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Rigel's R112 for Allergic Rhinitis Achieves Positive Safety Profile

Favorable Results May Strengthen Rigel's Opportunity in Population Treated for Allergic Rhinitis

South San Francisco, CA - December 11, 2003

Rigel Pharmaceuticals, Inc. (Nasdaq: RIGL) today announced positive clinical safety data for R112, an experimental drug to treat allergic rhinitis, the chronic nasal congestion that afflicts approximately 25 to 30 percent of the U.S. population. The multi-dose trial results indicate that R112 is well tolerated and demonstrates a favorable safety profile in the study population.

Three different dose regimens were studied in groups of six volunteers each and compared with placebo controls over a ten day treatment period. The safety data collected indicated that subjects treated with R112 were indistinguishable from the placebo controls across a wide range of clinical and laboratory safety tests. Specifically, the key findings of this study include: no local nasal irritation due to the administration of R112; and no significant laboratory abnormalities, including blood counts, blood chemistry, electrocardiography or spirometry (tests of airway function).

Based on the success of the multi-dose trial, Rigel plans to initiate phase II efficacy trials in early 2004. The randomized, placebo-controlled park study will provide more significant indications of R112's potential clinical value. The study will take place in two locations in different parts of the country, where patients will spend two days in an outdoor setting during the high-pollen season. Phase II results are expected in the second half of 2004.

"The successful development of R112 to treat allergic rhinitis would provide a tremendous market opportunity for Rigel, as the prevalence and awareness of the disease continues to grow," said Donald Payan, MD, Rigel's Chief Scientific Officer and Executive Vice President. "The positive safety results provide another clinical milestone toward demonstrating the potential of R112 as a first-line allergy therapeutic."

The market for drugs targeted at allergic rhinitis is potentially very large, as allergic rhinitis is the most common allergic condition and is increasing in prevalence. Currently, allergic rhinitis afflicts over 150 million people in North America, Europe and Japan alone. This condition is treated most often with inhaled steroids and antihistamines. Sales of these products exceed $2 billion annually in the U.S. and Rigel believes that, if shown to be safe and effective, R112 could become the agent of choice in treating allergic rhinitis, supplanting both steroids and single chemical mediator inhibitors (antihistamines or leukotriene antagonists) among allergic rhinitis sufferers. In an effort to leverage the optimal market potential for R112's novel approach to treating allergic rhinitis, Rigel hopes to complete a corporate partnership in 2004 to pursue this program.

How R112 Works and Its Possible Advantages
R112 enters mast cells, binds to an intracellular target and interrupts the signal from the IgE receptor, thus preventing downstream signaling and subsequent chemical mediator release. 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 in an allergic attack, potentially making R112 a more effective and comprehensive drug. Currently, steroids are the major class of drugs that are able to block multiple mediators in the allergic response, but these have a comparatively slow onset of action. In the phase I/II trial, R112 began to diminish chemical mediator release within minutes after allergen challenge. In addition, steroids need to be used selectively due to their global blocking of immune function. R112 is delivered intra-nasally and no systemic exposure to R112 has been detected in any intra-nasal administration in any human trials conducted to date.

About IgE and the Allergic Cascade
Allergic rhinitis and asthma are chronic inflammatory disorders of the airways. Upon exposure to allergens, such as pollen, an allergic response is triggered resulting in inflammation and airway obstruction. In some patients, allergens such as pollen trigger the production of IgE antibodies that then circulate in the blood. IgE antibodies bind to mast cells and cross-link with their receptors, causing an intracellular signal that results in the release of various chemical mediators, including prostaglandins, tryptase and histamine. When this process occurs repeatedly over time, it creates persistent inflammation of the airway passages, resulting in the chronic congestion and airway obstruction associated with allergic rhinitis and asthma, respectively.

About Rigel (http://www.rigel.com)
Rigel's mission is to become a source of novel, small-molecule drugs to meet large, unmet medical needs. Rigel has identified three lead product development programs: mast cell inhibition to treat immunologic diseases such as asthma/allergy and autoimmune disorders, antiviral agents to treat hepatitis C and ligases, a new class of cancer drug targets. Rigel has begun clinical testing of its first two product candidates, R112 for allergic rhinitis and R803 for hepatitis C, and plans to begin clinical trials of two additional drug candidates, for the treatment of rheumatoid arthritis and asthma, by the end of 2004.

This press release contains "forward-looking" statements, including statements related to the potential efficacy and applicability of drug candidates, the timing and subject matter of future clinical trials and possible corporate collaborations. 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 "plans," "believes," "hopes" and similar expressions are intended to identify these forward-looking statements. There are a number of important factors that could cause Rigel's results and plans to differ materially from those indicated by these forward-looking statements, including risks associated with the timing and success of clinical trials and the commercialization of product candidates, as well as other risks, detailed from time to time in Rigel's SEC reports, including its Quarterly Report on Form 10-Q for the quarter ended September 30, 2003 and Annual Report on Form 10-K, as amended, for the year ended December 31, 2002. Rigel does not undertake any obligation to update forward-looking statements.