The US Federal Trade Commission is expected to file a lawsuit Tuesday to block Amgen Inc.’s $27.8 billion deal to buy Horizon Therapeutics Plc, arguing the tie-up would harm competition for drug development, according to a person familiar with matter.

Amgen said it isn’t aware of any decision made by the agency.

“We will provide any appropriate updates when we have more information,” the company said in a statement. Representatives of Horizon didn’t immediately respond to a request for comment.

Horizon fell 15% in after-hours trading.

The FTC declined to comment.

Horizon Therapeutics Drops on Report FTC May Block Amgen Deal

The companies are both working to develop treatments for eczema and lupus.

Eczema is a chronic inflammatory skin condition that affects as many as 25 million people in the US. Amgen and Japan’s Kyowa Kirin Co. announced in 2021 that they were partnering to develop an antibody treatment, KHK4083, for severe eczema. Horizon has partnered with Q32 Bio on its own antibody treatment for eczema, ADX-914.

Lupus is a chronic autoimmune disease that attacks internal organs, affecting about 1.5 million people in the US, mostly women, according to the Lupus Foundation of America. Amgen has been working on developing a biologic drug, efavaleukin alfa, as a treatment for the most common type of lupus, while Horizon has focused on an antibody treatment, Daxdilimab.

The FTC has often allowed pharmaceutical mergers to move forward so long as the companies divest any overlapping treatments. But the agency under Chair Lina Khan has taken a tougher approach to deals, challenging Microsoft Corp.’s proposed acquisition of Activision Blizzard Inc. and Intercontinental Exchange Inc.’s deal to buy mortgage software rival Black Knight Inc.

The Capitol Forum reported earlier on the expected suit.

--With assistance from Michelle Fay Cortez and Ike Swetlitz.

(Updates with Amgen comment.)