Challenges and opportunities: what we are learning from the clinical natalizumab experience.

The approval of natalizumab for relapsing forms of multiple sclerosis, and the subsequent voluntary suspension of its use due to an unexpected viral infection, is a cautionary tale of how much we have to learn about how to prioritize and perform the necessary research and development of novel therapeutics for human diseases, the ethics of placebo-controlled trials and the relationships between researchers, regulatory authorities and the pharmaceutical industry.