UPDATE 2-U.S. Senator Warren 'particularly concerned' about Amgen, Indivior deals

(Adds comment from Amgen; updates share prices)

WASHINGTON, Jan 26 (Reuters) - Sen. Elizabeth Warren, an outspoken critic of corporate consolidation, wrote to the U.S. Federal Trade Commission to express concern about two pending pharmaceutical deals.

In a letter dated Wednesday, Warren said that she was focused on Amgen's plan to buy Horizon Therapeutics , and addiction specialist Indivior's plan to buy Opiant for $145 million, which was announced in mid-November.

"Given these companies' records of anti-competitive business practices, these acquisitions could cause further price increases on lifesaving drugs and prevent affordable alternatives from entering the market," she wrote in the letter to FTC Chair Lina Khan as well as Commissioners Rebecca Slaughter and Alvaro Bedoya, all of whom are Democrats.

Horizon Therapeutics closed down about 1.5% on Thursday while Opiant shed nearly 2%. Amgen closed down less than 1% on Thursday.

The agency shares the job of assessing mergers to ensure they comply with antitrust law with the Justice Department. It can sue to block planned acquisitions that it believes are illegal.

Warren said that both Amgen and Horizon Therapeutics "have engaged in brazen price increases," including on Amgen's Enbrel for arthritis and Horizon’s Krystexxa, a gout medication.

Amgen said it disagreed with Warren's analysis of the deal. "We have been working cooperatively with the Federal Trade Commission and remain confident there are no anti-competitive aspects of this transaction," the company said in a statement.

Warren noted that the FTC had settled with Indivior and its former parent over its attempt to protect its monopoly of the opioid addiction treatment Suboxone. Indivior makes Sublocade -- a slow-release treatment for opioid addiction that is administered monthly. Suboxone is given daily and also prevents an accidental overdose.

Opiant, original owner of opioid overdose remedy Narcan, is working on getting approval for a nasal spray version of nalmefene, believed to be a more effective opioid antagonist.

"The FTC should strongly consider Indivior's history of anti-competitive and deceptive practices when evaluating how Indivior might behave after this potential transaction is completed," wrote Warren.

Indivior said in its comment that nalmefene would be an "important additional option" to help people who have overdosed on an opioid but did not address Warren's concerns.

Warren noted that the FTC, which has long focused on healthcare mergers, said in 2021 that it would prioritize pharmaceutical acquisitions. She also asked the agency how it factored into its merger reviews efforts by the companies to extend patent protections, using strategies that are sometimes controversial. (Reporting by Diane Bartz; Editing by Chizu Nomiyama and David Gregorio)