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Rigel Begins U.S. Clinical Testing Of Allergy Drug

Efficacy Data Expected to be Available Mid-Year

South San Francisco, CA - January 13, 2003

Rigel Pharmaceuticals, Inc. (Nasdaq: RIGL) has initiated clinical testing in the United States for the company's first product, a drug to treat allergic rhinitis.

The experimental compound, R112, is the first of a new class of drug directed at immune blood cells called mast cells. Activation of these cells is one of the earliest steps in the initiation of an inflammatory response in allergy and asthma.

"Rigel's investigational drug represents a novel approach for intervening in the allergic process," said Harold S. Nelson, M.D., the study's lead investigator and professor of medicine at National Jewish Medical and Research Center in Denver, one of the leading asthma and allergy centers in the United States. "Because it has the potential to block all components of the mast cell response, and because it can be easily administered by nasal spray, R112 could offer a new strategy for treating allergic rhinitis as well as perhaps other allergic diseases of the respiratory system."

R112 works this way: When an allergen enters the body, a protein called immunoglobulin E (IgE) binds to mast cells, in essence turning them on and telling them to initiate an allergic reaction. Mast cells then release a host of chemical mediators, including histamine and leukotrienes, that create common allergy symptoms. When this process occurs repeatedly over time, it creates persistent inflammation of the nasal passages (rhinitis in medical terminology), resulting in chronic congestion.

R112 targets mast cells after they have been turned on by IgE and prevents the release of mediators. However, unlike common allergy drugs such as antihistamines or antileukotrienes that block only a single mediator, R112 is designed to block all of the major pathways that are triggered following mast cell activation. Because of this broad effect, Rigel anticipates that R112 could be used to treat the more chronic component of allergic rhinitis.

Rigel began clinical testing of R112 last September in Britain. In that initial safety study, conducted with normal volunteers, no significant adverse events were observed. The trial now underway at National Jewish Medical Center will evaluate the effectiveness of R112 in 20 patients with documented allergies. It is a randomized, double-blind, placebo-controlled "crossover" trial. In this design, patients serve as their own controls, in effect doubling the size of the study sample. Preliminary findings from the trial are expected towards the middle of the year.

Compelling Market Opportunity
The market for drugs aimed at chronic allergic rhinitis is potentially very large. Currently, this condition is most often treated with inhaled steroids such as Flonase®. Sales of these products exceed $2 billion annually in the United States alone. In addition, a number of the most popular inhaled steroids are due to lose patent protection in the next few years, creating an opportunity for new products to enter the marketplace. Therefore Rigel believes that, if shown to be safe and effective, R112 could readily become an alternative to chronic steroid use among allergy sufferers.

"We anticipate that this study will provide proof-of-concept for targeting mast cells in allergic disease," said Elliott Grossbard, M.D., Rigel's senior vice president of medical development. "This could then point the way to using a similar approach for treating asthma as well as autoimmune disorders, other conditions in which mast cells have been shown to play an important etiologic role."

Rigel already has advanced research and development programs underway in both asthma and autoimmunity. The company expects to begin clinical testing of a mast-cell drug for one of these indications later in the year.

About Rigel
Rigel's mission is to become a source of novel, small-molecule drugs to meet large, unmet medical needs. The company's business model is to develop a portfolio of drug candidates and to take these through phase II clinical trials, after which Rigel intends to seek commercialization partners for completion of clinical evaluation, regulatory approval and marketing. Rigel has identified three areas for its lead product research programs: mast cell inhibition to treat immunologic diseases such as asthma/allergy and autoimmune disorders, an antiviral agent to treat hepatitis C and ubiquitin ligases, a new class of cancer drug target. Rigel has begun clinical testing of its first product, for allergic rhinitis, and plans to follow this with two additional drugs in the clinic by the end of 2003. Rigel's approach to drug discovery is based on advanced, proprietary functional genomics techniques that allow the company to identify targets with a demonstrable role in a disease pathway and to efficiently screen for those that are likely to be amenable to drug modulation.

This press release contains "forward-looking" statements, including statements related to Rigel's business strategy, drug development programs and clinical trial plans. Any statements contained in this press release that are not statements of historical fact may be deemed to be forward-looking statements. Words such as "anticipates," "expects," "believes," "intends" and similar expressions are intended to identify these forward-looking statements. There are a number of important factors that could cause Rigel's results to differ materially from those indicated by these forward-looking statements, including the risks detailed from time to time in Rigel's SEC reports, including its Annual Report on Form 10-K for the year ended December 31, 2001. Rigel does not undertake any obligation to update forward-looking statements.