Change in disease activity is associated with TNF-α inhibitor serum levels in patients with axial spondyloarthritis in daily clinical practice

Objective To investigate, in daily clinical practice, TNF-alpha inhibitor serum trough levels in patients experiencing an increase in axial spondyloarthritis (ax-SpA) related symptoms. Secondly, to explore if these serum trough levels are associated with disease activity (DA) and/or change in DA. Methods Patients from the GLAS cohort treated with TNF-alpha inhibitors who had a serum trough level measurement during follow-up because of an increase in ax-SpA related symptoms between June 2015 and June 2018 were included. Serum trough levels were stratified in a therapeutic and below therapeutic range, based on published reference values of Sanquin in 2019. DA was assessed by ASDAS and BASDAI and change in DA (i.e. AASDAS or BASDAI compared to the visit before increasing symptoms). Results 31 patients had a serum trough level measurement because of increasing symptoms. These patients had a median treatment duration of 4.8 years (IQR 0.9-8.6). 22 (71%) had active disease according to ASDAS (score >= 2.1) and 15 (47%) had therapeutic drug levels. The increase in DA was significantly larger in patients with below therapeutic drug levels compared to patients with therapeutic levels (AASDAS: 0.94 +/- 0.81 vs. -0.07 +/- 126, p<0.05; Delta BASDA1: 1.72 +/- 1.73 vs. -0.53 +/- 1.8, p<0.005). No significant differences were found in absolute DA scores between patients with or without therapeutic drug levels. Conclusion In this observational study in daily clinical practice, approximately half of ax-SpA patients who experienced an increase in symptoms had below therapeutic TNF-alpha inhibitor serum trough levels. Change in DA and not absolute DA scores was significantly associated with drug levels.