This is a randomized, double-blind, parallel group, multicentre clinical study to evaluate the efficacy, safety, pharmacokinetics and immunogenicity of SB4 compared to Enbrel in subjects with moderate to severe Rheumatoid Arthritis (RA) despite Methotrexate (MTX) therapy. In some countries, after 52 weeks of treatment with either SB4 or Enbrel, subjects will be enrolled into an open label extension period. Subjects will receive SB4 for an additional 48 weeks.

Inclusion Criteria:

• Diagnosed as having RA according to the revised 1987 ACR criteria for at least 6 months but not exceeding 15 years prior to Screening
• Have moderate to severe active disease despite MTX therapy defined as having more than or equal to six swollen joints and more than or equal to six tender joints and either erythrocyte sedimentation rate (ESR, Westergren) ≥ 28 mm/h or serum C-reactive protein ≥ 1.0 mg/dL
• Must have been treated with MTX for at least 6 months prior to Randomisation and on a stable dose of MTX 10-25 mg/week given at least 4 weeks prior to Screening
• Female subjects who are not pregnant or nursing at Screening and who are not planning to become pregnant from Screening until 2 months after the last dose of investigational product

Exclusion Criteria:

• Have been treated previously with any biological agents including any tumour necrosis factor inhibitor

• Have a known hypersensitivity to human immunoglobulin proteins or other components of Enbrel or SB4

• Have a positive serological test for hepatitis B or hepatitis C or have a known history of infection with human immunodeficiency virus

• Have a current diagnosis of active tuberculosis

• Have had a serious infection or have been treated with intravenous antibiotics for an infection within 8 weeks or oral antibiotics within 2 weeks prior to Randomisation.

• Have any of the following conditions

  1. Other inflammatory or rheumatic diseases.
  2. History of any malignancy within the previous 5 years prior to Screening
  3. History of lymphoproliferative disease including lymphoma.
  4. History of congestive heart failure
  5. Physical incapacitation (ACR functional Class IV or wheelchair-/bed-bound).
  6. History of demyelinating disorders.