Biosimilar filgrastim (Sandoz) was approved in Europe in 2009 and, in 2015, was the first
biosimilar approved in the USA. These authorizations were based on the "totality of evidence" concept, an approach that considers data from structural and functional characterization and comparability analysis and non-clinical and clinical studies. For
biosimilar filgrastim, phase III confirmatory clinical studies were performed in the most sensitive population, patients with
breast cancer undergoing myelosuppressive
chemotherapy. In Europe and the USA, approval was granted for all indications of the reference
biologic. Hence,
stem cell mobilization and
severe chronic neutropenia indications were approved on the basis of extrapolation, with no clinical data available at the time of market authorization in the EU. Although extrapolation is well-accepted in
biologic development and regulatory contexts, it remains a misunderstood part of the biosimilarity concept in the medical community. Since approval, more than a decade of obtained clinical experience supports the totality of evidence and reassures clinicians regarding the efficacy and safety of
biosimilar filgrastim. This includes real-world data from MONITOR-GCSF, a multicenter, prospective, observational study describing treatment patterns and clinical outcomes of patients with
cancer (n = 1447) receiving
biosimilar filgrastim for the prophylaxis of
chemotherapy-induced
neutropenia in solid
tumors and
hematological malignancies. Evidence is also available from unrelated healthy donors and those with
severe chronic neutropenia. Together, the experience from a decade of use of
biosimilar filgrastim includes over 24 million patient-days of exposure, which can help reassure oncologists that extrapolation is based on strong scientific evidence and works in practice.