Inclusion Criteria:  Signed informed consent prior to any study-mandated procedure. Healthy male or female subjects aged between 18 and 55 years (inclusive) at Screening. Body mass index (BMI) of 18.0 to 30.0 kg/m2 (inclusive) at Screening. Ability to communicate well with the investigator, in a language understandable to the subject, and to understand and comply with the study requirements. Systolic blood pressure (SBP) 100-140 mmHg, diastolic blood pressure (DBP) 50-90 mmHg, and pulse rate 50-90 bpm (inclusive), measured on the same arm after ≥5 min in the supine position, at Screening and on Day -1. Estimated glomerular filtration rate calculated using the Cockcroft-Gault equation ≥ 90 mL/min at Screening. A female subject of childbearing potential must have a negative serum pregnancy test at Screening and a negative urine pregnancy test on Day -1 and must agree to consistently and correctly use (from Screening, during the entire study, and for at least 6 months after investigational product administration) a highly effective method of contraception with a failure rate of ≤1% per year, be sexually inactive, or have a vasectomized partner. If a hormonal contraceptive is used, it must be initiated at least 1 month before the treatment administration. A female subject of non-childbearing potential, must be post-menopausal (defined as 12 consecutive months with no menses without an alternative medical cause, confirmed by a follicle stimulating hormone [FSH] test), or must have a medical history of previous bilateral salpingectomy, bilateral salpingo-oophorectomy, hysterectomy, premature ovarian failure (confirmed by a specialist), XY genotype, Turner syndrome, or uterine agenesis. A male subject must use adequate contraception (e.g., condom) from investigational product administration up to at least 6 months after, unless he is vasectomized or sexually inactive. In addition, the subject must ensure that his female partner of childbearing potential agrees to consistently and correctly use for the same period a highly effective method of contraception with a failure rate of ≤1% per year. A male subject must agree to refrain from donation of semen from investigational product administration up to at least 6 months after.  Exclusion Criteria:  Previous exposure to FBL-MTX. Known hypersensitivity to MTX or any other FBL-MTX components. Clinically relevant findings on physical examination at Screening or on Day -1. Clinically relevant abnormalities on 12-lead ECG, measured after 5 min in a supine position, at Screening or on Day -1. Clinically relevant abnormalities on chest X-ray at Screening. Clinically relevant findings in clinical laboratory tests (hematology, clinical chemistry, and urinalysis) at Screening or on Day -1. Pre-existing blood dyscrasias, such as bone marrow hypoplasia, leukopenia, thrombocytopenia or significant anemia. QTc > 450ms in male and > 470ms in female. Aspartate aminotransferase (AST) or alanine aminotransferase (ALT) above the normal range. Any medical condition, acute, ongoing, recurrent or chronic, that presents a potential risk to the participant and/or that may compromise the objectives of the study. History of major medical or surgical disorders which, in the opinion of the investigator, are likely to interfere with the distribution, metabolism, or excretion of the investigational product. History of significant hypersensitivity, intolerance, or allergy to any drug compound, food, or other substance, unless approved by the investigator. History of tuberculosis. Presence of chronic or acute bacterial or viral infection. History or presence of an autoimmune disorder. History of known or suspected immunodeficiency syndrome Stomatitis, ulcers of the oral cavity and known active gastrointestinal ulcer disease. Any significant acute or chronic medical illness, including past or present liver disease. Previous clinically relevant history of fainting, collapse, syncope, orthostatic hypotension, or vasovagal reactions. Veins unsuitable for intravenous puncture on either arm (e.g., veins that are difficult to locate, access, or puncture, veins with a tendency to rupture during or after puncture). Participation in a clinical study involving investigational product administration within 3 months prior to Screening or in more than 2 clinical studies within 1 year prior to Screening. History or clinical evidence of alcoholism or drug abuse within the 3-year period prior to Screening. Excessive methylxanthines consumption, defined as ≥ 500 mg per day, at Screening. Nicotine intake (e.g., smoking, nicotine patch, nicotine chewing gum, or electronic cigarettes) within 3 months prior to Screening and inability to refrain from nicotine intake from Screening up to End-of-Study (EOS). Previous treatment with any prescribed medications or over-the-counter (OTC) medications (including herbal medicines such as St John's Wort, homeopathic preparations, vitamins, and minerals) within 3 weeks prior to investigational product administration. Previous treatment with vaccines within 4 weeks prior to investigational product administration. Loss of 250 mL or more of blood within 3 months prior to Screening. Positive results from urine drug and alcohol screen at Screening or on Day -1. Positive Hepatitis B surface antigen (HBsAg) or Hepatitis B core antibody (HBcAb) and/or HCV antibodies at Screening. Positive HIV serology results at Screening. Pregnant or lactating woman. Any other circumstances or conditions, which, in the opinion of the investigator, may affect full participation in the study or compliance with the protocol.