In a surprising development, the FDA is letting another deadline slip by before approving blood thinner Eliquis, said Bristol-Myers Squibb and Pfizer in a statement.

It’s unclear exactly what the agency needs before approving the product. No new trials are being asked for, but the agency wants clarification of information from already completed studies, the companies said. The drug, for patients with nonvalvular atrial fibrillation, could have had sales of $275 million in 2012, $2.3 billion in 2015 and $3.5 billion in 2020, “but these numbers may need to be revised,” wrote Sanford Bernstein analyst Tim Anderson in an investor note today.

Most analysts expect Eliquis will be the best-in-class drug among a new crop of oral anticoagulants designed to replace warfarin. With its original PDUFA date of March 28 having passed, the next one had been scheduled for this Thursday.

“Per FDA regulation, the agency may take up to six months to review the companies’ response to the CRL,” BMS’s Laura Hortas told MM&M in e-mailed comments, adding that the firms are already working with the agency and hope its review can be completed “within a shorter time frame.”

Eliquis (apixaban) is poised to become the third warfarin substitute to reach the US market, behind Boehringer Ingelheim’s Pradaxa (dabigatran) and Johnson & Johnson’s Xarelto (rivaroxaban). It was approved in the EU last year. Edoxaban, a Factor Xa inhibitor being developed by Daiichi Sankyo, is in Phase III for treating atrial fibrillation as well as for preventing clots.

BMS and Pfizer, both of which have lost best-selling drugs to patent expiry, are counting on the new cardiovascular pill to help provide fresh revenue. Pfizer’s Lipitor, which generated $7.7 billion in US sales last year, went off patent in November, while BMS’s Plavix, which had $6.8 billion in the US last year, saw expiry in May.

Data from the Aristotle trial had shown that Eliquis is superior to warfarin on both bleeding and efficacy in preventing stroke. ISI Group analyst Mark Schoenebaum said the FDA likely wants assurances that the companies’ data sets are accurate in that regard. “Although this is a material time delay, we don’t see an impact to our conclusion that Eliquis will eventually dominate the blood thinner space,” he wrote.

Schoenebaum expects the firms will submit a response to the FDA’s letter by year’s end, maybe sooner, triggering six-month review and US approval by mid-2013. 

“The timeline for approving Eliquis is not clear, but late 2012 seems possible,” wrote Anderson.

Meanwhile, Bristol-Myers Squibb and Pfizer had been prepared to launch Eliquis this week and “will adjust timelines accordingly,” said Hortas. Both “are ready to bring Eliquis to market quickly upon receipt of approval.”

Pfizer is also awaiting FDA approval of tofacitinib for treating rheumatoid arthritis. The pill won an FDA advisory panel’s backing in May.