The $32 per share acquisition - a 54 percent premium to Ardea's closing price on Friday - is worth $1 billion after deducting cash held by Ardea, the companies said on Monday.
The planned purchase shows Britain's second-biggest drugmaker delivering on a promise to step up deal-making to fix its drug pipeline.
Research head Martin Mackay told Reuters last month he was looking at acquisitions in the low billions of dollars.
We are building up momentum, Mackay said on Monday. We will continue to do these external deals. I think we will be talking about more as the year goes on.
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The main asset secured by AstraZeneca is an experimental drug called lesinurad, or RDEA594, that is in final-stage Phase III clinical tests for treating patients with chronic gout who have too much uric acid in their blood.
Although gout is not a huge market, independent analyst firm Decision Resources puts sales of drugs for the disease at $1.87 billion in 2019, with Ardea's product - which is likely to be submitted for approval in the first half of 2014 - vying for a sizeable chunk of that business.
Cowen analyst Steve Scala said lesinurad could be used in as many as 20 percent of gout patients, translating into a $550-$750 million sales opportunity, which could rise as the number of people with the disease increases.
AstraZeneca and Ardea said they expected the transaction to close in the second or third quarter, adding that shareholders representing around 30 percent of Ardea had agreed to vote in favour of the deal.
Ardea shares leapt 52 percent to $31.60 by 1450 GMT as investors banked on the deal going through.
AstraZeneca is facing competition from cheap generic versions of several key drugs, including its big-selling antipsychotic Seroquel that went off-patent last month, at a time when its pipeline of new medicines is relatively barren.
Earlier this month AstraZeneca also signed a collaboration deal to jointly develop and sell five biotech drugs currently in Amgen's
AstraZeneca is more desperate for new products than many of its rivals but other Big Pharma companies are also scouring the biotech landscape for bargains, especially in cases where biotech stocks have underperformed recently.
Britta Holt, a director at Fitch Ratings, believes small to medium-sized acquisitions are likely to continue to be on the agenda of cash-rich Big Pharma companies, which are hungry for new medicines to put through their marketing machines.
For AstraZeneca, the decision to pull the acquisition trigger follows a fallow period in the wake of its 2007 purchase of MedImmune for $15.6 billion - a deal that was slammed by investors and has failed to deliver significant new drugs.
Since then it has bought nothing for more than $400 million. But management is under pressure to do something more than simply slash costs, since it has forecast revenues will fall by more than 10 percent in 2012, due to patent expiries.
Quarterly results on April 26 were expected to show earnings per share down 20 percent and sales off 4.5 percent, according to consensus forecasts from Thomson Reuters.
Some shareholders have argued for a management shake-up, including the replacement of David Brennan as CEO - something that could be on the cards when new non-executive chairman Leif Johansson takes over in September.
Analysts at Barclays, who upgraded the stock to equal weight from underweight on Monday, said while AstraZeneca shares looked cheap, investors would need patience to see a turnaround.
The difficulty for management is that success of its new products will not be established instantaneously. Investor distrust around potential M&A is also something that can only be disproven over months and years, they wrote in a note.
AstraZeneca's rivals, meanwhile, have also been actively hunting for bolt-on deals.
At the same time, Amylin Pharmaceuticals
AstraZeneca, which had cash reserves of $7.6 billion at the end of December, has also been mentioned as a potential bidder for Amylin. Research head Mackay said the company looked at everything but declined to comment on Amylin specifically.
AstraZeneca shares fell 1.1 percent by 1040 GMT, broadly in line with a 0.9 percent fall in the European drugs sector <.SXDP>.
Ardea, which also has a cancer drug in Phase I and Phase II trials that is licensed to Bayer
(Editing by David Cowell)