Biosimilar medicines--new challenges for a new class of medicine.

Biosimilars are a new class of medicine. The European Medicines Agency has pioneered the legal, regulatory, and scientific framework for approval of these products. One of the foundational principles of the European framework is that biosimilars are expected to be similar, not identical, to the innovator biologics they seek to copy. This contrasts with the legal, regulatory, and scientific framework for copies of chemical medicines, generics, which are based on the expectation that the innovator and generic drug substance are identical. This article reviews the European biosimilar regulatory pathway and reviews some of the clinical data being made public following the approval, rejection, and withdrawal of biosimilar marketing applications.