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Rigel Completes Phase I Clinical Trial of R406 in Rheumatoid Arthritis

Initial Data Demonstrates Safety of R406 and Generates Useful Biomarker Data

South San Francisco, CA - March 31, 2005

Rigel Pharmaceuticals, Inc. (Nasdaq: RIGL) today announced the completion of its Phase I clinical study of R406, an orally-delivered product candidate for the treatment of rheumatoid arthritis (RA), a chronic inflammatory disease which progressively destroys joint cartilage and bone. The Phase I study evaluated the safety and pharmacokinetics of R406. Based on the preliminary results of the study, R406 was well tolerated at the dose levels that Rigel plans to use moving forward. The study also generated valuable pharmacokinetic/pharmacodynamic data establishing a strong correlation between R406 plasma levels and the inhibition of its target. This data will be helpful in guiding the future development of R406.

The study at Guy’s Drug Research Unit (GDRU) in London, England was conducted in two sequential parts. The first was a placebo-controlled, escalating single-dose human safety/pharmacokinetic clinical trial that included 35 volunteers. This was followed by a placebo-controlled, multiple-dose human safety clinical trial with an additional 24 volunteers. Based on the trial’s favorable safety data, Rigel believes that it can enter into broader, longer-term safety and efficacy trials with R406 for the treatment of RA.

“R406 is a fundamentally new approach to potentially halting the progressive destruction of bone and cartilage caused by RA,” said Elliott B. Grossbard, M.D., Rigel’s Senior Vice President of Medical Development. “We believe today’s safety and biomarker results are a promising and important milestone in the development of this potential novel, orally-delivered, and potent disease-modifying anti-rheumatic drug (DMARD) – one of very few in development.”

Future Plans
Based on the results of this study, Rigel intends to initiate a Phase I/II safety clinical trial of R406 for patients with RA, in the second half of 2005. Additionally, upon completion of the final statistical analysis of the results of the Phase I clinical study, Rigel expects to release further details of the clinical trial later this year as part of a scientific presentation of the study.

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. Approximately 2.1 million Americans suffer from this debilitating condition.

The current treatment options for RA have significant potential side effects and other shortfalls, including gastrointestinal complications and kidney damage. RA patients currently receive multiple drugs depending on the extent and aggressiveness of the disease. Most RA patients require some form of DMARD— including methotrexate, an anti-cancer agent or the TNF-blocking agents such as EnbrelTM. 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 the course of the disease, preventing its progression prior to major bone and cartilage destruction; this is the product goal for R406.

How R406 Works
R406 is a syk kinase inhibitor that has demonstrated inhibition of mast cell, macrophage 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 various animal models of arthritis in blocking the destruction of bone and cartilage that is the hallmark of RA. As announced today, R406 appears to be well tolerated in Phase I human clinical 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, rheumatoid arthritis, hepatitis C 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 as well as 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 "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 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 Annual Report on Form 10-K for the year ended December 31, 2004. Rigel does not undertake any obligation to update forward-looking statements.

Rigel Contact:
Rigel Pharmaceuticals
Raul Rodriguez
650-624-1302

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