Long-term treatment with adalimumab in psoriatic arthritis: serum adalimumab concentration, immunogenicity and the link with clinical response.

Objectives: An observational study to evaluate the relationship between serum concentrations of adalimumab and disease activity in patients receiving long-term adalimumab treatment for psoriatic arthritis.

Methods: Serum adalimumab and adalimumab antidrug antibodies were quantified by enzyme linked immunosorbent assay. Disease activity was assessed using Disease Activity Score (44 joint measures). Serum C-reactive protein was quantified using standard methods.

Results: A total of 30 patients were recruited. There were significant inverse correlations between serum adalimumab concentration and serum C-reactive protein (CRP) concentration [r = -0.43], the number of tender joints (r = -0.4), and Disease Activity Score (DAS44)-CRP (r = -0.36). Mean serum adalimumab levels were significantly higher in patients with DAS44-CRP <1.6 than in patients with DAS44-CRP ≥1.6.

Conclusions: Serum adalimumab could be an important tool that may improve the management of psoriatic arthritis in patients responding to long-term treatment.