Adalimumab serum levels and antidrug antibodies towards adalimumab in peripheral spondyloarthritis: no association with clinical response to treatment or with disease relapse upon treatment discontinuation

INTRODUCTION In this study, we evaluated the clinical relevance of serum drug levels and antidrug antibodies (ADAbs) with regard to response to treatment, as well as to relapse upon treatment discontinuation, in peripheral spondyloarthritis (pSpA) patients treated with adalimumab. METHODS The study included 26 pSpA patients treated with adalimumab for either 12 weeks (n = 12) or 24 weeks (n = 14) in a randomized controlled trial. Patients achieving inactive disease measured by Ankylosing Spondylitis Disease Activity Score (ASDAS) at the end of the treatment period were classified as responders. Clinical characteristics, serum trough adalimumab levels and ADAbs were assessed at the end of the treatment period and at follow-up (upon relapse or, in absence of relapse, at 16 weeks after discontinuation). RESULTS Serum adalimumab levels measured 2 weeks after the last adalimumab administration ranged from <0.002 to 23.0 μg/ml, with a median of 11.5 μg/ml. These levels were associated with neither response to treatment or disease activity measurements at the end of treatment nor with the occurrence of relapse and time to relapse after discontinuation of treatment. Antiadalimumab ADAbs were present in 23% of the patients at end of treatment and in 35% at follow-up after treatment discontinuation, indicating that ADAbs were masked by the presence of the drug in some patients. However, ADAbs at the end of treatment and at follow-up were not different between responders and nonresponders and were not associated with relapse upon discontinuation of treatment. CONCLUSIONS There is no clear association between adalimumab serum levels or antiadalimumab ADAbs with clinical response to treatment or with relapse upon treatment discontinuation in pSpA. TRIAL REGISTRATION Netherlands Trial Register ID: NTR1806 (registered 7 May 2009).