AbbVie Inc. says it is buying leukemia drugmaker Pharmacyclics for $21 billion. It is the first big acquisition for AbbVie since it last year pulled out of a deal to buy Irish drugmaker Shire after the U.S. government reduced the tax advantages of the offshore transaction. Pharmacyclics is known as the maker of Imbruvica, a drug that treats blood cancers. AbbVie said Thursday is offering $261.25 a share for Pharmacyclics under the cash and stock takeover. Pharmacyclics is recommending its shareholders accept the offer.