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Imke Redeker, Stefan Moustakis, Styliani Tsiami, Xenofon Baraliakos, Ioana Andreica, Björn Bühring, Jürgen Braun, Uta Kiltz

• \(\bf Background:\) Previous experiences with non-medical switching of adalimumab (ADA) in patients with chronic inflammatory rheumatic diseases (CIRD) come mainly from phase III extension of randomised clinical trials and little from routine care. \(\bf Objectives:\) To analyse treatment trajectories over 2 years in patients with CIRD conducting a non-medical switch from originator to biosimilar ADA. \(\bf Design:\) A retrospective observational cohort study was conducted with data from a third-level rheumatology centre in Germany. CIRD patients on originator ADA who switched to ADA biosimilar from October 2018 onwards were identified and followed until September 2020. \(\bf Methods:\) Patients’ characteristics were compared between the four \(\textit {a priori}\) defined treatment trajectories "continued biosimilar ADA therapy", "back-switch to originator ADA therapy", "switch to another biological disease-modifying anti-rheumatic drug (bDMARD) therapy" and "stopped bDMARD therapy/death/drop out". Factors associated with continuing biosimilar ADA therapy were analysed using Cox proportional hazards regression analyses. \(\bf Results:\) A total of 121 CIRD patients were included. Most patients (66.9%) continued therapy with biosimilar ADA over 2 years, with a treatment retention rate of 73.1%. Whereas 21 patients (17.4%) switched back to originator ADA, mainly due to adverse events, and 8 patients (6.6%) switched to a different bDMARD, mainly due to lack of effect. The estimated risk of withdrawal was lower for longer prior duration on originator ADA [hazard ratio (HR): 0.82; 95% CI: 0.69–0.97] and higher for higher C-reactive protein levels at baseline (HR: 1.18; 95% CI: 1.00–1.39). Male patients, older patients and those for whom originator ADA was their first bDMARD tended to have a lower risk of withdrawal. \(\bf Conclusion:\) Our results indicated that three of four patients continue biosimilar ADA over 2 years with lower risks of withdrawal for male sex, older age, longer prior duration on originator ADA and originator ADA as first bDMARD.