Although the new once-a-day inhaled treatment did not show superiority to GSK's existing twice-daily drug Advair, Britain's biggest drugmaker said the totality of the data gave it confidence about the medicine's prospects.
The convenience of Relovair's once-daily dosing should give it an edge, which GSK hopes will help it carve out billions of dollars in sales as a replacement for Advair, which has worldwide revenue of more than $8 billion (5 billion pound) a year.
But investors were underwhelmed by the details of the clinical studies and shares in GSK fell 3 percent by 1110 GMT, sharply underperforming a flat London stock market.
Mark Clark, an analyst at Deutsche Bank, said the data was poor and the failure to beat Advair in clinical outcomes would reduce Relovair's commercial potential and GSK's pricing power for the product.
This means any advantage is limited to its once- versus twice-daily format, he said.
Clark also believes GSK may now have trouble getting Relovair approved, since the results were inconsistent across trials and reports of fatal pneumonia in some of the studies were a potential concern.
Analysts at Shore Capital said the lack of superiority to Advair could limit Relovair's commercial positioning but added the data would probably not impact its chances of winning of approval.
Relovair, which is being developed with U.S.-based Theravance
It will also be submitted as a new treatment for asthma in Europe at the same time, while in the United States GSK will continue discussions with regulators on its use in asthma.
If all goes well, industry analysts believe Relovair should reach the market in 2013 and consensus forecasts point to sales of $1.88 billion by 2016, according to Thomson Reuters Pharma.
Darrell Baker, Glaxo's head of respiratory medicine development, said Relovair had reached an important milestone in its development.
Having undertaken an initial assessment of these data we believe they support our plan to seek global approvals of this once-daily medicine for the treatment of patients with COPD and asthma, he said in a statement.
GSK has previously built a 19 percent stake in Theravance 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 it will face generic competition immediately as respiratory drugs are notoriously difficult to copy.
Both Advair and Relovair are inhaled drugs that combine a steroid and a bronchodilator to open the airways. Bronchodilators provide short-term relief while steroids have a longer-term impact.
(Editing by Hans-Juergen Peters and David Holmes)