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Rigel Announces Selection of R348, A Novel JAK3 Inhibitor, For Advanced Preclinical Development

SOUTH SAN FRANCISCO, Calif. - October 17, 2006

Rigel Pharmaceuticals, Inc. (Nasdaq: RIGL) announced today that it has selected R348, an orally-available, potent and selective inhibitor of Janus Kinase 3 (JAK3), to enter preclinical studies to support an investigational new drug (IND) application planned for 2007.

In collaboration with researchers at Stanford University, Rigel’s R348 and other novel JAK3 inhibitors were evaluated in animal heart transplant models. These compounds were found to significantly extend survival of the transplanted organs, or allografts. In these models, treatment with R348 did not result in vasculopathy, the narrowing of the coronary arteries and the most common cause of death in heart transplant patients, or in anemia. Inhibition of JAK2, which is involved in a number of hematopoetic pathways, has been shown to result in anemia and R348 has demonstrated selectivity for JAK3 over JAK2 in relevant cellular readouts. Additionally, researchers at Rigel have evaluated R348 in a number of other animal models of autoimmune disease with promising results.

R348 also has been shown to be selective over other targets in an array of functional assays. In addition, R348 appears to effectively inhibit JAK3-dependent lymphocyte (white blood cell) proliferation, and has demonstrated a good pharmacokinetic profile and oral bioavailability in preclinical studies.

“JAK3 plays a key role in immune cell function and its selective inhibition could provide a targeted therapy for autoimmune diseases such as transplant rejection, multiple sclerosis, rheumatoid arthritis, graft-versus-host disease, and psoriasis. R348 has shown excellent selectivity and efficacy in animal models of immune-mediated disease, and we believe it may be a punctuational advance in treating these diseases,” said Donald G. Payan, M.D., executive vice president and chief scientific officer of Rigel.

About JAK3 Inhibition
JAK3 is a cytoplasmic tyrosine kinase with expression limited to T cells, B cells, natural killer (NK) cells, mast cells, and macrophages. JAK3 acts in the signal transduction cascade of numerous growth factors critical for lymphocyte activation and plays an important role in lymphocyte differentiation and proliferation. Based on its limited tissue distribution and the evidence for its role in immune cell function, JAK3 may represent an attractive drug target for immunosuppression. Inhibitors of JAK3 could play a role in the treatment of autoimmune diseases and the prevention of transplant rejection.

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 or phrases such as "plans," "intends," "may result," "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