LONDON, July 2 (Reuters) - An experimental antibody drug from Roche has produced promising results in mid-stage tests for asthma, offering a new way to fight the breathing disorder.

The encouraging response seen with quilizumab, which the Swiss group is developing as a treatment for moderate-to-severe asthma, underscores the growing focus on novel biotech medicines for hard-to-treat cases of the disease.

Other companies working on a variety of approaches using biotech medicines to fight the condition include AstraZeneca , GlaxoSmithKline and Sanofi. Analysts at Leerink said in March that sales of biotech asthma drugs could exceed $7.5 billion a year in the United States and Europe alone.

Roche already sells one biotech asthma drug in partnership with Novartis, called Xolair, which works by intercepting an inflammatory protein called immunoglobulin type E (IgE) linked to asthma.

Quilizumab also targets IgE but it does so in a different way. By focusing on a form of IgE found on the surface of certain cells, it inhibits the production of new IgE molecules.

Xolair, by contrast, does not affect production of the protein, so patients need regular doses to keep IgE levels at bay.

Researchers reported results from a Phase IIa clinical study with quilizumab in the journal Science Translational Medicine on Wednesday, showing it reduced airway constriction and prevented IgE production within four weeks of a single dose.

(Reporting by Ben Hirschler, editing by David Evans)