(Bloomberg) -- Amgen Inc. agreed to buy Horizon Therapeutics Plc for about $27.8 billion in its biggest-ever acquisition, deepening its commitment to treatments for autoimmune, inflammatory and rare diseases.

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Amgen will pay $116.50 a share in cash, the companies said in a statement Monday, for a premium of about 48% since Horizon Therapeutics disclosed on Nov. 29 that it was in early talks with three suitors. The announcement confirms an earlier report by Bloomberg News.

Horizon shares gained as much as 15% to $112 as of 11:47 a.m., their highest intraday level since April. Amgen’s fell 1.5%.

Horizon’s therapeutic focus overlaps with that of Amgen, the maker of Enbrel for autoimmune ailments like psoriasis and ankylosing spondylitis. Such therapies are often tested and used in a wide variety of indications after inititally reaching the market, which can add to sales. Horizon gets almost half of its $3.6 billion in annual sales from Tepezza, a treatment for a painful autoimmune condition called thyroid eye disease. In October, Amgen acquired another drug company with a similar immune focus, ChemoCentryx Inc.

Emerging from the exhausting focus on Covid-19, big drugmakers are resuming their search for innovative therapies, especially for those that treat rare diseases and cancer. The deal for Horizon is the biggest in pharma since AstraZeneca Plc bought Alexion Pharmaceuticals for $39 billion in 2020. Horizon is developing drugs for conditions including lupus, alopecia, arthritis and kidney transplant rejection.

Amgen in particular is contending with the threat of diminished revenue as drugs that currently make a lot of money for the company face the loss of patent protection in the coming years.