterine hormone-releasing system; bilateral tubal occlusion (must be documented); oral, intravaginal, transdermal, injectable, and implantable methods of hormonal contraception associated with inhibition of ovulation, vasectomized partner (must be documented); or sexual abstinence (only when it is the usual and preferred lifestyle of the participant)
9. Men must agree, from start of SR-878 administration until 90 days after SR-878 administration, to refrain from donating sperm and use a male condom when having sexual intercourse with a woman of childbearing potential, who is not currently pregnant, and advise her to use a highly effective contraceptive method
10. Disease status appropriate to be included in this exploratory safety study according to the investigator
Exclusion Criteria:
1. Abnormal findings in medical history and physical examination considered to be clinically relevant by the investigator
2. Clinically significant abnormal screening laboratory tests
3. Known active infection with human immunodeficiency virus (HIV), hepatitis B virus, tuberculosis, and/or hepatitis C virus (HCV)
4. Clinically relevant ECG (12 leads) abnormalities
5. Acute infectious diseases within 2 weeks before Screening
6. History or presence of any autoimmune disease other than RA, chronic inflammatory condition, or clinically significant (in the opinion of the Investigator) atopic allergy (eg, asthma, urticaria, eczematous dermatitis, allergic rhinitis)
7. Relevant history of other renal, hepatic, gastrointestinal, cardiovascular, respiratory, skin, hematological, endocrine, inflammatory, chronic infectious, or neurological diseases
8. History of anaphylaxis to drugs or major allergic reactions in general, which in the view of the investigator may compromise the safety of the participant
9. Malignancy in the last 5 years before Screening (except basal or squamous cell skin cancer)
10. History of any chronic inflammatory arthritis with onset prior to the age of 18 or history of acute inflammatory joint disease of a different origin than RA
11. Current or previous (within 30 days or, if applicable, 5 half-lives of the investigational medicinal product (IMP), whatever is longer, before Screening) participation in another clinical study with an investigational drug and/or medical device
12. Known hypersensitivity to the active substance or to any of the excipients of the IMP and auxiliary medicinal products
13. Drug and alcohol abuse
14. Prohibited medication as outlined below
15. Plans to undergo elective procedures or surgeries at any time after signing the informed consent form until end of the study
16. Employees of the sponsor, or employees or relatives of the investigator
17. Individuals committed to an institution by virtue of an order issued either by the judicial or the administrative authorities
18. Legal incapacity or limited legal capacity
