The U.S. Federal Trade Commission (FTC) has suspended its challenge of Amgen's $27.8 billion purchase of Horizon Therapeutics, allowing the FTC to consider whether the agency should settle the case, a filing late on Friday showed.

The pause is effective until Sept. 18.

The FTC filed a lawsuit on May 16 aimed at stopping the transaction in a rare move to block a large pharmaceutical deal.

The FTC had said it opposed the deal because of concern that Amgen would leverage its big selling drugs to pressure insurance companies and pharmacy benefit managers to give favorable terms for Horizon's two key products - the fast-growing thyroid eye disease treatment Tepezza and gout drug Krystexxa.

The Thousand Oaks, California-based company announced plans to buy Horizon in December last year, saying that its rare disease drugs would offer it some protection from the drug pricing provisions of the Inflation Reduction Act, which are aimed at drugs most widely used by the government's Medicare health plan.

Amgen and Horizon Therapeutics did not immediately respond to a Reuters' request for comment.

(Reporting by Urvi Dugar in Bengaluru; editing by Jason Neely and Diane Craft)