Rigel Reports Significant Progress in Aurora Kinase Program for the Treatment of Cancer

Rigel Selects R763 as Lead Candidate

South San Francisco, CA - July 22, 2004

Rigel Pharmaceuticals, Inc. (NASDAQ: RIGL) today announced progress in its aurora kinase inhibition program, targeting cancer cell proliferation. Rigel announced that it has identified R763 as a lead compound in the program and that it plans to file an investigational new drug (IND) application by Q3, 2005. Rigel scientists will present on its aurora kinase program and R763 at the Gordon Research Conference, Molecular Therapeutics of Cancer on July 22, 2004 in presentation and poster sessions. The presentation and poster are also available on Rigel’s website (www.rigel.com).

Aurora kinase plays a central role in the cell division process. An overabundance of aurora kinase can be linked to the transformation of normal cells into cancer cells and has been identified in many tumor types, including colon cancer, breast cancer and leukemia. R763 is a potent, highly-selective, small-molecule inhibitor of aurora kinase. Rigel discovered the R763 compound class using a high-content cell-based phenotype assay that simultaneously measures cell proliferation, apoptosis, cellular morphology and normal cell cycling.

“Aurora kinases are one of the most promising targets in oncology drug discovery,” said Elliott B. Grossbard, MD, Senior Vice President of Medical Development. “Rigel’s R763 is one of the most potent aurora kinase inhibitors known and potentially can be delivered both orally and intravenously, a valuable property not reported for previously described compounds.”

“Aurora kinase inhibition provides a promising, novel approach for the treatment of multiple human malignancies,” said David S. Alberts, MD of the Arizona Cancer Center of the University of Arizona. “Rigel’s program signals an important step toward unlocking the potential for a new class of drugs that could significantly impact the future of cancer treatment.”

Aurora Kinases and Cancer The overexpression of aurora kinase can cause cells to quickly form an abnormal number of chromosomes. As such, aurora kinase is frequently associated with various human cancers such as breast, bladder, colon, ovary, head and neck, and pancreas. Increased knowledge of aurora kinase and its regulation potential may be the basis for treating and even preventing cancer.

Cancer is the second leading cause of death in the United States. Nearly half of all men and a little over one-third of all women in the US will develop cancer during their lifetimes. Today, millions of people are living with cancer or have had cancer. Anyone can get cancer at any age; however, about 77% of all cancers are diagnosed in people age 55 and older.

About Rigel (www.rigel.com) Rigel's mission is to become a source of novel, small-molecule drugs to address large, unmet medical needs. The company has initiated three development programs: asthma/allergy, hepatitis C and rheumatoid arthritis. Rigel has begun clinical testing of its first two product candidates, R112 for allergic rhinitis and R803 for hepatitis C, and expects to begin clinical trials of R406 for the treatment of rheumatoid arthritis by the end of 2004, to be followed by clinical trials for drug candidates in oncology and asthma.

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 “will,” “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 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 Annual Report on Form 10-K for the year ended December 31, 2003 and its Quarterly Report on Form 10-Q for the quarter ended March 31, 2004. Rigel does not undertake any obligation to update forward-looking statements.

For additional information:
Melinda Bagatelos or Ayanna Anderson
Schwartz Communications, Inc.
415-512-0770
rigel@schwartz-pr.com

Raul Rodriguez
Rigel Pharmaceuticals, Inc.
650-624-1302
rrodriguez@rigel.com