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Rigel's R803 for HCV Achieves Favorable Safety Profile

Positive Results Demonstrate R803's Potential for the Treatment of Hepatitis C Virus

South San Francisco, CA - January 26, 2004

Rigel Pharmaceuticals, Inc. (NASDAQ: RIGL) today announced positive clinical safety data from a Phase I trial for R803, an experimental drug to treat Hepatitis C Virus (HCV), the blood-borne virus that affects nearly 170 million people worldwide. Clinical data indicates that R803 is well tolerated with no notable adverse effects reported in the dose levels that Rigel plans to use moving forward. Rigel plans to launch a Phase I/II efficacy clinical trial in the U.S. during the second quarter of 2004 in HCV-infected patients. This trial will monitor viral clearance and safety over numerous days of drug administration.

In the Phase I trial, an escalating dose regimen of R803 was studied in 42 volunteers and was compared with placebo controls. The safety data collected indicated that subjects treated with R803 were indistinguishable from the placebo controls across a wide range of clinical and laboratory safety tests, including clinical signs and symptoms, serial electrocardiography, and clinical chemistry and hematology studies. Pharmacokinetic data, which will aid in planning dosing in the next clinical study, was also collected. The trial was conducted in the U.K. and the results will be part of the U.S. IND package that Rigel expects to file with the FDA later in the first quarter of 2004.

“A leading issue for the millions of Americans infected with chronic HCV are the side effects of current therapies and their relatively limited efficacy,” noted Jules L. Dienstag, M.D., Professor of Medicine and Associate Dean for Academic and Clinical Programs at Harvard Medical School and a steering committee member of the upcoming study. “R803 continues to show promise as a unique first-line anti-HCV therapeutic, directly targeting HCV by interfering with the viral polymerase protein that is needed for replication.”

“The successful completion of Phase I safety trials represents another major step in Rigel’s efforts to advance the clinical development of R803,” said Elliott Grossbard M.D., Senior Vice President, Medical Development. “The availability of a new oral therapy that is convenient, safe and effective would be an important addition to currently available treatment options for patients with 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. R803 has been shown to be efficacious 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.

HCV: Current Treatments and Market Opportunity
Hepatitis C is an inflammation of the liver caused by the hepatitis C virus. As the most common blood-borne infection in the U.S., HCV affects 4 million Americans and 170 million individuals worldwide. Approximately 85 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 is with interferon alpha (IFN), usually in combination with ribavarin. IFN shows only a 20 percent to 40 percent success rate in patients who complete therapy, and significant side effects result 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 than 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 meet large, unmet medical needs. Rigel has identified four disease areas with which to focus its lead product development programs: asthma/allergy, virology, immunology and oncology Rigel has begun clinical testing of its first two product candidates, R112 for allergic rhinitis and R803 for hepatitis C, and plans to begin clinical trials of two additional drug candidates, for the treatment of rheumatoid arthritis and asthma, by the end of 2004.

This press release contains “forward-looking” statements, including statements related to Rigel’s plans to pursue clinical development of drug candidates and the timing thereof and the potential efficacy of drug 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 Quarterly Report on Form 10-Q for the quarter ended September 30, 2003 and Annual Report on Form 10-K, as amended, for the year ended December 31, 2002. Rigel does not undertake any obligation to update forward-looking statements.

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