* AbbVie to pay $261.25 per share in cash and stock
* Price at 13 pct premium to Pharmacyclics' Wednesday close
* Deal cuts reliance on Humira; expected to close mid 2015 (Adds Breakingviews link)
By Supriya Kurane and Ben Hirschler
March 5 (Reuters) - AbbVie Inc is to buy Pharmacyclics Inc for about $21 billion, giving it access to what is expected to be one of the world's top-selling cancer drugs and expanding its reach in the profitable oncology field.
The deal -- the latest example of a big drugmaker swooping on a biotech firm to refill its medicine pipeline -- confounds expectations that Pharmacyclics would sell out to Johnson & Johnson.
AbbVie will pay $261.25 per share in cash and stock, a 13 percent premium to Pharmacyclics stock's closing price on Wednesday. Back in 2008 and 2009, the shares dipped below $1.
The acquisition lessens Chicago-based AbbVie's dependence on its blockbuster rheumatoid arthritis drug Humira that accounts for most of its revenue but is expected to start to see sales decline from 2017 or 2018.
AbbVie failed last October to buy Dublin-based Shire Plc for $55 billion after the United States took steps to deter such tax-lowering deals.
Deutsche Bank analyst Robyn Karnauskas said the deal was positive for AbbVie as Pharmacyclics' blood cancer treatment Imbruvica would diversify the business beyond Humira.
"Imbruvica is not only complementary to AbbVie's oncology pipeline, it has demonstrated strong clinical efficacy across a broad range of hematologic malignancies," AbbVie Chief Executive Richard Gonzalez said in a statement.
Pharmacyclics expects U.S. sales of Imbruvica to hit $1 billion this year and by 2020 worldwide sales are forecast to reach $5.8 billion, according to consensus analyst estimates compiled by Thomson Reuters Cortellis.
AbbVie, which was spun out of Abbott Laboratories in 2013, said the deal would be "highly accretive" to its revenue and earnings by 2017.
Sunnyvale, California-based Pharmacyclics co-markets Imbruvica with Johnson & Johnson. Besides Imbruvica, it has three product candidates in development.
In a statement, J&J said: "We're looking forward to continuing our collaboration with the team at AbbVie to further develop and commercialize this important therapy for patients and their health care teams."
Media reports had said J&J was close to buying Pharmacyclics. Novartis AG was also interested in the company, a report said.
The acquisition is the latest in a spate of big pharma deals this year as many companies are strengthening their portfolios as old drugs go generic.
Last month, Pfizer Inc agreed to buy Hospira Inc for about $15 billion, and Canada's Valeant Pharmaceuticals International Inc agreed to buy Salix Pharmaceuticals Ltd for about $10 billion.
The deal, expected to close in the middle of the year, comprises about 58 percent cash and 42 percent AbbVie common stock. Pharmacyclics shareholders can opt for cash, AbbVie stock or a combination, AbbVie said.
Morgan Stanley advised AbbVie, while Centerview and J.P. Morgan worked for Pharmacyclics.
Reporting by Supriya Kurane in Bengaluru and Ben Hirschler in London; Editing by Gopakumar Warrier and Mark Potter