Washington-based Seagen is a pioneer of antibody-drug conjugates, which work like "guided missiles" designed for a targeted destructive effect and spare healthy cells.
The Pfizer logo is pictured on their headquarters building in the Manhattan borough of New York City, New York, U.S., November 9, 2020.
The drugmaker expects more than $10 billion in sales from Seagen products in 2030, and another $15 billion from its other recent acquisitions.
Pfizer said antitrust regulators could closely review the deal due to its size but eventually approve it.
Pfizer rival Merck & Co Inc (MRK.N) and Seagen were in advanced deal talks last year but those reportedly collapsed over antitrust concerns.