Irish drug-maker Shire has entered an agreement to acquire US biopharmaceutical firm Dyax for $5.9bn, in a bid to expand and extend its hereditary angioedema (HAE) portfolio.

The deal will give the company access to Dyax’s lead pipeline product, DX-2930, which is a Phase III-ready asset, offering potentially transformative prophylactic therapy for HAE.

The Irish firm will pay an initial $37.30 a share for Dyax, which represents a 35.5% premium to US biotech firms’ closing stock price on Friday.

Dyax shareholders will also be eligible to receive a non-tradable contingent value right (CVR) potentially worth $4 a share or an additional $646m, if Dyax’s DX-2930 drug is approved to treat HAE, a rare and potentially life-threatening disease that causes swelling.

After securing approval from the US Food and Drug Administration (FDA), DX-2930 is expected to be launched into the market in 2018.

"Through compelling proof-of-concept clinical data, this potentially transformative therapy has been shown to be both highly efficacious and convenient."

DX-2930 is a fully humanised monoclonal antibody targeting pKal with proof-of-concept Phase IB efficacy data, which showed a >90% reduction in HAE attacks compared to placebo in the 300mg/400mg arms in patients with more than two attacks in the three months prior to study entry.

If approved, DX-2930 has the potential to expand HAE-treated patients and achieve annual worldwide sales of up to $2bn with exclusivity beyond 2030.

Shire chief executive officer Flemming Ornskov said: "We have closely followed DX-2930’s progress in the evolving HAE landscape for some time, and we admire the work of the Dyax team in moving this next-generation therapy forward.

"Through compelling proof-of-concept clinical data, this potentially transformative therapy has been shown to be both highly efficacious and convenient, two key product attributes desired by both physicians and patients."

DX-2930 has received fast track, breakthrough therapy and orphan drug designations by the FDA, as well as secured orphan drug status in the EU.

By the end of this year, DX-2930 is expected to enter Phase III clinical trials, the company noted.

Ornskov said: "DX-2930 is a strategic fit within our HAE domain expertise, and we are well-positioned to advance the development, registration, and commercialisation of DX-2930 for the benefit of HAE patients.

"This transaction also offers other potential upside opportunities, including Dyax’s early-stage pipeline. Following the close of this transaction, we look forward to welcoming Dyax employees, who will bring to Shire substantial clinical and commercial expertise in HAE."

In August, Shire made an offer of around $30bn to acquire Baxalta , a drugmaker subsidiary of Baxter.

Shire has proposed to pay $45.23 per Baxalta share, while Baxalta has also confirmed that it received highly conditional and unsolicited proposal from the Irish firm to buy all of its outstanding common shares in an all-stock transaction.

Ornskov added: "I am also confident that our M&A expertise and the ongoing strength of our business will enable rapid and effective integration following the closing, as demonstrated by the success of our NPS and ViroPharma acquisitions.

"Even with this transaction, we will continue to have the financial firepower to pursue other value-added strategic acquisitions, including Baxalta."

Dyax is mainly focused on the development of plasma kallikrein (pKal) inhibitors to treat HAE.

The US firm has already developed and commercialised Kalbitor, which is approved for HAE acute treatment in patients 12 years of age and older, and was an early innovation in HAE treatment.