A drug sometimes used to treat asthma may hold the key to providing effective treatment for "smoker's lung", which kills more than 30,000 Britons a year.

Laboratory experiments suggest that the cheap and widely available drug theophylline may help combat chronic obstructive pulmonary disease (COPD), an umbrella term for a number of conditions including emphysema and chronic bronchitis.

The "smoker's lung" nickname originates from COPD's most common cause and it is so far incurable, leaving doctors to only treat symptoms which range from coughing and phlegm to debilitating shortness of breath. But researchers from Imperial College, London, now believe they understand why it is resistant to steroid treatment, often effective in treating other inflammatory conditions.

Inflammation is caused by cells producing chemical signals by switching on specific genes. Switching these back off, and therefore preventing inflammation, requires an enzyme known as HDAC2.

Professor Peter Barnes and colleagues, who will report their work to the British Endocrine Societies meeting in Harrogate tomorrow, discovered that steroids act as a bridge to recruit HDAC2 to the appropriate genes where it can then switch them off. Levels of the enzyme are extremely low in people with COPD, so there are not enough to switch off the inflammatory genes. But the researchers found that low doses of theophylline could raise the levels of HDAC2 and overcome the resistance to steroids. Trials on patients have begun.

Prof Barnes said: "COPD kills tens of thousands of people in the UK each year and currently we can only treat the symptoms, not the underlying problem... Our work has finally provided an explanation for steroid resistance in COPD, and has allowed us to identify ways to combat this.

"We hope that the clinical trials of theophylline will be successful so that we can finally offer an effective therapy to COPD sufferers - a staggering 6% of the population."