Pfizer and Bristol-Myers Squibb have suffered a setback on the news that regulators in the USA have delayed giving the green light to the firms' much-touted bloodthinner Eliquis.

The US Food and Drug Administration has issued a complete response letter for Eliquis (apixaban) for the prevention of stroke and systemic embolism in patients with nonvalvular atrial fibrillation. In the CRL, the agency has requested additional information on data management and verification from Pfizer and B-MS' 18,201-patient Phase III study, called Aristotle.

The companies noted that the FDA has not requested any new studies. B-MS chief scientific officer Elliott Sigal said the firm still believes that Aristotle and another large trial, Averroes, "have established the therapeutic profile for Eliquis and demonstrated a meaningful advance over the standard of care", namely warfarin.

In March, the agency extended the action date by three months for Eliquis (apixaban) and following the CRL, Pfizer and B-MS will have a bit of catching up to do on two new bloodthinners for stroke - Boehringer Ingelheim's Pradaxa (dabigatran) and Bayer/Johnson & Johnson's Xarelto (rivaroxaban), which like Eliquis is an oral Factor Xa inhibitor.

However a number of analysts believe Eliquis, with its strong efficacy  and safety profile, notably regarding bleeding episodes, will become the market leader, despite the delays. In a research note, Tim Anderson at Sanford Bernstein said the FDA's stance is surprising "given the widespread perception that Eliquis is a best-in-class product" relative to Pradaxa and Xarelto, while Mark Schoenebaum at ISI Group wrote that "although this is a material time delay, we don’t see an impact to our conclusion that Eliquis will eventually dominate the bloodthinner space".

In May 2011, Eliquis was approved in the European Union for the prevention of venous thromboembolic events in adults who have undergone elective hip or knee replacement surgery.