IntroductionThe objective of this study was to evaluate the long-term safety and efficacy of tabalumab, a monoclonal antibody that neutralizes membrane-bound and soluble B-cell activating factor, in rheumatoid arthritis (RA) patients.MethodsPatients with RA who completed one of two 24-week randomized controlled trials (RCTs) participated in this 52-week, flexible-dose, open-label extension study. Patients in RCT1 received intravenous placebo, 30-mg tabalumab, or 80-mg tabalumab every three weeks, and patients in RCT2 received subcutaneous placebo or 1-, 3-, 10-, 30-, 60-, or 120-mg tabalumab every four weeks (Q4W). Regardless of prior treatment, all patients in this study received subcutaneous 60-mg tabalumab Q4W for the first three months, then a one-time increase to 120-mg tabalumab Q4W (60/120-mg group) and a one-time decrease to 60-mg Q4W per patient was allowed (60/120/60-mg group).ResultsThere were 182 patients enrolled: 60-mg (n¿=¿60), 60/120-mg (n¿=¿121), and 60/120/60-mg (n¿=¿1). Pre-tabalumab baseline disease activity was generally higher for the 60/120-mg group. There was a higher frequency of serious adverse events and treatment-emergent adverse events, as well as infections and injection-site reactions, in the 60/120-mg group. One death unrelated to study drug occurred (60/120-mg group). For both groups, total B-cell counts decreased approximately 40% from baseline of the RCT originating study. Both groups demonstrated efficacy on American College of Rheumatology criteria improvement ¿20%, ¿50%, and ¿70%, European League Against Rheumatism Responder Index in 28 joints, Disease Activity Score in 28 joints¿C-reactive protein, and Health Assessment Questionnaire¿Disability Index with 52 weeks of treatment relative to baseline pre-tabalumab disease activity.ConclusionsWith long-term, open-label tabalumab treatment, no unexpected safety signals were observed, and B-cell reductions were consistent with previous findings. Despite differences in RCT-originating studies, both groups demonstrated an efficacy response through the 52-week extension.Trial registrationClinicalTrials.gov NCT00837811. Registered 3 February 2009.