Eculizumab, a humanized monoclonal antibody targeting complement C5, is the first approved drug for complement-mediated diseases and indicated to treat paroxysmal nocturnal hemoglobinuria, atypical hemolytic uremic syndrome, myasthenia gravis, and neuromyelitis optica spectrum disorder. The introduction of eculizumab has improved the prognosis of paroxysmal nocturnal hemoglobinuria and atypical hemolytic uremic syndrome to near-normal life expectancy and quality of life. Administration of eculizumab resulted in a rapid and sustained reduction in hemolytic activity and a consequent risk of thrombosis in paroxysmal nocturnal hemoglobinuria, and thrombotic microangiopathy in atypical hemolytic uremic syndrome, respectively. Nevertheless, many patients still have difficulty accessing eculizumab treatment because of its high costs. Biosimilars to reference eculizumab may increase patient access to treatment by creating market competition and eventually decreasing treatment costs. Clinical use of biosimilars in Europe in the last 15 years has demonstrated that they are as safe and effective as their reference products, and can also drive cost reductions and increase patients' access to treatment. This review aims to increase awareness about the importance of biosimilars of reference eculizumab and their entry for use in patients with paroxysmal nocturnal hemoglobinuria or atypical hemolytic uremic syndrome based on the accumulated experience of other previously approved biosimilars, and to provide an overview of the stringent biosimilar development pathway in general and the concept of extrapolation in particular.
