Background: Many patients may have resistance to TNF-blockers. These drugs may induce
neutralizing antibodies. The determination of the drug levels of TNF-blockers and Anti-Drug
Antibodies (ADAs) against TNF-blockers may help to make clinical decisions.
Objectives: The objective of this study was to associate and predict the drug levels of TNFblockers
and ADAs in relation to disease activity in patients with Spondyloarthritis (SpA) and
Rheumatoid Arthritis (RA).
Methods: Cross-sectional study including patients fulfilling ASAS classification criteria for SpA
and 2010 ACR-EULAR classification criteria for RA. These patients were treated with Adalimumab
(ADA), Infliximab (IFX), and Etanercept (ETN). A bivariate analysis and the chi-square test
were performed to evaluate the association of ADAs and drug levels with activity measures for
SpA and RA. Five regression models analyzing drug levels, ADAs and disease activity measures
using a multiple linear regression were performed in order to evaluate the prediction of ADAs and
drug levels in relation to disease activity.
Results: In SpA, IFX levels were associated with BASDAI (Bath Ankylosing Spondylitis Disease
Activity Index) (p=0.034). In RA, total drug levels were associated with DAS28-ESR (28 joint
Disease activity Score-erythrocyte sedimentation rate), (p=0.008), DAS28-CRP (p=0.042), CDAI
(Clinical Disease Activity Index) (p=0.047) and SDAI (Simple Disease activity index), (p=0.017).
ADA levels had association with CDAI (p=0.002) and SDAI (p=0.002). IFX levels were associated
with a DAS28-ESR (p=0.044), DAS28-CRP (p=0.022) and SDAI (p=0.022). ADAs were associated
in SpA with BASDAI (p=0.027). Drug levels and ADAs did not predict disease activity in
patients with SpA or RA.
Conclusion: ADAs and drug levels of anti-TNF are associated with disease activity measures in
patients with SpA and RA. However, they cannot predict clinical activity in these conditions.