Genentech has halted a clinical trial of its injectable treatment for severe asthma, Xolair, in the prevention of reactions caused by peanut allergies.

Xolair (omalizumab) is the first-ever biological drug to treat asthma and works by inhibiting immunoglobulin E, an inflammatory mediator that is implicated in allergic reactions. Genentech is hoping that this mechanism could make it useful for blocking the severe allergic reactions, which affect cause 30,000 emergency room admission each year in the USA and lead to 150-200 deaths.

According to a Wall Street Journal Online report, Genentech said the problems with the programme lay with the design of the trial, rather than the product.

As part of the initial screening visit, participants in the Phase II trial were given capsules containing either trace amounts of peanut flour or wheat flour, and to qualify for entry had to react only to the peanut flour. But two children reacted with severe hypersensitivity even on this initial challenge, and Genentech was unprepared to continue with the project.

None of the patients in the study had yet to receive Xolair, which was approved for severe asthma in 2003 in the USA and last year in Europe, so the decision has no bearing on the safety of the drug. But it illustrates the difficulties in testing potential treatments for patients with severe allergic reactions, and puts back prospects of a therapy for peanut allergy by several years, according to the WSJ.

Xolair has been a slow burner for Genentech in asthma, but saw sales rise by more than 70% to $320 million dollars last year as physicians grew more confident with using it to treat asthmatics.