The objective of this trial is to investigate the efficacy (American College of Rheumatology 20% : ACR20) superiority of two dose regiments of CDP870 versus placebo in combination with MTX in active RA patients who have an incomplete response to MTX. The pharmacokinetics and immunogenicity profile of CDP870 will also be investigated to assess the extrapolability of foreign data to the Japanese population.

Inclusion Criteria:

• Subjects must have a diagnosis of adult-onset RA of at least 6 months but not longer than 15 years in duration as defined by the 1987 American College of Rheumatology classification criteria.

• Subjects must have active RA disease as defined by:

  • At least 9 tender joints and 9 swollen joints
  • ESR of 30 mm/hour or CRP of 1.5 mg/dL

• Subjects must have received treatment with MTX for at least 6 months prior to the start of study drug administration. The dose of MTX must have remain fixed for at least 2 months prior to the study and the dose of MTX should be within 6 to 8 mg/week.

Exclusion Criteria:

• Patients who have a diagnosis of any other inflammatory arthritis
• Patients who have a secondary, non-inflammatory type of arthritis (eg, osteoarthritis, fibromyalgia)
• Patients who currently have, or who have a history of, a demyelinating or convulsive disease of the central nervous system (eg, multiple sclerosis, epilepsy)
• Patients who have NYHA (New York Heart Association) Class III or IV congestive heart failure
• Patients who currently have, or who have a history of, tuberculosis
• Patients who have a high risk of infection (with a current infectious disease, a chronic infectious disease, a history of serious infectious disease)
• Patients who currently have, or who have a history of, malignancy
• Female patients who are breastfeeding or pregnant, who are of childbearing potential
• Patients who previously received treatment with 2 or more anti-TNFα drugs or who previously failed to respond to treatment with 1 or more aint-TNFα drugs.