AstraZeneca's sales fell by a bigger-than-expected 13% in the first quarter as patent expiries took a heavy toll, underscoring the turnaround challenge facing Britain's second largest drugmaker.

The biggest damage was caused by loss of exclusivity on antipyschotic medicine Seroquel and heart drug Atacand in many markets, and on cholesterol fighter Crestor in Canada.

Sales of Brilinta - a new heart drug on which AstraZeneca is pinning high hopes - picked up modestly in the three months to end-March to $51 million from $38 million in the last quarter of 2012.

The group reiterated its expectation for a mid-to-high single digit percentage fall in revenue this year, with earnings declining significantly more due to increased operating costs.

Sales in the quarter of $6.39 billion generated "core" earnings, which exclude certain items, down 25% at $1.41 a share, the company said on Thursday.

Analysts had, on average, forecast sales of $6.51 billion and earnings of $1.31, according to Thomson Reuters I/B/E/S.

Chief Executive Pascal Soriot, who joined from Roche last October, set out a far-reaching plan last month to return the group to growth by axing one in 10 jobs and reorganising its drug research operations.

His plan promises no quick fixes, although he aims to double the number of drugs in late-stage development by 2016 and says he will scour the industry for bolt-on acquisitions with which to replenish the company's medicine cabinet.