European regulators laid out the rules for copying biotech's blockbuster monoclonal antibody (mAb) therapies, paving the way for biosimilars developers to access the $36.4 billion market. The draft guidelines, published by the European Medicines Agency (EMA) in November, outline the process biosimilars developers must follow to gain approval for a mAb once a patent on the pioneer drug has expired. The studies and tests needed for approval are “less demanding than expected,” comments Huub Schellekens at the departments of Pharmaceutical Sciences and Innovation Studies at Utrecht University, The Netherlands. The EMA will require in vitro pharmacokinetic and phamacodynamic studies to demonstrate that a biosimilar mAb is functionally equivalent to a reference mAb. In some cases, in vivo nonclinical studies may also be necessary. “The need for these studies should be decided on a case-by case basis,” the guideline states. Factors that may warrant the need for such studies are, for instance, processing and formulation differences or insufficient evidence that a biosimilar is as safe and effective as the branded product. The EMA is willing to accept a drug's adverse event profile as proof of biosimilarity, and data from one clinical trial could be sufficient for approval in two different indications if the mechanism of action is the same. “This really opens the door [to biosimilars],” Schellekens points out. The guideline is available for public comment until May 31.