Association of Pharmacological Biomarkers with Treatment Response and Longterm Disability in Patients with Psoriatic Arthritis: Results from OUTPASS

Objective.To identify (1) whether tumor necrosis factor inhibitor (TNFi) drug levels/anti-drug antibodies (ADAb) are associated with treatment response and disability in patients with psoriatic arthritis (PsA); and (2) the factors associated with TNFi drug levels.Methods.Patients were recruited from a national multicenter prospective cohort with longitudinal serum samples and 28-joint count Disease Activity Scores (DAS28)/Health Assessment Questionnaire (HAQ) measurement over 12 months.Results.Adalimumab (ADA) drug levels were significantly associated with ΔDAS28 (β 0.055, 95% CI 0.011–0.099; p = 0.014) and inversely with HAQ over 12 months (β −0.022, 95% CI −0.043 to −0.00063). Factors significantly associated with ADA drug levels were ADAb levels and body mass index.Conclusion.Drug level testing in ADA-initiated PsA patients may be useful in determining treatment response/disability over 12 months.