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Irvine-based drug maker Allergan Inc. was sued today by activist investor Bill Ackman’s Pershing Square Capital Management LP.

Ackman, who owns about 10% of the Botox maker, and Canada-based Valeant Pharmaceuticals International Inc. are teaming up on a hostile takeover attempt. Allergan has consistently rebuffed Valeant’s advances, including rejecting a cash and stock deal valued at about $53 billion earlier this week.

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Ackman and Pershing want a federal court to confirm that their effort to call a special meeting of Allergan shareholders to oust six Allergan directors does not trigger a “poison pill” adopted by Allergan’s management earlier this year. That poison pill limits Pershing Square’s stake in Allergan to less than 10% and could restrict talks between investors.

Allergan said in a letter to Pershing Square that “it would nonetheless be all but impossible to provide full and complete responses” to questions set forth by the investor regarding the lawsuit.

Allergan’s shares were up 1% to a market value of $48.5 billion. Valeant shares were flat with a market value of $40.4 billion.