Barclays analyst Douglas Tsao says that while he'd probably stop short of calling it "mission impossible," he sees "significant challenges" in the development of a biosimilar to Allergan's (AGN) Botox. Following a deep dive into the key issues Mylan (MYL) will likely face in developing a biosimilar Botox, the analyst and concluded that it seems "technically difficult and faces a long road ahead." Even a best case scenario puts timing out to 2023, Tsao tells investors in a research note. The analyst has an Overweight rating on Allergan with a $230 price target.