(The Center Square) – The Washington Attorney General’s Office has joined a lawsuit filed by the Federal Trade Commission, or FTC, against a prescription drug manufacturer to prevent it from acquiring a biotech company.

The lawsuit was filed by FTC in May against the California-based biopharmaceutical company Amgen after it announced its plan to buy Horizon Therapeutics for $27.8 billion.

The FTC complaint argues that the biotech company “currently enjoys a monopoly on the medicines that treat thyroid eye disease and chronic refractory gout. If consummated, the Acquisition would enable Amgen to leverage its portfolio of blockbuster drugs to foreclose actual or potential rivals to Horizon’s top-selling medications, thereby substantially lessening competition in the markets for the sale of FDA-approved drugs to treat TED and CRG.”

The lawsuit notes that Amgen has previously purchased rights to several drugs that have “successively expanded Amgen’s product portfolio, thereby increasing its leverage in negotiations over its products’ availability and reimbursement rates.”

In a statement, Attorney General Bob Ferguson wrote that he decided to join the lawsuit because “monopolies harm consumers by undermining competition. They also create an imbalanced playing field that harms other businesses.”

In a statement, Amgen argued that the basis for the FTC complaint “is entirely speculative and does not reflect the real world competitive dynamics behind providing rare-disease medicines to patients. 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. The medicines offered by Amgen and Horizon generally treat different diseases and patient populations, and there are no overlaps of competitive concern.”

Other states to join the lawsuit are New York, Illinois, California, Minnesota, and Wisconsin.