* Mylan had hoped for generic approval by March 28
* Hikma seeks okay for competing generic by May 10
* GlaxoSmithKline shares gain, while Hikma falls
By Ben Hirschler
LONDON, March 29 (Reuters) - GlaxoSmithKline shares gained on Wednesday as lack of news about U.S. approval of a generic copy of its blockbuster inhaled lung drug Advair buoyed speculation that a threat to profits might be delayed.
The U.S. Food and Drug Administration (FDA) had been due to decide by March 28 whether to approve the first substitutable generic copy of the two-part drug for asthma and chronic lung disease, made by generics firm Mylan.
In the event, neither the FDA nor Mylan made any statement.
"It is possible Mylan might still make an announcement when the U.S. market opens, but I don't think anybody would be surprised if the review period for their product is extended," said Deutsche Bank analyst Richard Parkes.
GSK shares were 0.5 percent higher by 0755 GMT, outperforming a flat European drugs sector.
Options trading in Mylan had been busy on Tuesday, although many investors appeared to believe Mylan was unlikely to win a green light this week.
Mylan's version of Advair would be the first complex inhaled combination generic product to be approved by the U.S. agency and a number of analysts believe the likelihood of an on-time approval is therefore quite low.
"With such a commercially important decision it was always likely that the approval process wasn't going to be straightforward," said Trinity Delta analyst Mick Cooper.
A second generic version of Advair from Hikma Pharmaceuticals is also awaiting an FDA approval decision by May 10. Hikma shares fell around 1 percent.
Dealing with the threat of competition to Advair, which has generated more than $1 billion in annual sales since 2001, is the first big challenge facing GSK's new chief executive, Emma Walmsley, who takes over at the end of this month.
GSK has told the market that core earnings per share, in constant currencies, will be flat to slightly lower in 2017, if substitutable Advair generics arrive in the United States by mid-year. If they don't launch, EPS should rise between 5 and 7 percent.
If generics do arrive by mid-2017, the company has forecast Advair's U.S. sales will be around 1 billion pounds ($1.24 billion), down from 1.83 billion in 2016.
GSK is not alone in worrying about generic Advair. Novartis' generics unit Sandoz, which is further behind in developing its form of Advair, has also tried to stall generics by taking issue with FDA rules governing such copies.
$1 = 0.8070 pounds Editing by Louise Heavens