The purpose of this study is to assess the efficacy, safety and pharmacokinetics of maintenance treatment with 3mg/kg, 6mg/kg or 10mg/kg of TA-650 in combination with methotrexate (MTX) after three infusions (weeks-0, 2, 6) of 3mg/kg in Rheumatoid Arthritis (RA) showing an insufficient response to MTX.