* Relovair meets goals in six-month COPD lung function tests
* New drug a successor to 5 bln-pound-a-year seller Advair
* Drug partnered with Theravance may reach market in 2013
* Sales in 2015 seen at $1.34 bln - Thomson Reuters Pharma (Adds sales forecast, details, background)
LONDON, June 2 (Reuters) - GlaxoSmithKline's (GSK.L) planned once-daily successor to its blockbuster lung treatment Advair proved successful in two six-month trials testing lung function, the company said on Thursday.
The British drugmaker and its partner Theravance THRX.O are banking on Relovair to carve out billions of dollars in sales as a replacement for Advair, GSK's top selling product, which had revenue of 5.14 billion pounds ($8.4 billion) in 2010.
Detailed results of the two Phase III studies will only be made available later, when the results are presented at a future scientific meeting, but GSK said the results clearly supported continued development of the inhaled medicine.
Darrell Baker, Glaxo's head of respiratory medicine development, said successful completion of the two clinical studies marked "an important milestone" in the development of Relovair for chronic obstructive pulmonary disease (COPD).
GSK and Theravance are now waiting for results from larger 12-month studies before filing the new medicine for regulatory approval.
If all goes well, industry analysts believe Relovair should reach the market in 2013 and consensus forecasts point to sales of $1.34 billion by 2015, according to Thomson Reuters Pharma.
GSK increased its stake in Theravance to 19 percent last November in a vote of confidence of its respiratory collaboration with the U.S. firm.
Advair is set to lose patent protection in key markets but it is not certain that it will face generic competition immediately as respiratory drugs are notoriously difficult to copy.
Both Advair and Relovair are inhaled drugs that combine a corticosteroid and long-acting beta agonist to open the airways.
Reporting by Ben Hirschler; editing by Paul Sandle