The flurry of mergers taking place among mid-sized European drug companies in recent months seems to have inspired their peers across the Atlantic.

Now, Gilead Sciences has taken the plunge with a $2.5 billion buyout of Myogen that would give it ownership of ambrisentan, a drug for pulmonary artery hypertension in Phase III testing, and reduce its reliance on HIV drugs.

HIV drugs account for 80% of Gilead’s product sales, and the company is currently cash-rich because it makes the flu drug Tamiflu (oseltamivir) for Roche, which has seen sales explode on fears of a pandemic in recent months.

Gilead’s $52.50-per-share offer comes shortly after the biotech bought out respiratory drug specialist Corus Pharma for $365 million, adding Cayston (aztreonam) for cystic fibrosis-related pulmonary infections to its portfolio of Phase III drug candidates.

Myogen has said it expects to file for approval of ambrisentan before the end of the year and, if approved, analysts have predicted that the drug could in time become a $1 billion product.

Datamonitor said in a report published earlier this year that it expects the global PAH market to grow from $636 million in 2004 to just over $2 billion by 2014, driven by new product and formulation launches and increased treatment rates. Up to 75% of PAH sufferers are currently not recognised and treated.

But PAH is fast becoming a competitive therapeutic category, with Actelion’s Tracleer (bosentan) and Pfizer’s Revatio (sildenafil) already jostling for market share and other rivals such as Encysive’s Thelin (sitaxsentan) awaiting approval. And some analysts seemed to be concerned that with ambrisentan Gilead is moving away from its established territory in infectious disease therapy.

Perhaps reflecting these concerns, Gilead’s shares closed down more than 6% at $64.28 yesterday. In contrast, Myogen soared 47% to $51.44 as investors in the stock welcomed the benefit of combining with a company that has established commercial operations in place.

There were also suggestions that Gilead was paying a little too much for Myogen, although chief executive John Martin insisted that the premium was justified as there is a scarcity of good drug candidates in late-stage development.

“Myogen represents a unique scientific and strategic fit with our company, bringing to Gilead a late-stage product candidate that addresses an area of significant unmet medical need and further enhances our growing focus on pulmonology, as initially established through our recent acquisition of Corus,” he said.

Myogen already has a revenue stream from the sale of Flolan (epoprostenol), an intravenous PAH drug it licenses from GlaxoSmithKline in the US market, and darusentan – a follow-up to ambrisentan with potential in the treatment of resistan hypertension – in Phase II. This has also been tipped as a potential blockbuster.