Horizon Therapeutics plc (HZNP)

Horizon Therapeutics plc was a biotechnology company that developed and sold medicines for rare autoimmune and severe inflammatory diseases. Its best-known products were Tepezza, the first approved treatment for thyroid eye disease; Krystexxa, used for chronic gout that does not respond to other therapy; and Uplizna, for neuromyelitis optica spectrum disorder. It also sold a portfolio of older primary care and orphan drugs.

The company began as Horizon Pharma, based in the Chicago area, and listed on NASDAQ in 2011 under the symbol HZNP. In 2014 it combined with Vidara Therapeutics and reincorporated in Ireland, moving its legal headquarters to Dublin while keeping major US operations in Deerfield, Illinois.

Horizon grew mainly by buying drugs and companies, including Raptor Pharmaceutical, the rights to Krystexxa through Crealta, the Tepezza developer River Vision, and Viela Bio, which brought Uplizna. In 2019 it changed its name to Horizon Therapeutics to reflect its focus on rare diseases, and Tepezza's launch in 2020 drove rapid revenue growth.

In December 2022 Amgen agreed to buy Horizon for $116.50 per share in cash. The US Federal Trade Commission sued to block the deal in May 2023 over concerns about product bundling, then settled in September 2023 after Amgen agreed to conduct limits. The acquisition closed on October 6, 2023, valuing Horizon's equity at about $27.8 billion. Horizon's shares were delisted from NASDAQ when the deal closed, and its products became part of Amgen's rare disease business.