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Rigel’s R788 Slows Progression of Murine Lupus in Preclinical Studies

Lead drug candidate may help patients with Systemic Lupus Erythematosus (SLE)

SOUTH SAN FRANCISCO, Calif. - April 29, 2008

Rigel Pharmaceuticals, Inc. (Nasdaq: RIGL) announced today that its lead product candidate, R788, has successfully treated lupus prone mice and significantly improved their survival as reported in a recently published study of the drug candidate. R788 (fostamatinib disodium) is an orally bioavailable syk kinase inhibitor, which has shown clinically significant results in treating patients with rheumatoid arthritis and immune thrombocytopenic purpura in clinical trials. A third clinical trial of R788 in patients with B-cell lymphoma will be completed later this year. Rigel also expects to initiate a Phase 2 clinical trial in lupus in the second half of 2008.

The study, which evaluated the potential of R788’s effect on the immune cascade in an in vivo lupus model, has been published in Arthritis and Rheumatism and is titled - “An Orally Bioavailable Spleen Tyrosine Kinase Inhibitor Delays Disease Progression and Prolongs Survival in Murine Lupus” (May 2008, Volume 58, No. 5, p.1433).

“These results are impressive and consistent with R788’s mechanism of action,” said Donald G. Payan, M.D., executive vice president and president of discovery and research at Rigel. “Given this mechanism, R788 has the potential to treat a broad range of immune-related disorders, a number of which we are advancing in the clinic.”

Summary of results
The study evaluated the effects of three doses of R788 versus a control group and an untreated group of lupus-prone mice. The mice in the R788 groups orally received 10mg/kg, 20 mg/kg or 40 mg/kg twice a day, for 240 days. Baseline, periodic and terminal measurements of renal enzymes and proteinuria (presence of proteins in the urine associated with kidney malfunctions), blood urea nitrogen, and other tests were done on all subjects in the treatment and control groups.

At the completion of the study, only 2 of the 29 mice in the 40 mg/kg group had elevated proteinuria compared to 21 of the 30 mice in the control group. All 29 (100%) of the mice treated with 40 mg/kg of R788 survived the duration of the study, compared to 14 of the 30 (47%) mice in the control group. The mice treated with 10 mg/kg and 20 mg/kg of R788 demonstrated results that were between those of the control group and the 40mg/kg group. In a separate study, where treatment was initiated after the onset of disease, the researchers noted that the majority of the animals (~95%) given the 40 mg/kg dose had elevated proteinuria levels that decreased following the onset drug treatment.

Systemic Lupus Erythematosus (SLE)
Lupus is an autoimmune disease, which affects nearly 2 million Americans, the majority of whom are women (90%). The disease affects various parts of the body including the kidneys, skin, joints, heart, lungs, and brain. Its effects can be mild, limited to a couple of organs in the body and occasional flare-ups, or can cause serious and life-threatening complications. Like other autoimmune diseases the primary characteristic of lupus is inflammation, which causes swelling, pain, loss of function and may ultimately destroy the involved organ if left untreated. Current therapies for lupus treat the symptoms of the disease and include non-steroidal anti-inflammatory drugs (NSAIDS), corticosteroids, and, in severe cases where organ damage is at risk, immunosuppressant drugs.

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/autoimmune diseases and cancer, as well as viral and metabolic diseases. Our goal is to file one new investigational new drug (IND) application in a significant indication each year. Rigel has achieved this goal every year 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. Rigel has product development programs in inflammatory/autoimmune diseases such as rheumatoid arthritis, thrombocytopenia and asthma, as well as in cancer.

This press release contains "forward-looking" statements, including statements related to the preclinical data and plans and potential efficacy of R788. 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 "believe," "may," "can," "support," "indicate," 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, potential problems that may arise in the clinical testing and approval process and Rigel's need for additional capital, as well as other risks detailed from time to time in Rigel's SEC reports, including its Form 10-K for the year ended December 31, 2007. Rigel does not undertake any obligation to update forward-looking statements.

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

Media Contact: Susan C. Rogers, Alchemy Consulting, Inc.
Phone: 650.430.3777
Email: susan@alchemyemail.com