The purpose of this study is to examine the safety of a single dose of RG2077 in patients with systemic lupus erythematosus (SLE) who are currently receiving cyclophosphamide. This study will also determine if RG2077 is effective in decreasing disease activity in these patients. Study hypothesis: CTLA4-Ig mediates a T cell costimulatory blockade that effectively induces an antigen-specific nonresponsiveness in T cells.
Inclusion Criteria: * Diagnosis of SLE by American College of Rheumatology (ACR) criteria * Concurrent treatment with intravenous cyclophosphamide (500 to 1000 mg/m2) for at least one of the following manifestations of lupus: World Health Organization (WHO) class III, IV, or V lupus nephritis; British Isles Lupus Assessment Group (BILAG) score of A for vasculitis; BILAG score of A for cytopenia; BILAG score of A for nervous system * Stable medication regimen for at least 4 weeks prior to study entry * Weight between 40 kg (88.2 lbs) and 125 kg (275.6 lb) * Willing to use acceptable forms of contraception Exclusion Criteria: * Moderately severe anemia (hemoglobin less than 8 mg/dL) * Neutropenia (absolute neutrophil count less than 1,500/mm3) * Thrombocytopenia (platelets less than 50,000/mm3) * Positive tuberculin (PPD) test without evidence of prior treatment or administration of bacille Calmette-Guérin (BCG) vaccine * Active infections, including HIV and hepatitis B or C * Receipt of a live vaccine within 3 months of study entry * End-stage renal disease with creatinine clearance less than 20 ml/min/1.73 m2 * History of cancer. Patients with a history of carcinoma in situ and treated basal and squamous cell carcinomas are not excluded. * Pregnant or breastfeeding