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Rigel’s R788 Shows Promise for Treating B-Cell Lymphoma and Leukemia in Preclinical Studies

Results Presented at the American Society of Hematology (ASH) 48th Annual Meeting and Published in Journal of Experimental Medicine

SOUTH SAN FRANCISCO, Calif. and Orlando, Florida - December 12, 2006

Rigel Pharmaceuticals, Inc. (Nasdaq: RIGL) today announced the presentation of preclinical results demonstrating that R406/R788, a syk kinase inhibitor, inhibited the proliferation of multiple diffuse large B-cell lymphoma cell lines. R788 is the oral solid dosage formulation of R406. The verbal presentation took place at the American Society of Hematology (ASH) 48th Annual Meeting and Exposition being held this week in Orlando, Florida. The company also announced the publication of additional preclinical results in the December 25th issue of the Journal of Experimental Medicine, demonstrating that R406 blocks the proliferation of tumorigenic pre-B cells in leukemia cell lines. Rigel has filed an Investigational New Drug Application (IND) for R788 in lymphoma and plans to initiate clinical trials in 2007.

ASH Abstract #226: “Tonic B-Cell Receptor Signaling Promotes the Survival of Diffuse Large B-Cell Lymphomas: Identification of SYK as a Rational Treatment Target”
Syk kinase appears to amplify B-cell antigen receptor (BCR) signaling, which may be an important survival mechanism in certain B-cell lymphomas, and implicates syk as a possible rational therapeutic target. R406/R788 is a novel syk kinase inhibitor that blocks the activation of mast cells, B-cells and macrophages by blocking IgG signaling. In this study, researchers at the Dana-Farber Cancer Institute and Brigham and Women's Hospital, in conjunction with researchers at Rigel, showed that R406 inhibited proliferation and was cytotoxic in multiple diffuse large B-cell lymphoma cell lines, confirming that R406/R788 may be useful in the treatment of certain B-cell lymphomas.

Diffuse large B-cell lymphoma, the most common type of non-Hodgkin’s lymphoma (NHL), is a cancer of the immune system that is usually aggressive and marked by rapidly growing tumors in the lymph nodes, spleen, liver, bone marrow, or other organs.

“More than 54,000 patients are diagnosed with lymphoid malignancies each year. Patients with diffuse large B-cell lymphomas have an average five-year survival rate of only 50 percent,” said Margaret A. Shipp, M.D., director of the Dana Farber/ Harvard Cancer Center Lymphoma Program, and senior investigator in the ASH study. “New treatments are needed for these aggressive tumors, which is why we are excited about syk-inhibition as a possible novel target.”

“Deregulated syk kinase inhibits differentiation and induces growth factor-independent proliferation of pre-B cells,” Journal of Experimental Medicine, Volume 203, Issue #13
In this study, deregulated syk kinase activity allowed growth factor-independent proliferation of B lymphocytes and transformed bone-marrow derived pre-B cells that were then able to induce leukemia in an animal model. Treatment of the syk-transformed pre-B cells with R406 demonstrated that R406 allowed these cells to differentiate normally, indicating that inhibiting syk kinase may be a useful approach for the treatment of leukemia.

“The results of both of these studies confirm that syk is a potential target for the treatment of lymphoma and leukemia, which each represent a complex group of different blood-related cancers,” stated Donald G. Payan, M.D., executive vice president and chief scientific officer of Rigel. “We have filed an IND for R788 in lymphoma, and look forward to initiating clinical trials in early 2007.“

About R406/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 currently being evaluated in two Phase II studies, in rheumatoid arthritis and immune thrombocytopenic purpura (ITP). 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 and in a murine model of ITP, increased platelet counts significantly.

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 to pursue clinical development of product candidates and the timing thereof. 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 "continue," "could," "may," “promise,” “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, 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, 2006. Rigel does not undertake any obligation to update forward-looking statements.

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

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