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Rigel Initiates Phase I/II Trial of R803 for the Treatment of Hepatitis C Virus

Hepatitis C Virus Affects an Estimated 170 Million People Worldwide, with Three to Four Million New Infections Every Year

South San Francisco, CA - May 24, 2004

Rigel Pharmaceuticals, Inc. (NASDAQ: RIGL) today announced the initiation of a multi-dose Phase I/II clinical trial for R803, an experimental drug to treat the blood-borne liver disease Hepatitis C Virus (HCV). The goals of this trial are to assess safety, tolerability and pharmacokinetics of multiple dose administrations of R803 in patients with HCV. The trial will also explore the effectiveness of various dosing regimens of R803 in reducing viral levels. Results of this trial are expected by the fourth quarter of 2004 and will allow Rigel to enter into the broader, longer-term trials, which are necessary for FDA approval.

The Phase I/II multi-dose clinical trial is a double-blind, placebo-controlled, dose-ascending study in subjects with diagnosed HCV. There will be eight groups of subjects, with each subsequent group receiving an increasing dose or increasing number of days of treatment. Subjects will be dosed for two to four days, plus the morning dose on the following day. In January, Rigel released the results of a Phase I clinical trial, which evaluated the safety of R803 in healthy volunteers. No clinically adverse effects were attributed to R803 during this trial at relevant dose levels.

“HCV is a serious global epidemic, and since the current therapies have significant limitations, new therapies are needed to treat this infection,” commented Elliott B. Grossbard, M.D., Senior Vice President, Medical Development of Rigel. “Rigel’s R803 represents a novel approach in the treatment of HCV, and is one of the first direct antiviral agents to reach human clinical trials.”

“Because of its unique mechanism of action, Rigel’s R803 has great promise as a potential therapeutic in the fight against HCV,” noted Donald G. Payan, M.D., Chief Scientific Officer and Executive Vice President of Rigel. “Rigel is committed to the development of this compound and to furthering treatment options for those affected by HCV.”

Rigel’s R803, a non-nucleoside HCV polymerase inhibitor, is an oral, small-molecule compound. To date, R803 has demonstrated potent efficacy in inhibiting viral replication in cell-based assay systems and in live virus assays. In these models, R803 has been shown to be active against various genotypes of HCV, including genotype 1, the most common in North America and Europe. In various assays, R803 appears to act within days to reduce viral levels significantly. In addition, as a result of R803’s novel viral binding site, resistance may be slow to develop. In cell-based systems, R803 has demonstrated synergy when used with interferon alpha (IFN). This observation may allow the use of a reduced dose of IFN, potentially minimizing the significant side effects of that drug.

HCV: Current Treatments and Market Opportunity
Hepatitis C is an inflammation of the liver caused by HCV. As the most common blood-borne infection in the U.S., HCV affects approximately 4 million Americans and 170 million individuals worldwide. Approximately 80 percent of those with acute illness will go on to develop chronic hepatitis, a condition that has been linked to cirrhosis, hepatocellular carcinoma (liver cancer) and liver failure. HCV accounts for 30 percent of end-stage liver disease and liver cancer and is the leading cause of liver failure, which can result in the need for liver transplantation. Public health officials in the U.S. and abroad have mobilized to address this medical crisis by identifying detection guidelines for HCV and implementing therapies to eradicate chronic infection.

Currently available HCV therapies are only modestly effective at treating the disease. The most prevalent treatment regimen utilizes IFN, usually in combination with ribavarin. IFN shows only a 20 to 40 percent success rate in patients who complete therapy, with significant side effects resulting in up to half the patients either quitting treatment or moving to a lower dose regimen. Moreover, IFN is least effective against HCV genotype 1, the strain responsible for 70 percent of chronic HCV infection cases in the U.S. Rigel believes that its approach is substantially different from that of IFN; instead of working to boost the immune system, experiments indicate that R803 directly, rapidly, selectively and potently targets HCV by interfering with a viral polymerase protein that is needed for replication.

With the current high prevalence and projected increase in cases of HCV and related diseases, and with the limited success of currently available therapies, Rigel believes that the potential for new direct HCV therapeutics is large and that R803 has the potential to be at the forefront of this opportunity.

About Rigel (www.rigel.com)
Rigel's mission is to become a source of novel, small-molecule drugs to address large, unmet medical needs. We have 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 asthma and oncology.

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 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

Rigel Contact:
Raul Rodriguez (650-624-1302)

Media Contact:
Melinda Bagatelos or Ayanna Anderson, 415-512-0770, rigel@schwartz-pr.com