* Macitentan the big hope as Tracleer nears patent expiry
* Drug improved 6-minute walking distance by 23 metres
* Reduced the risk of all-cause mortality by 36 pct
* Halved risk of death and hospitalisation due to PAH
(Adds details on study, analyst)
ZURICH, Oct 24 (Reuters) - A new lung and heart drug from Actelion prolonged overall survival by more than a third in a clinical trial, reassuring Europe's leading biotech company that it has a viable successor to its current top seller.
The Swiss company is banking on macitentan to replace Tracleer, which also treats pulmonary arterial hypertension (PAH) and currently accounts for around 90 percent of group sales. Tracleer goes off patent from 2015 and faces growing competition from Gilead's Letairis.
The latest data presented at a meeting of the American College of Chest Physicians in Atlanta late on Tuesday showed that a 10 milligram dose of the drug reduced the overall death rate by 36 percent.
Macitentan also halved the risk of death or hospitalisation due to PAH for patients on a 10 mg dose compared to a placebo.
ZKB analysts said the data should help convince doctors to switch patients onto macitentan from Tracleer.
Pulmonary arterial hypertension (PAH) is a progressively worsening condition in which the arteries carrying blood to the lungs narrow, overburdening the heart and raising blood pressure. The cause is unknown and the disease has no cure.
Actelion said this week it had filed macitentan - which it plans to sell under the brand name Opsumit - for U.S. regulatory approval. If the Food and Drug Administration grants the drug a priority review, a decision could be reached within eight months, spokesman Roland Haefeli said.
The Actelion study showed patients who took the drug at its highest dose showed an improvement of 23 meters in a six-minute walk test. But the distance fell short of Bayer's drug riociguat, which helped patients improve their walking distance by 36 metres on average.
Deutsche Bank analysts, however, said comparison of walking distance benefit was ``invalid and irrelevant'' and they saw limited threat from riociguat.
``Confirmation that there is no ''devil in the detail`` should help further convince (that) Actelion can negotiate its Tracleer patent-cliff,'' they said.
The data did not show a significant risk of liver damage, a major downside of Tracleer. Side effects included headache, anaemia and nasopharyngitis, also known as the common cold.
Actelion said in April that the drug lowered the risk of death and the disease worsening by 45 percent, sending the shares up 18 percent.
In morning trade the shares were down 1.2 percent.
The stock has gained more than 40 percent so far this year after the company survived a bid from activist investor Elliott Advisors to seize control in 2011. This compares with a 11 percent rise in the European healthcare index.
(Editing by Edwina Gibbs and Louise Heavens)