Background Etanercept levels are associated with clinical response in rheumatoid arthritis, however, in a small group of 53 patients with ankylosing spondylitis (AS) this association was not found (de Vries MK et al. ARD 2009).
Objectives To investigate the relationship between etanercept levels and clinical response in a large cohort of AS patients.
Methods Prospective observational cohort study of 170 consecutive AS patients treated with etanercept, monitored during 24 weeks of follow-up. Etanercept trough levels were determined retrospectively using an enzyme linked immunosorbent assay (ELISA) designed by Sanquin Research, Amsterdam. Response to etanercept treatment was defined as a Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) response (50% improvement or an absolute improvement of 2 points on a BASDAI, scale 0-10) and disease activity was measured using AS Disease Activity Score, including CRP (ASDAS).
Results Of 170 patients, 122 (71.8%) were male and 130 (82.8%) were HLA-B27 positive. At baseline median BASDAI was 6.0 (IQR 4.8-7.1) and mean ASDAS was 3.5 (SD 1.0). After 24 weeks 98 (57.6%) patients were BASDAI responder (last observation carried forward), mean ASDAS (SD) for responders was 1.6 (0.9) vs. 2.4 (0.9) for non-responders (p<0.001). Of 170 patients 19 dropped out: 16 (84.2%) due to treatment failure or adverse events and 3 (15.8%) due to other reasons. At 24 weeks of treatment etanercept levels were significantly higher in patients with inactive to moderate disease activity (ASDAS <2.1, median (IQR) 3.7 (2.5-5.5)) compared to patients with high to very high disease activity (ASDAS ≥2.1, (median (IQR) 2.3 (1.2-3.7)) (p=0.01). For BASDAI response no significant association with etanercept levels was found. When patients were categorized into quartiles according to the height of etanercept levels at 24 weeks, the lowest quartile (etanercept level < 1.8 mg/L) comprised 35.1% of all patients with ASDAS ≥2.1. The highest quartile (etanercept level > 5.0 mg/L) comprised 33.8% of all patients with ASDAS <2.1.
Conclusions Lower etanercept levels in AS are associated with high disease activity at 24 weeks of treatment.
Disclosure of Interest E. Kneepkens: None Declared, C. Krieckaert: None Declared, M. Nurmohamed Grant/research support from: Abbott, Roche, Pfizer, Consultant for: Abbott, Roche, Pfizer, MSD, UCB, SOBI and BMS, Speakers bureau: Abbott, Roche, Pfizer, I. van der Horst-Bruinsma: None Declared, G. Wolbink Grant/research support from: Pfizer, Speakers bureau: Pfizer, Amgen