Celgene's success with Otezla is a stark contrast to Gilead Sciences' experience with Zydelig, a treatment for chronic lymphocytic leukemia. After winning approval in 2014, safety concerns have put a lid on Zydelig's sales. In Q3, Zydelig's revenue totaled only $39 million, a far cry from the billion-dollar blockbuster potential forecast by industry watchers when the drug launched.