Ireland-based and London-listed drugmaker Shire has agreed what is being called a “defensive” deal to buy US biotech firm Dyax, which has a breakthrough drug said to threaten one of its own.

Initally, Shire will pay $5.9 billion, or $37.30 per share, for the company in an all-cash deal. Dyax shareholders potentially could receive an additional $4 per share – adding $646 million to the total – if the development drug, DX-2930, is approved by regulatory authorities for treatment of hereditary angioedema by 2019.

The drug was fast-tracked by the US Food and Drug Administration (FDA) in July following completion of early-stage trials, in which it was shown to reduce by more than 90% the number of attacks of the potentially life-threatening hereditary disease that attacks the immune system and causes swelling.

The deal with the biotech firm, which would allow Shire to take control of phase 3 trials with DX-2930, comes on the heels of a stalemate in Shire’s attempt to acquire Baxalta, a recent spin-off from Baxter International. That company’s management thus far has resisted the takeover bid worth more than $30 billion.

Analysts are said to have warned that DX-2930 could cannibalize sales of Shire’s Cinryze, a drug to treat the same ailment, acquired with the $4.2 billion purchase of ViroPharma last year.

Shire CEO Flemming Ornskov said he likes to buy firms with trials at stage 3 level in order to shape the tests and “create the best profile for the product.” Ornskov has projected a launch of DX-2930 by 2018 and potential peak sales of $2 billion annually.

With Dyax, Shire also will gain access to the biotech firm’s Kalibitor, a post-attack treatment for hereditary angioedema with annual sales of more than $68 million.