A Dose Escalation Study to Assess Safety, Tolerability and Pharmacokinetics of ASP2409 Following a Single Intravenous Dose in Patients With Rheumatoid Arthritis on Methotrexate

Inclusion Criteria: * Subject weighs at least 50 kg. * Subject has a body mass index (BMI) of ≤ 35 kg/m2. * Subject's 12-lead electrocardiogram (ECG) results are normal at Screening and Day -1 or, if abnormal, the abnormality is not clinically significant * Female subject must be either: * Of non-childbearing potential: 1. post-menopausal (defined as at least 1 year without any menses) prior to Screening, or 2. documented surgically sterile or status post hysterectomy (at least 1 month prior to Screening). * Or, if of childbearing potential: 1. must have a negative serum pregnancy test at Screening and a negative urine pregnancy test on Day -1. 2. must use two forms of birth control (at least one of which must be a barrier method) starting at Screening and throughout the Treatment and Observation Period, and for ≥ 120 days after final study drug administration. 3. Acceptable forms include: 1. Established use or oral, injected or implanted hormonal methods of contraception. 2. Placement of an intrauterine device (IUD) or intrauterine system (IUS). 3. Barrier methods of contraception: condom or occlusive cap (diaphragm or cervical/vault cap) with spermicidal foam/gel/film/ cream/suppository. * Female subject must not be breastfeeding at Screening or during the Treatment and Observation period and for ≥ 120 days after final study drug administration. * Female subject must not donate ova starting at Screening and throughout the Treatment and Observation period and for ≥ 120 days after final study drug administration. * Male subject must not donate sperm starting at Screening and throughout the Treatment and Observation period and for at least ≥ 120 days after final study drug administration. * Male subject and their female spouse/partners who are of childbearing potential must be using highly effective contraception consisting of two forms of birth control (one of which must be a barrier method) starting at Screening and continue throughout the Treatment and Observation period and for ≥ 120 days after final study drug administration. * Subject has Rheumatoid Arthritis (RA) that was diagnosed according to the 1987 revised criteria of the American College of Rheumatology (ACR) ≥ 6 months prior to Screening. * Subject meets the ACR 1991 revised criteria for Global Functional Status in RA, Class I, II or III at Screening. * Subject MUST be on concomitant methotrexate (MTX): * for ≥ 3 months prior to study drug infusion, AND * at a stable dose (10 - 25 mg/week) for ≥ 28 days prior to study drug infusion and throughout the study. * Subjects on the following medications must remain on a stable regimen: non-steroidal anti-inflammatory drugs (NSAIDs), selective cyclooxygenase-2 (COX-2) inhibitors, hydroxychloroquine (Plaquenil®), sulfasalazine, oral corticosteroids (≤ 10 mg of prednisone, or equivalent, daily) or low dose opioids (≤ 30 mg of oral morphine, or equivalent, daily) for ≥ 28 days prior to Screening and remain so throughout the Treatment and Observation Period. (The start of Plaquenil and sulfasalazine must be ≥ 2 months prior to study drug infusion.) * Subject is highly likely to comply with the protocol and complete the study. Exclusion Criteria: * Subject has an ongoing clinically significant systemic disease such as uncompensated heart failure, uncontrolled diabetes mellitus, severe hepatic failure or severe pulmonary disease. * Subject has a history of any malignancy except for adequately-treated, non-melanoma skin cancer and adequately-treated in-situ cervical cancer. * Subject has a history of severe allergic or anaphylactic reactions. * Subject has a history of consuming more than 14 units of alcoholic beverages per week or has a history of alcoholism or drug/chemical/substance abuse within past 6 months prior to Screening (Note: one unit = 12 ounces of beer, 4 ounces of wine or 1 ounce of spirits). * Subject has a positive test for alcohol or drugs of abuse (excluding drugs prescribed to subject) at Screening or Day -1. * Subject has/had a viral, bacterial (including upper respiratory infection), or fungal (non-cutaneous) infection within 1 week prior to Day -1. * Subject has a past history of serious opportunistic infection. * Subject is known positive for human immunodeficiency virus (HIV) antibody. * Subject has a positive tuberculosis (TB) skin test or Quantiferon Gold test at Screening. * Subject has a positive test for hepatitis C antibody, or positive test for hepatitis B surface antigen (HBsAg), or positive hepatitis B core antibody at Screening. * Subject's laboratory test results at Screening: * alanine aminotransferase (ALT) and/or aspartate aminotransferase (AST), are ˃ 2 times the upper limit of normal, AND/OR * are outside the normal limits and considered by the Investigator to be clinically significant with regard to the remaining per-protocol laboratory tests. * Subject received any live or live attenuated vaccine within 30 days prior to study drug infusion. * Subject received any systemic immunosuppressant agent, other than (MTX) or stable steroid regimen, within 2 months prior to study drug infusion. * Subject has previously received any antibody or therapeutic biologic product within 56 days or 5 half-lives, whichever is longer, prior to study drug infusion. * Subject has previously participated in any interventional clinical study or has received an experimental agent within 56 days or 5 half-lives, whichever is longer, prior to study drug infusion. * Subject is participating in another clinical trial or has participated in another dose group of the current trial. * Subject has had any significant blood loss, donated one unit (450 mL) of blood or more, or received a transfusion of any blood or blood products within 60 days or donated plasma within 7 days prior to clinic admission on Day -1. * Subject has taken Orencia® (abatacept), Nulojix® (belatacept) or any other CTLA4-Ig molecule. * Subject has any other condition which precludes the subject's participation in the trial. * Subject has a history of prolonged QT syndrome.