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Rigel’s R406/788 Shows Promise for Treating Immune Thrombocytopenia, Hemolytic Anemia and Myeloid Leukemia in Preclinical Studies

Company Announces Presentations at the American Society of Hematology (ASH) 47th Annual Meeting

SOUTH SAN FRANCISCO, Calif. - December 07, 2005

Rigel Pharmaceuticals, Inc. (Nasdaq: RIGL) today announced that two presentations related to R406/788 will be made at the American Society of Hematology (ASH) 47th Annual Meeting and Exposition, which is taking place December 10-13 at the Georgia World Congress Center in Atlanta, Georgia.

R788, the oral solid dosage formulation of R406, is a novel syk kinase inhibitor that blocks the activation of mast cells, B cells and macrophages by blocking IgG signaling. In the first study, R788 was found to be protective from immune thrombocytopenia and hemolytic anemia in a mouse model. This study suggests that R788 may be useful in the treatment of Immune Thrombocytopenic Purpura (ITP) as well as improving autoimmune hemolytic anemia (AIHA). Both ITP and AIHA are autoimmune-based hematological diseases characterized by the destruction of formed blood cells by auto-antibodies.

In a second study, R406 inhibited tumor growth in a dose-dependent manner in a xenograft mouse model with a human acute mylelogenous leukemia (AML) FLT3 cell line, demonstrating that R406 may be a beneficial in FLT3-type AML. AML is characterized by the overproduction of immature white blood cells, which causes deficits of other normal blood cells. AML is the most common form of leukemia with over 10,000 patients diagnosed annually. With current therapy, only 20-40% of patients survive at least 5 years without any relapse. FLT3-mutant type AML is found in approximately 20-30% of all AML patients, and the prognosis for these patients is even poorer. Rigel’s R406/788 is also a known inhibitor of FLT3.

Presentation details are as follows:

Presentation Time: Sunday, December 11, 2005 at 9:15 a.m. EST
Title: Inhibition of Immune Thrombocytopenic Purpura (ITP) by an Orally Bioavailable Inhibitor of Syk Kinase Activity
Session Type: Poster Session 369-II

Presentation Time: Monday, December 12, 2005 at 8:00 a.m. EST
Title: An Orally Bioavailable Inhibitor of FLT3 and Syk Kinases Prevents Tumor Growth in Subcutaneously Implanted Human Tumor Xenografts and Promotes Cell Death of FLT3 Mutant AML Cells
Session Type: Oral Session

About R406/788
R788 is Rigel’s lead product candidate. It has a novel mechanism of action, blocking IgG receptor signaling in macrophages and B-cells. Phase I trial results, completed to date, have demonstrated that R406/788 is well-tolerated, established a good biomarker, and showed good pharmaceutical properties. Rigel is developing R788 for rheumatoid arthritis and possibly other diseases. In preclinical studies, Rigel’s compound greatly diminished the swelling and tissue destruction associated with RA. In other preclinical models, R406/788 was shown to have potential in treating various cancers and hematological diseases. Rigel plans to retain ownership of R788 and will begin efficacy studies of the drug in 2006.

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 move one new product candidate for a significant indication into the clinic 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 allergy/asthma, rheumatoid arthritis and cancer.

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 "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 pre-clinical studies and clinical trials, 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, 2005. 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