Subjects with a stable methotrexate (MTX) dose enter the run-in period in which certolizumab pegol (CZP) will be administered at a dose of 400 mg (2 injections) at Weeks 0, 2, and 4 and at a dose of 200 mg with placebo (1 injection placebo, 1 injection CZP) at Weeks 6, 8, 10, 12, 14 and 16. The dose of MTX should be stable for at least 2 months prior to the Baseline visit and will remain stable throughout the trial, unless there is a need to reduce the dose for reasons of toxicity.~At the Week 18 visit, subjects who were ACR20 (American College of Rheumatology 20% Improvement) responders at Week 16 will be randomized in a double-blinded way to receive either 400 mg CZP given every 4 weeks and placebo given every 4 weeks given as two injections (alternating CZP and placebo every two weeks) plus MTX, 200 mg CZP and placebo administered every 2 weeks (one injection of each) plus MTX, or Placebo administered as two injections every 2 weeks plus MTX. Non-responders will be withdrawn from the study.
During the run-in period, CZP will be administered at 400 mg (2 injections) at Wks 0, 2, and 4 and 200 mg with placebo (1 injection placebo, 1 injection CZP) at Wks 6, 8, 10, 12, 14 and 16. At Wk 18 patients will be grouped as responders or non-responders based on results of the ACR20 at Week 16.