BRUSSELS Pfizer's $43 billion bid for Seagen will be decided by EU antitrust regulators by Oct. 19, a regulatory filing showed on Friday.

U.S. pharmaceutical company Pfizer announced the deal, its biggest, in March for Washington-based Seagen, a pioneer of antibody-drug conjugates, which work like "guided missiles" designed to destroy cancer while sparing healthy cells.

The European Commission can either clear the deal after a preliminary review or it can start a four-month long investigation if it has serious concerns.

Pfizer sought EU antitrust approval on Thursday.

(Reporting by Foo Yun Chee; editing by Grant McCool)