* Trial aimed to extend range of use for PAH drug Opsumit

* Analyst says sales potential was 100-200 mln Sfr/year

* Shares pare losses after falling more than 1 pct (Adds comment from analyst, shares)

By John Miller

ZURICH, Jan 23 (Reuters) - Actelion's Opsumit drug missed the main target in a late-stage study of patients with pulmonary arterial hypertension due to Eisenmenger Syndrome, the Swiss drugmaker said on Monday.

Actelion had been conducting the Phase III trial on 226 patients as part of its push to expand clinical use of Opsumit -- its follow-on medicine to blockbuster PAH drug Tracleer that has lost patent protection -- for patients suffering from different forms of the deadly heart-lung ailment.

Falling short of the primary endpoint -- exercise capacity as measured in a six-minute walk test -- could weaken Opsumit's revenue potential and further complicate U.S. healthcare giant Johnson & Johnson's efforts to acquire Europe's biggest biotech group, although analysts downplayed the magnitude of the impact.

"The potential revenue would have been around 100 to 200 million Swiss francs, it would have been one of the smaller complementary indications," Sibylle Bischofberger, a Zuercher Kantonalbank analyst, wrote in a note to investors. "As a result, we see the study result as merely slightly negative."

Analysts at Jefferies called the adverse impact "likely minor," in part because Tracleer is already approved in Europe to treat Eisenmenger-related PAH.

Actelion shares opened more than 1 percent lower before trimming losses. They were down 0.4 percent at 0830 GMT.

Patients with Eisenmenger Syndrome have untreated congenital heart defects that lead to pulmonary hypertension, where blood vessels in the lung arteries become stiff and narrow.

While conceding that Opsumit failed to reach the study's primary objective, Actelion called its results "difficult to interpret".

"Although the results point towards a benefit of treatment with macitentan (Opsumit), we do not see a significant treatment effect on the primary endpoint of exercise capacity as measured in the 6 minute walk test," said Nazzareno Galie, head of the Pulmonary Hypertension Center at the University of Bologna's Institute of Cardiology.

The result may have been influenced by an "unexpected improvement" in the study's placebo arm that had not shown up in previous PAH studies, Galie said, adding that researchers must "fully analyze the data to understand what could have caused this phenomenon".

Opsumit was Actelion's second-biggest selling drug behind Tracleer in the third quarter, with sales of 218 million Swiss francs.

(Editing by Michael Shields)