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Rigel Announces Data on Its Multiple Cancer Therapy Targets at AACR Meeting

Company Focused on New Pathways That Regulate Cell Proliferation

SOUTH SAN FRANCISCO, Calif., - April 09, 2002

Rigel Pharmaceuticals, Inc (Nasdaq: RIGL) announced today at the 93rd annual meeting of the American Association of Cancer Research (AACR) that data presented on its multiple potential cancer therapy targets are helping to shed new light on pathways that may play a role in uncontrolled cell reproduction that is the hallmark of malignant cells. Rigel believes that this insight into the role of pathways is the first step in uncovering potential new ways to treat many types of cancer.

"The research showcased at AACR is typical of Rigel's functional approach to drug discovery," said Donald G. Payan, the company's executive vice president and chief scientific officer. "We have identified new cell regulatory mechanisms that directly affect cell division. These novel potential drug targets will serve to accelerate our focused drug development programs."

In the new research, Rigel identified several proteins that influence cell arrest, i.e., the switching off of a cell's ability to replicate, and the proteins that regulate these cells. One of these regulators, called LETM-1, caused potent cell arrest at the earliest stages of reproduction. Another regulator, part of the Mi-2 histone deacetylase complex, allowed cells to reproduce even after they were exposed to a protein known to induce cell arrest.

Rigel researchers also presented findings about a class of proteins known as ubiquitin ligases, which currently are the subject of intense industry-wide interest in cancer research and a major focus of the company's research. Ubiquitin ligases are enzymes that mediate the degradation of proteins, which in turn affects many important cellular functions, including cell division. Researchers in Rigel's ubiquitin ligase drug discovery program have identified small molecule inhibitors of this class of enzymes that exhibit antiproliferative activity against several tumor cell types. These findings further validate ubiquitin ligases as potential targets for cancer therapy.

These new targets for potential oncology therapeutics come out of Rigel's novel approach to drug discovery. The company uses advanced genomics technologies to identify protein targets that have a significant and demonstrable role in a disease pathway. By understanding the disease pathway, Rigel focuses on the subset of proteins that are specifically implicated in the disease process rather than studying proteins that are not likely to make good drug targets.

The normal life cycle of a cell involves positive regulators that enhance the function of a particular protein, and negative regulators that downgrade or eliminate the function. In the case of cancer, the normal checks and balances in cell replication break down, causing cells to divide unchecked. The goal is to identify those regulators that affect cell arrest. These regulators can fall into two categories: those that play a role in active cell arrest and those that interfere with pathways that block cell arrest. The research announced by Rigel at AACR identified both types of regulators, helping to open multiple avenues for potential drug development.

About Rigel Pharmaceuticals, Inc.

Rigel Pharmaceuticals, Inc. (www.rigel.com) is a drug discovery and development company that uses advanced functional genomics tools to discover novel drug targets that can be used to develop orally administered small molecule drugs. Rigel's technology is designed to identify molecules that play an important role in regulating a human cell's response to disease by testing a very large number of proteins in a very large number of cells to determine which proteins will change a cell's response to the disease. Rigel currently has ten product development programs underway, with five programs being proprietary programs in the areas of asthma/allergy, rheumatoid arthritis and inflammatory bowel disease, cancerous tumor growth and hepatitis C. Rigel expects to begin clinical trials during 2002 with one or more drug candidates from these five programs. In addition to the Rigel-owned programs, the company has five joint programs with its corporate partners in the product development areas of asthma/allergy, autoimmunity, transplant rejection and two programs in cancerous tumor growth. With Rigel's support, one of the company's partners is conducting an additional program in chronic bronchitis at its premises. Rigel has multi-year collaborations with Pfizer Inc., Cell Genesys, Inc., Johnson & Johnson Pharmaceutical Research & Development, L.L.C. and Novartis Pharma A.G. Rigel is based in South San Francisco, California.

This press release contains "forward-looking" statements, including statements related to the Company's drug development programs. 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 "believes," "anticipates," "plans," "expects," "will" and similar expressions are intended to identify forward-looking statements. There are a number of important factors that could cause the results of the Company to differ materially from those indicated by these forward-looking statements, including the risks detailed from time to time in the Company's SEC reports, including its Annual Report on Form 10-K for the year ended December 31, 2001. The Company does not undertake any obligation to update forward-looking statements.