Abstract
Objectives. To determine the influence of patient characteristics and disease activity on adalimumab (ADA) concentrations; to assess the relationships between ADA concentrations, the presence of antidrug antibodies (ADAb), and disease activity in rheumatoid arthritis (RA); and to determine the association between cytokine concentrations and ADA concentrations. Methods. A cross-sectional study of people with RA receiving ADA for at least 4 weeks was undertaken. Disease activity was assessed by the Disease Activity Score in 28 joints (DAS28), with responders defined as DAS28 <= 3.2. Serum and plasma were obtained for ADA concentrations and ADAb, and a panel of cytokines were obtained for a subgroup. ADA concentrations were compared between demographic and clinical subgroups using ANOVA. The independent associations between clinical and demographic features were analyzed using a general linear model. Variables significantly associated with ADA concentrations from the univariate analyses were entered into multivariate analyses. Results. Of the 156 participants, 69.2% were female and the mean age was 57.4 (SD 12.7) years. Multivariate analysis revealed that higher C -reactive protein (P < 0.001) and higher weight (P < 0.004) were independently associated with lower ADA concentrations. ADA concentrations were higher in those with DAS28 <= 3.2 compared to those with DAS28 > 3.2 (median 10.8 [IQR 6.4-20.8] mg/L vs 7.1 [IQR 1.5-12.6] mg/L, P < 0.001). There was a significant negative correlation between interleukin 6 (IL -6) and ADA concentrations (r = -0.04, P < 0.01). Conclusion. ADA concentration correlates negatively with markers of inflammatory disease activity in RA, including IL -6. ADA concentration in the range 5 to 7 mg/L over the dose interval are associated with better disease control.