Background: Since the late '90s, infliximab (Remicade^®) is being used successfully to treat patients with several non-infectious immune mediated inflammatory diseases (IMIDs). In recent years, infliximab biosimilars, including Remsima^® were introduced in clinical practice. Aim: To investigate the interchangeability of Remicade^® (originator infliximab) and its biosimilar Remsima^® in patients with rare immune-mediated inflammatory diseases (IMIDs). Methods: This two-phased prospective open label observational study was designed to monitor the transition from Remicade^® to Remsima^® in patients with rare IMIDs. All included patients were followed during the first 2 years. The primary endpoint was the demonstration of non-difference in quality of life and therapeutic efficacy, as measured by parameters including a safety monitoring program, physicians perception of disease activity (PPDA) and patient self-reported outcomes (PSROs). Secondary outcomes included routine blood analysis, pre-infusion serum drug concentration values and anti-drug antibody formation. Results: Forty eight patients treated with Remicade^® were switched to Remsima^® in June-July 2016 and subsequently monitored during the first 2 years. The group consisted of patients with sarcoidosis (n = 17), Behçet's disease (n = 12), non-infectious uveitis (n = 11), and other diagnoses (n = 8). There were no significant differences in PPDA, PSROs, clinical and laboratory assessments and pre-infusion serum drug concentrations between the groups. De novo anti-drug antibodies were observed in two patients. Seven patients with sarcoidosis and five with another diagnosis developed a significant disease relapse (n = 7) or adverse events (n = 5) within 2 years; 10 of these patients discontinued Remsima^® treatment, one withdrew from the study and one received additional corticosteroid therapy. Conclusions: We observed no significant differences in PSROs, PPDA and laboratory parameters after treatment was switched from Remicade^® to Remsima^®. However, disease relapse or serious events were observed in 12 out of 48 patients when treatment was switched from Remicade^® to Remsima^®. The choice to switch anti-TNF alpha biologics in patients with rare IMIDs, particularly in sarcoidosis, requires well-considered decision-making and accurate monitoring due to a possibly higher incidence of disease worsening.

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