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Rigel Initiates Clinical Trial in Rheumatoid Arthritis

Initial Trial Designed to Establish Safety of R406

South San Francisco, CA - December 21, 2004

Rigel Pharmaceuticals, Inc. (Nasdaq: RIGL) today announced that it has initiated a phase I clinical trial of R406, a product candidate for the treatment of rheumatoid arthritis (RA). The goal of this trial is to establish the safety and pharmacokinetics of R406.

The escalating single-dose, placebo-controlled clinical human safety trial will include 36 volunteers and will be followed by a multiple dose study including an additional 24 people. Results of the trial are expected by the second half of 2005 and, if favorable, will, allow Rigel to enter broader, longer-term safety and efficacy trials in patients suffering from RA, the chronic inflammatory disease that causes pain, swelling, loss of function in the joints and the destruction of bone and cartilage for nearly 2.1 million people in the U.S.

R406 is a novel, oral syk kinase inhibitor that blocks the activation of mast cells, macrophages and B cells that promote swelling and an inflammatory response. Data from preclinical studies indicate that R406 is effective across a broad spectrum of rodent arthritis models and was without obvious toxicities at doses above the anticipated effective dose.

“R406 has shown significant potential as a novel, potent and orally delivered disease-modifying anti-rheumatic drug (DMARD),” said Donald G. Payan, M.D., Rigel's Chief Scientific Officer and Executive Vice President. “R406 addresses multiple RA disease factors and is very selective, potentially providing greater efficacy and fewer and less severe side effects than currently available treatments.”

“Rigel has developed a potentially new class of DMARD to complement and possibly replace some of the currently available agents,” said Ernie Brahn, M.D., Professor of Medicine and Rheumatology Program Director at the UCLA School of Medicine. “Successful results in animal models with R406 showing that it prevented structural joint damage further underscore its potential as a novel treatment of RA.”

Rheumatoid Arthritis: Current Treatments and Market Opportunity
Rheumatoid arthritis is a chronic inflammatory disease that affects multiple tissues, but typically produces its most pronounced symptoms in the joints. It is often progressive and debilitating, preventing people from living life symptom-free. Ultimately the chronic inflammation of joints leads to the destruction of the soft tissue and erosion of the articular surfaces of the bone.

The current treatment options for RA have significant potential side effects and other shortfalls, including gastrointestinal complications and kidney damage. RA patients receive multiple drugs depending on the extent and aggressiveness of the disease. Most RA patients require some form of DMARD—those include methotrexate, an anti-cancer agent or the TNF-blocking agents such as Enbrel. The TNF- blocking agents only inhibit the inflammatory mediator, TNF, and are all delivered via injection. Rigel believes that there is a significant opportunity for an oral, safe DMARD that can be used earlier in course of the disease, preventing its progression prior to major bone and cartilage destruction.

How R406 Works
R406 is a syk kinase inhibitor that has demonstrated inhibition of mast cell and B cell activation by both IgE and IgG, thus blocking inflammatory mediators, including TNF, various interleukins and other mediators. R406 has been shown to be effective in animal models of arthritis and appears to be well tolerated in preclinical studies. Rigel believes that R406 may become a first-line DMARD with greater efficacy, safety and improved delivery and patient compliance than injectable treatments, and thus may be used early in the course of the disease before significant bone and cartilage damage occurs.

About Rigel (www.rigel.com)
Rigel's mission is to become a source of novel, small-molecule drugs to address large, unmet medical needs. We have four research and development programs investigating treatments for asthma/allergy, hepatitis C, rheumatoid arthritis and oncology. Our strategy is to initiate clinical trials with at least one new product candidate annually and to pursue partnerships with pharmaceutical and biotechnology companies for late-stage clinical development and commercialization of those product candidates.

This press release contains "forward-looking" statements, including statements related to Rigel's plans to pursue clinical development of product candidates and the timing thereof and the potential efficacy of product candidates. 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 "will," "plans," "intends," "expects" 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 risks that early-stage drug discovery and development might not successfully generate good product candidates and the inherent difficulty in selecting the right drug target and avoiding unwanted side effects, 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, 2004. Rigel does not undertake any obligation to update forward-looking statements.

Contacts:
Rigel Contact: Raul Rodriguez, 650-624-1302

Media Contact: Melinda Bagatelos or Ayanna Anderson, 415-512-0770
rigel@schwartz-pr.com