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Rigel Announces Initiation of Phase I Study Evaluating R763 in Solid Tumors

--Rigel to Receive $3 Million Milestone Payment from Serono --

SOUTH SAN FRANCISCO, Calif. - September 19, 2006

Rigel Pharmaceuticals, Inc. (Nasdaq: RIGL) announced today that its partner Serono has begun enrolling patients in a Phase I study evaluating the safety and tolerability of R763, a highly potent, orally available multi-Aurora kinase inhibitor, for the treatment of patients with refractory solid tumors. Serono licensed development and commercialization rights to Rigel's Aurora kinase inhibitor program, including R763 for cancer, in October 2005. Rigel will receive a $3 million milestone payment from Serono triggered by the initiation of the Phase I study.

"This is the first of several Phase I studies to be conducted in order to evaluate the safety and initial efficacy of R763 in different types of cancers, including solid tumors and hematological tumors, and in combination with standard therapies" said Donald G. Payan, M.D., executive vice president and chief scientific officer of Rigel. "We look forward to reviewing the data from this trial and subsequent trials as indications of R763’s potential."

The study will evaluate R763 as a single agent therapy for the treatment of refractory solid tumors. The multi-center, U.S. study will enroll 60 to 80 patients with refractory tumors who have exhausted standard therapeutic options and volunteered for an experimental agent. The primary endpoints of the study are safety and tolerability. The study will also look at the pharmacokinetic profile of R763, and will measure biomarkers to determine the activity of the compound against the target kinase and on signaling networks.

Aurora Kinase and Cancer
The over-expression of Aurora kinase can cause cells to rapidly develop an abnormal number of chromosomes. Elevated levels of Aurora kinase are frequently associated with various human cancers, such as cancers of the breast, bladder, colon, ovary, head and neck, and pancreas. Inhibition of Aurora kinase disrupts cell division and promotes programmed cell death (apoptosis). Increased knowledge of Aurora kinase and its regulation may result in novel approaches to the treatment of cancer.

Rigel's lead oncology drug candidate, R763, is a highly potent inhibitor of Aurora kinase, which has been shown to effectively inhibit proliferation and trigger apoptosis in several tumor cell lines including cervix, colon, lung, pancreas and prostate. Rigel discovered R763 using its proprietary cell-based PAD (Proliferation, Apoptosis and DNA content) assays applied to tumor cell lines.

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, 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," “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: Andrea Campbell, WeissComm Partners, Inc.
Phone: 281.360.7239
Email: acampbell@weisscommpartners.com