Shire Pharmaceuticals

(SHPGY)

tumbled in early trading after the drugmaker's chief executive unexpectedly announced his resignation.

Shire said Chief Executive Rolf Stahel will step down once the company has found a replacement, adding that the resignation was by mutual agreement and was unrelated to the company's strategy.

Still, Stahel oversaw the company's recent acquisition strategy and his departure was being viewed with some urgency by investors, who offered down the shares nearly 8% to $23.80.