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Background and Objectives: Long-term treatment of patients with rheumatoid arthritis with tumor necrosis factor-α inhibitors leads to initial changes in disease activity that can predict a late treatment response. This observational and retrospective study aimed to determine when it is possible to foresee the response to therapy in the case of long-standing rheumatoid arthritis comparing also the efficacy of the original biologics with their biosimilars. Methods: A total of 1598 patients were recruited and treated with the original biologics, adalimumab and etanercept, or with biosimilars. Patients were monitored over a period of 48 months and disease activity scores (28-Joint Disease Activity Score, Simplified Disease Activity Index, and Clinical Disease Activity Index) were measured every 6 months. Results: No differences in disease activity levels were observed in etanercept versus biosimilars (GP2015/SB4) and adalimumab versus biosimilar (GP2017) patient groups. All scores significantly decreased in all treatments during the first 18 months of therapy, and after 24 months reached a minimum that lasted up to 48 months. Conclusions: We conclude that biosimilars of adalimumab and etanercept have equivalent effectiveness over a long period of time compared to their originator drugs, and also that the levels of disease activity after 6 months of tumor necrosis factor-α inhibitors (originator drugs and biosimilars) might predict the response to therapy at 4 years in patients with long-standing rheumatoid arthritis. [ABSTRACT FROM AUTHOR]