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Rigel Announces Initiation of Phase II Study Evaluating R788 in Rheumatoid Arthritis

SOUTH SAN FRANCISCO, Calif. - September 06, 2006

Rigel Pharmaceuticals, Inc. (Nasdaq: RIGL) announced today that it has initiated enrollment and dosing in a Phase II study evaluating the efficacy and safety of its lead product candidate, R788, an oral syk kinase inhibitor, for the treatment of patients with rheumatoid arthritis (RA).

"As an oral agent with the potential to modify the course of RA, R788 is a very attractive product candidate for the treatment of this disease," said Elliott B. Grossbard, M.D., senior vice president of medical development of Rigel. "We are hopeful that we will see clinical efficacy for R788 in this Phase II trial."

This multi-center, randomized, double-blind, placebo-controlled, ascending dose, dose ranging study will evaluate up to three doses of R788. The study is expected to enroll approximately 180 patients in the U.S. who have had RA for a minimum of 12 months and who have active disease despite receiving adequate doses of methotrexate.

The primary endpoint of this study is to determine the percent of ACR 20 responders at the end of 12 weeks. An ACR 20 response requires at least a 20% improvement in a number of different measures of disease activity, and is one of the standard measures to assess efficacy in RA clinical trials. Secondary endpoints include other measures of disease activity as well as safety.

About R788
R788 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. It is being developed to treat RA. Phase I trial results have demonstrated that R788 is well-tolerated and showed good pharmaceutical properties when administered both alone and in combination with methotrexate. Earlier Phase I studies generated valuable pharmacokinetic/pharmacodynamic data, establishing a strong correlation between drug plasma levels and the inhibition of the drug target. In preclinical studies, Rigel's compound greatly diminished the swelling and tissue destruction associated with RA. In addition, in a murine model of ITP, the drug increased platelet counts significantly.

Rheumatoid Arthritis: Current Treatments and Market Opportunity
Approximately 2.1 million people in the U.S. suffer from RA and the worldwide market for innovative RA drugs is projected to reach $10 billion by 2015*. RA 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 a symptom-free life. 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 potentially significant shortcomings and/or side effects. Some 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 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 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 R788 in RA.

About Rigel (www.rigel.com)
Rigel is a clinical-stage drug development company that discovers and develops novel, small-molecule drugs for the treatment of inflammatory diseases, cancer and viral diseases. Our goal is to file one new investigative new drug (IND) application in a significant indication each year. We have achieved this goal since 2002. Our pioneering research focuses on intracellular signaling pathways and related targets that are critical to disease mechanisms. Rigel’s productivity has resulted in strategic collaborations with large pharmaceutical partners to develop and market our product candidates. We have product development programs in inflammatory/autoimmune diseases such as rheumatoid arthritis, thrombocytopenia, and asthma and allergy, as well as in cancer.

This press release contains "forward-looking" statements, including statements related to Rigel's plans with respect to clinical development of product candidates, the market opportunity for its product candidates, and expansion of its product portfolio. 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," “promising,” "expects," “anticipates” 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 Form 10-Q for the quarter ended June 30, 2006. Rigel does not undertake any obligation to update forward-looking statements.

Contact: Raul Rodriguez
Phone: 650.624.1302
Email: invrel@rigel.com

Media Contact: Carolyn Bumgardner Wang, WeissComm Partners, Inc.
Phone: 415.946.1065
Email: carolyn@weisscommpartners.com