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Rigel Discovers Novel Angiogenesis and Tumor Formation Roles of Axl Kinase

--Studies Published in Cancer Research--

SOUTH SAN FRANCISCO, Calif. - October 18, 2005

Rigel Pharmaceuticals, Inc. (Nasdaq: RIGL - News) today announced that Cancer Research, the journal of the American Association for Cancer Research (AACR), published findings from studies done at Rigel that identified and characterized a role for the receptor tyrosine kinase Axl in angiogenesis and tumor growth. Scientists at Rigel found that Axl signaling controls diverse processes in endothelial cells, including growth, survival, migration, and morphologic differentiation – mechanisms that are central to tumor growth and formation.

“We think that using a small molecule inhibitor to disrupt Axl signaling may independently target angiogenesis and tumor growth, providing a highly effective method to treat solid human tumors,” said Donald G. Payan, M.D., executive vice president and chief scientific officer of Rigel. "While anti-vascular endothelial growth factor (VEGF) monoclonal antibodies have displayed anti-angiogenic activity, they are not sufficient to block tumor growth in a majority of patients. Axl inhibition could potentially achieve both aims of targeted cancer therapy.”

As part of an effort to develop novel therapeutic strategies for cancer treatment, Rigel researchers developed a unique genetic screening protocol to discover genes that regulate cell migration in primary human endothelial cells. Their findings indicate that Axl regulates processes vital for both neovascularization and tumorigenesis. In vitro testing showed that Axl is a key regulator of multiple angiogenic behaviors including endothelial cell migration, proliferation and tube formation. In an animal model of human angiogenesis, Axl expression is important for the formation of functional blood vessels. Furthermore, researchers observed that Axl was required for the human breast carcinoma cells to form a tumor in vivo.

Treatment Gap in Angiogenesis Inhibition
Angiogenesis, or the growth of new blood vessels, is a strictly regulated process in healthy adults. However, with diseases such as cancer, diseased cells produce abnormal amounts of angiogenic growth factors and can cause aberrant angiogenesis. In the case of cancer, new blood vessels feed tumor growth and facilitate metastasis. Targeted inhibition of angiogenesis using the VEGF antibody Avastin® (bevacizumab) has shown efficacy in treating cancer. Still, tumors frequently progressed while patients were receiving therapy, indicating that inhibition of VEGF signaling alone may not be sufficient to block the growth of new blood vessels. Blocking processes complementary to those driven by VEGF may therefore be important in fully inhibiting angiogenesis in human disease.

About Rigel (www.rigel.com)
Rigel is a late-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 three 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 June 30, 2005. Rigel does not undertake any obligation to update forward-looking statements.

Avastin® is a registered trademark of Genentech.

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