Pfizer Inc.’s proposed $43 billion takeover of Seagen Inc. will undergo an in-depth antitrust review by the Federal Trade Commission.

Seagen disclosed in a regulatory filing that both companies received a so-called second request seeking additional information on the deal from the antitrust agency. In June, the companies withdrew their filing and submitted it again to give the FTC more time to conduct their initial review.

The request is normal in major deals, though it could signal potential antitrust difficulties. It may lengthen the review period by months or years, depending on its complexity. The companies still expect to close the deal by the end of 2023 or early next year.

Pfizer announced that it would buy the cancer-drug maker in March, the largest takeover so far this year. Seagen is a leader in developing a type of medicine called antibody-drug conjugates, which uses antibodies to deposit a strong concentration of drug directly at a tumor site — an effort to increase efficacy with fewer side effects.

If the deal goes through, Seagen’s portfolio will double Pfizer’s pipeline of early-stage experimental cancer therapies, Pfizer has said.

The companies said when the deal was announced that they expected close FTC scrutiny, but didn’t foresee any antitrust difficulties. However, in May, the FTC challenged Amgen Inc.’s proposed purchase of Horizon Therapeutics Plc. It was the first time the agency has sought to block a pharmaceutical acquisition in more than a decade and raised doubts about the viability Pfizer’s plan to buy Seagen.

--With assistance from Jonathan Roeder and Catherine Larkin.