Shares in Horizon Therapeutics fell sharply after reports emerged that the US Federal Trade Commission (FTC) is preparing a lawsuit to block its nearly $28 billion takeover by Amgen, according to multiple media outlets.

The action would be the FTC’s first attempt to thwart a pharmaceutical industry deal in more than a decade. In January, the FTC requested more details regarding the purchase.

The deal, which was reached in December after Sanofi and Johnson & Johnson withdrew their bids for Dublin-based Horizon, has received an official response from Amgen, which said, “We have been working cooperatively over the past several months to address the questions raised by the FTC’s investigative staff and believe we have overwhelmingly demonstrated that this combination poses no legitimate competitive issues.”

Since US Senator Elizabeth Warren mentioned it in a letter to the FTC expressing worries about “rampant consolidation in the pharmaceutical industry and its impact on drug affordability and access,” there have been whispers about the potential for regulatory opposition.

With revenues of more than $400 million in the first three months of the year, Tepezza, a treatment for thyroid eye disease, is on track to become a multibillion-dollar seller, and Amgen hopes to acquire a few other already-marketed prescription cures through the purchase.

There are concerns that other significant purchases will be challenged, such as Merck’s $43 billion push for Seagen, if the Amgen-Horizon deal is rejected.