Shire Plc (LON: SHP), the drugmaker that’s made a $30 billion unsolicited offer to buy Baxalta Inc. (NYSE: BXLT), agreed to buy biotechnology company Dyax Corp. (Nasdaq: Dyax) for at least $5.9 billion in cash to gain a promising treatment for a rare genetic disorder.

Shire will pay $37.30 for each Dyax share, the companies said in a statement on Monday. That’s a 35 percent premium over the stock’s Friday closing price. Dyax investors may also get an additional $646 million, or $4 for each share, if a compound known as DX-2930 receives approval for treating hereditary angioedema, or HAE, a disease that can be debilitating and sometimes fatal.

Dyax’s DX-2930 could generate annual sales of as much as $2 billion globally, and would have patent protection and other exclusivity pacts beyond 2030, the companies estimated. The treatment will be enrolled in a late-stage clinical trial by the end of the year, and has won fast track, breakthrough therapy, and orphan drug designations by the U.S. Food and Drug Administration.

Shares of Shire fell 1.1 percent to 4,871 pence as of 11:38 a.m. London time. The stock has climbed 18 percent over the past year, surpassing the almost 14 percent gain in the Bloomberg Europe Pharmaceutical Index.

The acquisition will curb Shire’s earnings for two years and then be accretive starting in 2018, assuming that U.S. regulators approve the use of DX-2930 that year. Dyax also sells Kalbitor for the treatment of acute attacks of HAE in patients 12 years of age and older.

Dyax, which is based in Burlington, Massachusetts, canceled a research and development meeting it was scheduled to hold in New York on Monday. Dublin-based Shire and Dyax will instead host a conference call to discuss the transaction at 8:00 a.m. New York time.

Shire will fund the acquisition, which the companies expect to complete in the first half of 2016, using a $5.6 billion term loan and its $2.1 billion revolving credit facility. The transaction is backed by the boards of both companies, but requires Dyax shareholders’ approval.

“Even with this transaction, we will continue to have the financial firepower to pursue other value-added strategic acquisitions, including Baxalta,” Shire Chief Executive Officer Flemming Ornskov said in the statement.

Shire on Aug. 4 publicly announced its offer to buy Baxalta for about $30 billion in stock to bolster its focus on rare diseases. Baxalta, a business spun off by Baxter International Inc. the previous month, had already rejected Shire’s offer and has continued to rebuff the company since then.

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