Amgen Inc will buy rare disease drugmaker Horizon Therapeutics Plc in a deal valued at US$27.8 billion, boosting the biotech firm’s rare diseases portfolio and marking the biggest acquisition in the sector this year.

The company will pay US$116.50 in cash for each Horizon share held, a premium of nearly 20% to the stock’s last close.

Amgen gains several approved drugs through the deal, which could help counter the impact from rising competition for its top-selling arthritis drug, Enbrel, from newer treatments. Enbrel and some other drugs are expected to lose patent protection in the next few years.

The deal should help Amgen come close to achieving its 2030 sales goal, said BMO Capital Markets analyst Evan Seigerman.

Amgen emerged as the sole bidder after rivals Sanofi and Johnson & Johnson pulled out of the race.

Shares of Horizon jumped 14.4% to US$111.37 premarket. They have gained 23.5% through Friday close since the company said in late November that it was in preliminary talks with Amgen, Sanofi and Johnson & Johnson for potential takeover offers.

Amgen's shares fell 2.4% to US$272.

Thyroid eye disease treatment Tepezza is Horizon’s largest selling drug and generated net sales of US$1.66 billion in 2021, while Krystexxa, which is used to treat gout not controlled by other medicines, brought in sales of US$565.5 million.

Sales of Amgen’s Enbrel have declined over the last four quarters, tumbling 14% in the latest reported quarter to US$1.1 billion.

The offer values Horizon at US$27.8 billion on a fully diluted basis, according to the company, which includes ordinary shares to be vested. Based on Reuters calculations, it values Horizon at US$26.40 billion.

Amgen has entered into a US$28.5 billion debt agreement with Citibank and Bank of America in connection to the deal.

The offer values Horizon at US$28.3 billion, including debt.

  - Reuters