Objective: Adalimumab has proven effective in psoriasis; however, secondary failure may result from the drug's immunogenicity. Prevalence data on the immunogenicity of biologicals, and of adalimumab in particular, are highly variable. We investigated the prevalence of anti-adalimumab antibodies and the association with clinical indexes and tumour necrosis factor α (TNFα) serum levels in psoriatic patients. Design: Case-control, longitudinal. Setting: Single centre. Participants: Patient groups: I (n=20) receiving biological therapies after switching from adalimumab; II (n=30) ongoing adalimumab therapy; III (n=30) novel adalimumab therapy; IV (n=15) biological therapies other than adalimumab. Healthy subjects: (group V; n=15) never treated with immunosuppressants or biologicals. Interventions: All groups were tested at enrolment. Group II was also tested at 12 months, and group III at 1, 3, and 6 months. Primary and secondary outcome measures: Standard clinical evaluations (Psoriasis Area Severity Index (PASI)), blood samples and two-site ELISAbased measurement of serum adalimumab trough levels, anti-adalimumab antibodies and TNFα. Results: The false-positive rate was 23% for adalimumab detection and 22% for anti-adalimumab antibodies in patients naïve to adalimumab. Spurious positivity for anti-adalimumab antibodies (one-timepoint positivity in group III during follow-up) accounted for 33% of the total. The prevalence of antidrug antibodies was highest (87%) in group I patients. No correlations were found between the presence of anti-adalimumab antibodies or adalimumab levels and changes in PASI scores. Conclusions: High variability of results, high prevalence of false-positives and lack of association between anti-adalimumab antibodies and TNFα level/PASI score limit this assay's usefulness. Accurate clinical evaluation is key to early identification of treatment failures.
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