Gilead Sciences has been boosted by the news that US regulators have given the go-ahead for Letairis, a treatment for a debilitating lung disease.

The US Food and Drug Administration has granted approval of Letairis (ambrisentan), an endothelin receptor antagonist, for the once-daily treatment of pulmonary arterial hypertension. Patients with PAH suffer from shortness of breath as the heart struggles to pump against high pressure, causing them to ultimately die of heart failure. The disease afflicts approximately 100,000 patients in the USA, and 200,000 worldwide.

The FDA’s approval is based on two Phase III clinical trials (ARIES-1 and ARIES-2) involving 393 patients, and an increase in walk distance after a standard six-minute test was observed after four weeks of treatment with each dose regimen of Letairis, with a dose-response observed after 12 weeks of treatment. Using the drug also significantly delayed time to clinical worsening of PAH and its most common side effects included leg and ankle swelling, nasal congestion, facial flushing and sinusitis. However, the FDA noted that there are risks of liver injury and birth defects, so Letairis is available only through a special restricted distribution programme.

Letairis, which will now compete with Actelion’s Tracleer (bosentan) and Pfizer’s Revatio (sildenafil), the same active ingredient as Viagra, could potentially have sales of over $1 billion a year, say analysts. Gilead got control of the drug through its $2.5 billion acquisition of Myogen in October last year, and GlaxoSmithKline holds the rights to commercialise ambrisentan for PAH outside the USA. A marketing application was filed with the European Medicines Agency earlier this year.

…but Encysive’s Thelin is rejected yet again

One drug that Gilead will not be up against is Encysive Pharmaceuticals Thelin (sitaxsentan sodium), as the FDA has, for a third time, rejected the firm’s PAH drug.

The agency has issued a third approvable letter for Thelin, saying that Encysive's development programme did not demonstrate the evidence of effectiveness needed for the green light. The FDA did note, however, that there is some evidence that Thelin improves exercise tolerance in PAH and encouraged the company to conduct an additional study.

The decision has stunned Encysive and chief executive Bruce Given said that “we believe we adequately addressed the issue raised by the FDA in the second approvable letter and we are deeply disappointed in their decision”. He added that the firm “remains committed to Thelin in Europe, Australia and Canada, where it has been approved for sale," but the FDA’s decision means that the company “may be required to make significant reductions in its infrastructure and workforce in the USA.”