David BrennanThe boss of AstraZeneca has announced he is stepping down as the drug maker slashed its full-year profit hopes following a series of patent setbacks.

David Brennan, who has been in the post for six years, will leave on June 1 after coming under fire for not acting sooner to tackle a shortage of blockbuster drugs as patents on existing best sellers expire.

The UK's second biggest drugs maker has struggled in recent months after it lost patent cover on anti-psychotic drug Seroquel and tests on ovarian cancer drug olaparib proved it to be ineffective.

The struggle at AstraZeneca was underlined by the results for the first three months of the year, which blamed a "loss of exclusivity" for a 38% decline in pre-tax profits to 2.1 billion US dollars (£1.3 billion) and an 11% drop in revenues to 7.3 billion US dollars (£4.5 billion).

Executive director and chief financial officer Simon Lowth will act as interim chief executive from June 1 until a permanent successor is in place for the group which has lowered its target for full-year earnings per share.

Announcing his retirement, Mr Brennan said: "After more than six years as chief executive of this great company I have decided that now is the right time to step down and allow a new leader to take the reins."

He added: "I am confident that AstraZeneca will continue to have a positive impact on the lives of patients around the world and by doing so will deliver real value to our shareholders."

Mr Brennan, appointed to his current role in January 2006 and one of the longest serving chief executives in the pharmaceutical sector, started his career in 1975 at healthcare firm Merck where he was a sales representative in the US Division.

The group said the loss of exclusivity on several key brands accounted for eight percentage points of the 11% revenue decline, which included the impact of losing the Seroquel patent.

Astra's so-called "patent cliff" - when revenues drop off as patents expire - is particularly steep as breast cancer drug Arimidex has already lost patent protection while acid reflux treatment Nexium and biggest seller Crestor will lose patent protection in the next four years.