CEO David Brennan will step down in June

AstraZeneca posted a steep 19 percent drop in first quarter earnings Thursday prompting the surprise resignation of CEO David Brennan after six years at the helm of Britain's second-biggest drugmaker.

The London-based group said CFO Simon Lowth will take over as interim chief executive in June as the group reported earnings per share of $1.81 off of $7.35bn in sales. AstraZeneca also lowered its full year earnings outlook to between $5.86 and $6.15 per share from $6.00 and $6.30. The target was $7.28 per share in 2011.

Brennan has been under pressure to revive sales at profits at AstraZeneca, which has been hit hard by the loss of patents on key cancer drugs such as Arimidex and Casodex. The appointment of Leif Johansson as AstraZeneca's new chairman in March increased speculation that the board was searching for a new leader after six years with Brennan at the helm.

AstraZeneca shares plunged 5 percent in the opening minutes of London trading to change hands at 2,699.50 pence. The shares have fallen nearly 10 percent so far this year.