Background: Immunogenicity, specifically the onset of antibodies against Tumour Necrosis Factor (TNF) blocking agents, seems to play an important role in non-response to treatment with these drugs.
Objectives: To assess the relation of clinical response of Ankylosing Spondylitis (AS) to etanercept with etanercept levels, and the presence of antibodies to etanercept.
Methods: AS patients were treated with etanercept 25 mg twice weekly, according to the international ASAS consensus statement. Sera were collected at baseline, after 3 and 6 months of treatment. Clinical response was defined as a 50% improvement or as an absolute improvement of 2 points on a (0-10 scale) BASDAI score. Functional etanercept levels were measured by a newly developed ELISA, measuring the binding of etanercept to TNF. Antibodies against etanercept were measured with a two-site assay and antigen binding test. Clinical data were used to correlate disease activity with serum etanercept levels.
Results: 53 consecutive patients were included. After 3 months of treatment 40 patients (76%) fulfilled the response criteria. Mean etanercept levels were 2.7 mg/l and 3.0 mg/l after 3 and 6 months respectively. Characteristics and etanercept levels of responders and non-responders were similar. No antibodies to etanercept were detected with any of the assays.
Conclusion: Etanercept levels of responders and non-responders were similar and no antibodies to etanercept were detected with any of the assays. This study indicates that etanercept is much less immunogenic compared with the other TNF blocking agents.