Horizon Therapeutics plc and Amgen Inc. have reached an agreement on the terms of a cash offer for Amgen’s newly formed private company, Pillartree Ltd., to acquire Horizon in a deal valued at approximately $28 billion.   Amgen will gain Horizon’s thyroid eye disease drug Tepezza, chronic gout treatment Krystexxa and Uplizna, a newer therapy to treat neuromyelitis optica spectrum disorder. Horizon’s commercial medicines generated approximately $2 billion in sales for the first nine month...