GlaxoSmithKline is banking on a major clinical trial to revive its flagging respiratory medicine business, with billions of dollars of sales riding on a positive result. 

Data from the so-called SUMMIT study, designed to show GSK's Breo can prolong lives of patients with chronic lung disease, are expected as early as next month, nine years after a similar study with GSK's older drug Advair failed by a whisker.

This time GSK reckons it has a better chance, since the new trial, with 16,500 patients, has more statistical power than the 6,100-patient Advair trial, known as TORCH. GSK has also chosen higher-risk patients with heart issues for the new study.

At present, inhaled drugs such as Breo that combine a steroid and a long-acting beta agonist (LABA) are known to help patients breath more easily but their effect on survival is unclear.

If it succeeds, Breo would be the first drug to show a survival benefit in patients with chronic obstructive pulmonary disease (COPD), potentially turning around its fortunes.

While Breo was approved for COPD in 2013 and won a U.S. green light for asthma in April, it has been slow to take off commercially.

Sometimes known as "son of Advair", once-a-day Breo has a dosing advantage over twice-daily Advair. But the market for inhaled lung drugs is fiercely competitive and Breo has struggled at a time when GSK has been forced to cut Advair prices.

HSBC analyst Stephen McGarry thinks Breo could eventually achieve peak annual sales of $5.0 billion if SUMMIT succeeds, or $1.8 billion if it fails.

Other analysts are more cautious and the current consensus is for sales of $1.55 billion in 2020, according to Thomson Reuters Cortellis.

GSK's partner Theravance is entitled to royalties of 15 percent on the first $3 billion of annual sales and 5 percent beyond that.

Britain's GSK has been a world leader in respiratory medicine since launching the Ventolin inhaler back in 1969, but weak sales in recent quarters are a mounting concern for its investors.

Courtney Crimm, a pulmonologist who oversees the SUMMIT trial at GSK, believes a successful result would oblige doctors to rethink prescribing strategies.

"If this study is positive, it could lead to a paradigm shift," he told Reuters, noting that 50 to 70 percent of COPD patients also have cardiovascular risk factors.

Furthermore, because SUMMIT recruited patients with "moderate" COPD it might prompt wider use of the medicine in patients with less severe breathing difficulties, he said.

Current medical guidelines recommend steroid and LABA combinations only for people with more severe breathing problems. Those who are less seriously affected are often given an alternative drug such as Pfizer and Boehringer Ingelheim's Spiriva.