Rigel Pharmaceuticals, Inc. To Receive $5 Million Milestone Payment From Serono International S.A. For R763

SOUTH SAN FRANCISCO, Calif., Jan. 26 /PRNewswire-FirstCall/ -- Rigel Pharmaceuticals, Inc. (Nasdaq: RIGL - News) announced today that it will receive a $5 million milestone payment from Serono triggered by the progress on the Investigational New Drug (IND) application for R763, for which Phase I clinical trials in cancer patients shall commence later this year. R763 is a highly potent, orally available multi-Aurora kinase inhibitor. Serono licensed development and commercialization rights to Rigel’s Aurora kinase inhibitor program, including R763 for cancer, in October 2005.

R763 has been shown in vitro and in in vivo tumor xenograft models to potently inhibit proliferation and trigger apoptosis in several leukemia and tumor cell lines including the cervix, colon, lung, pancreas and prostate.

“This is an important milestone for our collaboration with Serono, who will take R763 into clinical development in 2006. Given the broad applicability R763 has demonstrated on many types of solid tumors and leukemias, we’re excited to see this compound move further along in development,” said Donald G. Payan, M.D., executive vice president and chief scientific officer of Rigel.

On October 25, 2005, Rigel announced that it had granted Serono an exclusive license to develop and commercialize product candidates from Rigel’s Aurora kinase inhibitor program. The license is worldwide, except for Japan, which Serono has an option to include at any time within two years following signature of the agreement. Under the terms of the agreement, Rigel is eligible to receive up to $160 million in total as well as royalties on any eventual product sales of R763 and other Aurora kinase inhibitors developed under the agreement. Serono will be responsible for the further development and commercialization of R763, as well as any other product candidates arising from Rigel’s Aurora kinase inhibitor program. Rigel has achieved milestones for total payments of $30 million from Serono to date, including $10 million in initial payments, $15 million in equity participation and this $5 million milestone payment for the IND.

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 arrests cell division and promotes programmed cell death (apoptosis). Increased knowledge of Aurora kinase and its regulation may result in future treatments for cancer.

Rigel’s lead oncology drug candidate, R763, is a highly potent inhibitor of Aurora kinase, which has been shown to potently 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 move one new product candidate for a significant indication into the clinic 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 allergy/asthma, rheumatoid arthritis and 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,” “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 pre-clinical studies and clinical trials, 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, 2005. Rigel does not undertake any obligation to update forward-looking statements.

Contact: Raul Rodriguez Phone: 650.624.1302 Email: invrel@rigel.com

Media Contact: Carolyn Bumgardner Wang, WeissComm Partners, Inc. Phone: 415.946.1065 Email: carolyn@weisscommpartners.com

Source: Rigel Pharmaceuticals, Inc.

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