(Reuters) - AstraZeneca said on Thursday it would buy Actavis' branded respiratory drug business in the United States and Canada for an initial $600 million (394 million pounds) as it reported disappointing fourth-quarter earnings.

AstraZeneca, which is seeking such deals to drive growth, will also pay another $100 million after Actavis agreed to changes in ongoing collaborations between the two firms.

Britain's second-biggest drugmaker said 2015 sales would decline by a mid single-digit percent at constant exchange rates.

But adjusted or "core" earnings per share (EPS) are forecast to increase by a low single-digit percent this year, due to lower spending and plans to find partners on certain projects.

"We'll behave as a biotech company would when it comes to products that are not core," Chief Executive Pascal Soriot told reporters, citing a recent Alzheimer's deal with Eli Lilly as a model for future cost-sharing.

The weak quarterly results knocked the shares back 1.9 percent by 1110 GMT.

Citi analyst Andrew Baum said AstraZeneca appeared to have "kitchen-sinked" the quarter, or taken a deliberate hit to earnings by bringing forward some drug development costs. Deutsche's Mark Clark said this should help secure 2015 profits.

The group, which fended off a $118 billion bid by Pfizer last year, saw sales in the fourth quarter fall 2 percent to $6.68 billion, generating core earnings down 38 percent at 76 cents a share.

Industry analysts had on average forecast sales of $6.79 billion and earnings of 82 cents, according to Thomson Reuters.

Buying the Actavis respiratory drugs builds on AstraZeneca's acquisition of Almirall's lung drugs last July and gives it global rights to inhaled medicines containing aclidinium, which helps open airways. Actavis had owned North American rights under an earlier agreement with Almirall.

Respiratory is a key therapeutic area for AstraZeneca, which has predicted a 75 percent jump in annual sales to $45 billion by 2023.

"We are on track to return to growth by 2017 and are well positioned to deliver our long-term goals," Soriot said.

AstraZeneca's promising cancer drugs have won over many investors but it faces near-term profit pressure from loss of exclusivity on older drugs, with the arrival of generic copies of heartburn and ulcer medicine Nexium in the United States a big challenge in 2015.

After reporting core 2014 EPS of $4.28, Soriot and his team have good reason to ensure earnings hold up this year, since management incentives only vest fully if earnings remain at or above $4.20.