Letairis LEAPS Forward

Letairis ambrisentan was the driver of Gilead Sciences Inc. $2.5 billion cash acquisition of Myogen Inc. in 2006. But like Tracleer bosentan from Actelion Ltd., Letairis ended up with a boxed warning about the risk of liver injury.

As a result, Tracleer's six-year head start made it the leading endothelin receptor antagonist (ERA) in pulmonary arterial hypertension, and Gilead has struggled to differentiate Letairis in the marketplace...