AstraZeneca Plc has agreed to buy Alexion Pharmaceuticals Inc. for $39 billion in cash and stock, adding a specialist in the treatment of rare diseases and immunology to its portfolio of medications for cancer and other illnesses.

The offer values Alexion at $175 a share, a 45% premium to the closing price on Friday. It would be the largest deal for AstraZeneca since it was founded in a 1999 combination of British and Swedish companies, entrenching its position among the world’s 10 biggest drugmakers.

Acquiring Alexion would bolster AstraZeneca in areas such as the treatment of blood disorders, building on Chief Executive Officer Pascal Soriot’s turnaround. Since taking over in 2012 he has pushed the U.K. company further into lucrative areas such as oncology. AstraZeneca, along with the University of Oxford, has also developed a Covid-19 vaccine that’s shown effectiveness in large trials, despite questions around the study results.

High-priced medicines for exotic diseases can generate billions in sales from a relatively small group of patients. Snapping up drugmakers that focus on them has been a popular way for larger pharmaceutical companies to purchase sales growth in recent years.

“It’s an important step in the history of the company,” Soriot said on a call with reporters. “It’s a tremendous opportunity for us to accelerate our development of immunological” therapies.

AstraZeneca was on the hunt for acquisitions earlier this year and made a preliminary approach to Gilead Sciences Inc., Bloomberg reported in June. AstraZeneca fended off a takeover bid from Pfizer Inc. about six years ago.

Alexion has specialized in developing drugs that selectively inhibit immune factors to fight diseases that involve the body’s protective system. Soliris, the company’s biggest product, is a monoclonal antibody approved in several countries for treatment of immune-related conditions such as paroxysmal nocturnal hemoglobinuria.

Monoclonal antibodies have garnered more attention of late because two such drugs made by Eli Lilly & Co. and Regeneron Pharmaceuticals Inc. have been given emergency authorization in the U.S. for the treatment of Covid-19.

Soliris had about $4 billion in 2019 revenue. More recently, Alexion launched Ultomiris, another monoclonal antibody. The company announced plans in April to conduct late-stage tests of that drug in severely ill patients with Covid.

Alexion has been pressured in the past to put itself on the block. Activist investors Elliott Management Corp. opposed a deal the company struck earlier this year to acquire Portola Pharmaceuticals, saying the transaction didn’t make strategic sense and didn’t fit with Alexion’s focus on rare diseases.

In the sale to AstraZeneca, Alexion shareholders will receive $60 in cash and 2.1243 AstraZeneca American Depositary Shares for each Alexion share, the U.K. company said in a press release Saturday. The U.K. company will fund the acquisition with a $17.5 billion financing facility from Morgan Stanley, JPMorgan Chase & Co. and Goldman Sachs Group Inc.

Despite the premium, the price is attractive for AstraZeneca, and its agreement with Alexion could draw out other offers, SVB Leerink analyst Geoffrey Porges said in a note. Alexion shareholders could hold out for $200 per share or seek a higher cash portion, he said.

“We believe that in the coming days and weeks the debate about this transaction will center on whether this is enough, and whether other bidders might emerge, rather than whether this was too much,” Porges wrote.

The current deal includes a fee of $1.2 billon if Alexion agrees to sell itself to another bidder, while AstraZeneca faces a $1.4 billion breakup penalty. The acquisition is expected to close in the third quarter of 2021 and Alexion shareholders would own 15% of the combined companies.

Alexion had $5.9 billion in sales for the 12 months ending with the third quarter of 2020, with a 24% growth rate, AstraZeneca said.

AstraZeneca shares have risen 7% this year and almost 70% over the past three years for a market value of 107 billion pounds ($142 billion).

AstraZeneca said it intends to establish its headquarters for rare diseases in Boston. There are no plans for a major reduction in employment, Soriot said. The combined companies should have a dozen blockbuster drugs — those with more than $1 billion in sales — by 2023, up from nine as of 2019, Chief Financial Officer Marc Dunoyer said on a call.

The deal will add to earnings immediately, with annual synergies of about $500 million projected three years after completion, the company said.

Evercore Partners International LLP and Centerview Partners UK LLP are AstraZeneca’s lead financial advisers. Bank of America Corp. worked with Alexion.