Pfizer Inc (PFE.N) is buying Anacor Pharmaceuticals Inc (ANAC.O) in a US$5.2 billion deal to add an eczema gel to its portfolio, a month after the U.S. drug major scrapped plans to buy Allergan Plc AGN.N.

Anacor shares rose 54 per cent to US$98.60 in premarket trading on Monday, just below the offer price of US$99.25 per share in cash. The net-of-cash deal value assumes conversion of Anacor's outstanding convertible notes, the companies said in a joint statement.

Pfizer, which is also said to be in talks to buy cancer drug maker Medivation Inc (MDVN.O), seems to be focusing on smaller deals after its US$160 billion bid for Dublin-based Allergan failed.

The Anacor deal will give Pfizer access to a non-steroidal topical gel, crisaborole, which is currently under review by the U.S. Food and Drug Administration for the treatment of mild to moderate eczema.

The equity value of the deal is US$4.45 billion, based on 44.87 million Anacor's outstanding fully diluted shares as of March 31, according to Thomson Reuters data.

Pfizer said it believed crisaborole has the potential to reach or exceed peak sales of $2 billion.

In the past 15 years, there have been no new molecules approved for eczema - or atopic dermatitis - a common, relapsing, inflammatory skin disorder that affects about 18-25 million people in the United States.

Anacor also holds the rights to Kerydin, a topical treatment for toenail fungus, marketed in the United States by Novartis AG's (NOVN.S) Sandoz unit.

Pfizer's current inflammation and immunology drugs portfolio includes Enbrel and Xeljanz, which target auto-immune diseases.

The company needs to buy new medicines or biotech companies to boost its earnings growth, analysts have said.

Enbrel's patents for Europe expired in February last year and its U.S. patents are set to expire in 2028.

Pfizer shares were down slightly at US$33.20 before the bell.

Up to Friday's close, Anacor shares had fallen nearly 60 per cent from their record high of US$156.93 touched in July.

Centerview Partners and Guggenheim Securities were Pfizer's financial advisers and Citi advised Anacor.

Wachtel Lipton Rosen & Katz is Pfizer's legal adviser for the deal and Davis Polk & Wardwell LLP advised Anacor.