AbstractPurposeTo observe the efficacy, safety and immunogenicity of biosimilar infliximab (CT‐P13) in patients with refractory non‐infectious uveitis (NIU).MethodsRetrospective observational study. Indication for treatment was active uveitis unresponsive to classical immunosuppressants. Clinical data were retrospectively analyzed at baseline and 3 months of treatment. The main outcome measures were uveitis activity, adverse effects and serum biosimilar infliximab trough concentrations as well as anti‐infliximab antibodies (ATI).ResultsNine patients received biosimilar IFX after failing classical immunosuppressive treatment. Mean age was 53 years. The most common diagnosis was Birdshot (n = 4), followed by sarcoidosis (n =3 ), Behcet (n = 1), and Sympathetic Ophthalmia (n = 1). The duration of treatment ranged from 18 to 192 weeks. Five patients without active disease and one patient with activity had IFX levels >9 μg/mL. Two patients with active disease and 1 patient in remission had undetectable levels of IFX. Two patients with undetectable levels of IFX had positive anti‐infliximab antibodies (ATI) levels; both patients showed activity. The other patient with undetectable levels of IFX had no ATI and was in remission. None of the patients with IFX levels >9 μg/mL had positive ATI levels. One patient dropped out of treatment due to infusion‐related skin eruptions.ConclusionBiosimilars are pharmacologically highly similar to an approved biological pharmaceutical ‘reference’ product, but are not exact copies. Even if limited to a small group, this study provides evidence that low biosimilar IFX trough levels associate with increased antibodies in most, but not in all cases.The clinical use of measuring biosimilar infliximab (IFX) trough levels and antibodies against IFX (ATIs) remains unclear.