Shares of Pfizer (PFE) and Bristol-Myers Squibb (BMY) tumbled early Monday, as the FDA asked for more information on a key drug up for approval. The companies hoped that the agency would approve their stroke-prevention drug Eliquis this month, but instead the FDA sent a letter asking for more details on how researchers conducted a certain clinical trial. It did not ask for new studies, which would have entailed a longer delay. Eliquis is one of a