Comparison of non-radiographic axial spondyloarthritis and ankylosing spondylitis patients--baseline characteristics, treatment adherence, and development of clinical variables during three years of anti-TNF therapy in clinical practice

Abstract

Background: The relationship between non-radiographic axial spondyloarthritis (nr-axSpA) and ankylosing spondylitis (AS) is currently debated. Using observational data from the South Swedish Arthritis Treatment Group register, we thus aimed to compare clinical development and treatment adherence between nr-axSpA and AS patients during three years of anti-TNF (tumor necrosis factor) therapy in clinical practice, and to explore the impact of inflammatory activity measured by CRP (C-reactive protein) at treatment initiation.

Methods: Nr-axSpA and AS patients (n = 86/238) in southern Sweden, commencing anti-TNF therapy 1999-2011, were followed during three years. Anti-TNF cessation was defined as stopping therapy, without restarting another anti-TNF agent within three months. Differences in the three year developments of patient's visual analogue scale (VAS) scores for global health and pain, EuroQol 5-Dimensions utility, evaluator's global disease activity assessment, CRP, and ESR (erythrocyte sedimentation rate) were assessed by repeated ANOVA. Anti-TNF adherence was compared by Log rank test and Cox regression. In a subanalysis, the same outcomes were studied after splitting both groups into patients with/without baseline CRP elevation.

Results: Nr-axSpA patients were more often female and had lower acute phase reactants at baseline. Apart from CRP, which remained lower in the nr-axSpA group throughout follow-up (p = 0.004), no between-group differences were detected regarding clinical developments (p >0.1 for all comparisons) or anti-TNF adherence (hazard ratio: 1.1 (95% CI 0.7 to 1.8) for the nr-axSpA vs. AS group) during three years. Elevated baseline CRP was similarly associated with superior clinical outcomes and treatment adherence in both groups.

Conclusions: With the exception of constantly lower CRP levels in the nr-axSpA group, three years anti-TNF therapy resulted in similar clinical outcomes and treatment adherence in nr-axSpA and AS patients, thus strengthening the hypothesis that these diagnoses represent different aspects/phases of the same disease.

Fig. 1

Clinical developments during three years of anti-TNF therapy. Mean (95 % CI) developments in visual analog scale (VAS) global, VAS pain, EuroQ ol 5-Dimensions (EQ-5D) utility, Evaluator’s global assessment of disease activity, erythrocyte sedimentation rate (ESR), and C-reactive protein (CRP) in the non-radiographic axial spondyloarthritis and ankylosing spondylitis groups over time, using last observation carried forward imputed data

Fig. 2

Adherence to anti-TNF therapy. Kaplan-Meier survival curves from initiation of anti-TNF therapy, showing adherence to anti-TNF treatment in the non-radiographic axial spondyloarthritis (nr-axSpA) and ankylosing spondylitis (AS) groups, respectively. Cessation of anti-TNF therapy was defined as stopping therapy without restarting another anti-TNF agent within 3 months. Numbers of patients still on anti-TNF treatment at the respective time points are shown below the graph

Fig. 3

Clinical developments according to C-reactive protein (CRP) status at baseline. Mean (95 % CI) developments in visual analog scale (VAS) global, VAS pain, EuroQ ol 5-Dimensions (EQ-5D) utility, and Evaluator’s global assessment of disease activity during 3 years of anti-TNF therapy in patients with non-radiographic axial spondyloarthritis (nr-axSpA) and ankylosing spondylitis (AS), who had CRP elevation (CRP >3.0 mg/l) or did not have CRP elevation (CRP ≤3.0 mg/l) at baseline

Fig. 4

Adherence to anti-TNF therapy according to C-reactive protein (CRP) status at baseline. Kaplan-Meier survival curves from initiation of anti-TNF therapy, showing adherence to anti-TNF treatment in patients with non-radiographic axial spondyloarthritis (nr-axSpA) and ankylosing spondylitis (AS) who had CRP elevation (CRP >3.0 mg/l) or did not have CRP elevation (CRP ≤3.0 mg/l) at baseline. Cessation of anti-TNF therapy was defined as stopping therapy, without restarting another anti-TNF agent within 3 months. Numbers of patients still on anti-TNF treatment at the respective time points are shown below the graph