AD 452 is a new drug which is being developed for use in adult patients with RA. It is believed that AD 452 may modify the underlying disease of RA as well as improving RA symptoms, and in order to establish its efficacy and safety, AD 452 is being tested in a 3 month study. Patients enrolled in this study will already be taking methotrexate for their RA and they will remain on methotrexate throughout the study. An earlier clinical study in 98 subjects with RA on stable background therapy investigated the pharmacokinetics, safety and tolerability of AD 452 taken for one month. The drug was well tolerated and no significant drug related adverse events were reported.