Anacor Pharmaceuticals Inc had received an offer from a third party before it agreed to be bought by Pfizer Inc in a $5.2 billion deal, regulatory filings showed.

Pfizer agreed to buy Anacor on May 16 to add an eczema gel to its portfolio, a month after the U.S. drug major scrapped plans to acquire Allergan Plc.

At the time, some analysts said they had expected Anacor to be acquired by Allergan, which has a stronger presence in the dermatology market, or by Novartis AG's Sandoz unit, which sells Anacor's toenail fungus drug in the United States.

Anacor said in a regulatory filing on Thursday that Pfizer originally offered $95 per share on May 6 but its financial adviser, Citi, recommended the company improve the bid, given Anacor had received another offer.

On May 13, Pfizer communicated to Citi its "best and final" offer of $99.25 per share, while the other bidder said it was ready to offer $90 per share, and an additional $500 million of crisaborole generated net sales of more than $1 billion in the first eight quarters post launch.

Anacor's crisaborole is currently being reviewed by the U.S. Food and Drug Administration.

Some analysts have also said the relatively low termination fee of about $181 million could persuade Anacor to accept a superior proposal.

Anacor's shares ended the day up about 1 percent at $100.07 on Thursday. They had closed above Pfizer's offer price for eight of the nine days since May 16.