Summary Background Infliximab biosimilars have become available for treatment of inflammatory bowel disease (IBD). However, data showing long‐term safety and effectiveness of biosimilars in IBD patients are limited. Aim To study prospectively the switch from infliximab innovator to biosimilar in an IBD cohort with 12 months follow‐up to evaluate safety and effectiveness. Methods Adult IBD patients from two hospitals treated with infliximab innovator (Remicade; Janssen Biotech,  Horsham ,  Pennsylvania, USA) were switched to infliximab biosimilar (Inflectra; Hospira, Lake Forest, Illinois, USA) as part of routine care, but in a controlled setting. Blood samples were taken just before the first, second, fourth and seventh infusion of biosimilar. Infliximab trough levels, antibodies‐to‐infliximab (ATI), CRP and ESR were measured and disease activity scores were calculated. Results Our cohort consisted of 133 IBD patients (64% CD, 36% UC). Before switching we found widely varying infliximab levels (median 3.5 μg/mL). ATI were detected in eight patients (6%). Most patients were in remission or had mild disease (CD: 82% UC: 90%). After switching to biosimilar, 35 patients (26%) discontinued therapy within 12 months, mostly due to subjective higher disease activity (9%) and adverse events (AE, 9.8%). AE included general malaise/fatigue (n = 7), arthralgia (n = 2), skin problems (n = 2) and infusion reactions (n = 2). No differences in IFX levels, CRP, and disease activity scores were found between the four time points (P ≥ .0917). Conclusions We found no differences in drug levels and disease activity between infliximab innovator and biosimilar in our IBD cohort, indicating that biosimilars are safe and effective. The high proportions of discontinuers were mostly due to elective withdrawal or subjective disease worsening.

Although all assays are suitable for TDM, significant differences were observed in both imprecision and agreement. Therapeutic consequences were acceptable when patients were divided in treatment categories, but this could be improved by assay standardization.

The objective of this study is to apply therapeutic drug monitoring (TDM) as an objective tool to monitor the switch from infliximab innovator (INX) to infliximab biosimilar (INB) in our diverse rheumatic cohort in daily clinical practice. All rheumatic patients on INX treatment (Remicade®) and ≥18 years were switched to INB (Inflectra®) as part of routine care, but in a controlled setting. Patients were monitored by taking blood samples just before the first infusion of INB (T1), and after the second (T2), fourth (T3), and seventh (T4) infusion of INB. T4 reflects the patients' status after ∼12 months. Infliximab trough levels, antibodies-to-infliximab (ATI), C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), and validated disease activity scores (if possible) were measured. Our population consisted of 27 patients with seven different rheumatic diseases who had received INX for 143 (58-161) months (median (IQR)). Half of the patients (52%) received concomitant immunosuppressives. We found widely varying infliximab levels, with only 56% within the proposed therapeutic range of 1-5 μg/mL. One patient had very high ATI levels (>880 au/mL), and two had low ATI levels (≤30 au/mL). After switching to INB, seven patients (26%) discontinued the therapy, partially due to subjective reasons. No difference in infliximab levels, CRP levels, and disease activity scores was found between the four time points (p ≥ 0.2460). In conclusion, no pharmacokinetic or clinical differences were found between INX and INB in our diverse rheumatic cohort. TDM is a helpful tool to monitor patients switching from INX to INB.