* Q1 net profit 128 mln Sfr vs 98.5 mln forecast
* Opsumit sales of 15 mln in first quarter
* Expect selexipag results in the next few weeks
* Maintains 2014 and 2015 guidance for now
* Says will be in a position to review guidance by mid-year
* Shares indicated up 3.5 pct (Adds comments by CEO, analyst)
ZURICH, April 17 (Reuters) - Actelion Ltd's launch of it new lung drug has got off to a good start and as a result the Swiss firm might raise its earnings forecast at mid-year, it said on Thursday after reporting a better than expected first-quarter result.
Europe's biggest biotech drugs company is banking on a strong ramp-up in sales of the new drug, Opsumit, a treatment for pulmonary arterial hypertension (PAH), to boost core earnings in the low single-digit percentage range this year.
But the company said it would review this earnings forecast by mid-year following a "good start" to the year.
"We really think that we should guide up, but how much up that's a question and that's what we will be in a position to do in the next quarter," Chief Executive Jean-Paul Clozel told Reuters.
The Swiss company notched up 15 million Swiss francs ($17 million) in sales for Opsumit in the first quarter, but stressed this figure does not reflect underlying demand for the drug since the company offers a portion of the medicine for free in the United States under a patient access programme.
Jefferies analyst Peter Welford described the more than 2,300 patients already on the drug, mostly in the United States, as "impressive" and suggested annual sales in excess of 165 million Swiss francs ($187 million).
"It's a very very significant success, you can imagine that 2,300 patients a few months after the launch was something I was not expecting," Clozel said.
Shares in Actelion, which trade at 17.7 times forecast earnings compared with a peer medium of 14.5 times, were indicated up 3.5 percent in premarket trade.
BETTER THAN TRACLEER?
The company expects Opsumit's peak sales to eclipse those of its current flagship product Tracleer, which also treats PAH but goes off patent from 2015 and has faced growing competition from Gilead's Letairis. Jefferies' Welford forecasts peak sales for Opsumit of $2.2 billion.
Sales of Tracleer, rose 2 percent in the quarter to 383 million francs, due to strong demand in countries where Opsumit is not yet on the market.
In countries where Opsumit has launched, such as the United States, weekly new Tracleer prescriptions are down by two thirds or more, Actelion's spokesman Roland Haefeli said. The drug is also available in Canada, Germany, Austria, Switzerland, Denmark and Norway.
Actelion reported an 8 percent rise in first-quarter product sales to 469 million francs, generating a net profit of 128 million francs, up 30 percent on the year.
Analysts in a Reuters poll had forecast product sales of 435 million and net profit of 98.5 million.
Investors are now turning their attention to Phase III trial results for selexipag, another PAH drug, and Actelion said it would present top-line results "in the next few weeks."
The company is also conducting a late-stage trial for Cadazolid, an anti-biotic for patients suffering from diarrhoea associated with clostridium difficile which was granted fast-track status by U.S. health regulators in February.
For now Actelion maintained its forecast for a low single-digit percentage increase in core earnings at constant currencies this year. It also expects single-digit core earnings growth in 2015. ($1=0.8816 Swiss francs) (Reporting by Caroline Copley; Editing by Greg Mahlich)