Study X052070 will evaluate the safety and pharmacokinetics (PK) of XOMA 052 administered to patients with active, stable, moderate to severe rheumatoid arthritis (RA).

It is hypothesized that administration of XOMA 052 is likely to improve inflammatory control in subjects with RA.

Inclusion Criteria:

• American College of Rheumatology (ACR) diagnostic criteria for RA
• Moderate to severe disease, defined as follows - At least six tender and six swollen joints (28 joint count) AND ESR > 28 mm/hr or CRP > 1.0 mg/dL
• Current duration of RA at Screening ≥ 6 months and ≤ 20 years
• RA and other medical conditions must be stable.
• Age ≥ 18 and ≤ 75 at Screening
• Weight ≥ 80 lbs (36.3 kg) and ≤ 275 lbs (125.0 kg)
• For females with child-bearing potential, a negative serum pregnancy test

Exclusion Criteria:

• Major surgery within 28 days prior to Day 0
• Joint replacement surgery within 60 days prior to Day 0 or joint replacement surgery planned within 9 months following Day 0
• Known HIV antibody, or hepatitis B surface antigen
• History of malignancy within 5 years prior to Screening other than carcinoma in situ of the cervix, or adequately treated, non-metastatic squamous or basal cell carcinoma of the skin
• Immunodeficiency
• History or symptoms of a demyelinating disease
• History of severe allergic or anaphylactic reactions to humanized or murine monoclonal antibodies. Respiratory distress (dyspnea, oxygen desaturation with pO2 < 90% or onset of acute respiratory distress syndrome), flank or back pain, and/or hypotension may be signs of anaphylaxis.
• History of tuberculosis, positive PPD test, active atopic disease requiring medication, or asthma. A subject who has had a positive PPD test but has completed a course of treatment for tuberculosis, had a documented vaccination against tuberculosis, or had a negative QuantiFERON -TB test result is eligible.
• Chronic obstructive pulmonary disease (COPD), asthma, or other pulmonary disease requiring more therapy than using one inhaler 4× daily
• Significant systemic involvement secondary to RA (e.g. vasculitis, pulmonary fibrosis)
• Liver disease (e.g., hepatitis, cirrhosis) or abnormal hepatic function. If the diagnosis of liver disease was based on positive Hep C serology due to prior vaccination, the subject is eligible.
• Pregnant or planning to become pregnant during the course of the study, or breast-feeding