Rigel Demonstrates Success of its Proprietary Fluorescent Libraries in Human Functional Genomic Screens

Lead Article in the May Issue of Chemistry and Biology

South San Francisco, CA - May 01, 2001

Rigel Pharmaceuticals, Inc. (Nasdaq: RIGL) today announced that new research published in the journal, Chemistry & Biology, shows that the Company's proprietary intracellular peptide libraries can cause functional changes in human diseased cells. The research, led by Rigel scientists David C. Anderson, Ph.D., and Donald G. Payan, M.D., is featured on the cover of the May issue of the peer-reviewed publication.

"This research shows that our functional screens work in a variety of human cells and are able to generate multiple peptide hits with the different fluorescent libraries delivered to the cell with Rigel's retroviral technology," said Dr. Payan, Rigel's chief scientific officer. "This is important because peptides which successfully change cellular function could have many uses in the drug discovery and development process, as their binding partners may represent potential new drug targets. Rigel has used this patented technology very successfully to discover numerous novel peptides and drug targets, both in our partnered and retained programs, and we are moving them rapidly through development."

The research was designed to test several of Rigel's proprietary peptide libraries, each of which has a different structural bias, to demonstrate their ability to change a cell's function in a desired way (e.g., from a diseased state to a normal state). In order to do this, researchers expressed peptide library members inside human diseased cells. Attached to each of these peptides was green fluorescent protein (GFP), which helps stabilize the peptides and serves as a fluorescent tag allowing localization of the peptides within the cell. Once the GFP-expressing peptide was inside the cell, cells were examined to identify those containing novel localization sequences (i.e., peptides that were not uniformly localized). This non-uniform localization represents a functional change, and may be due to binding to localized macromolecules or to transporters within the cell.

This technology can screen large intracellular peptide libraries and identify peptides that cause functional changes with defined effects on very complex or poorly understood disease-related cellular processes. During the past two years, Rigel has identified numerous functional targets with this technology.

A patent covering this technology was exclusively issued to Rigel in November of 2000, further strengthening the Company's growing intellectual property portfolio. Rigel now has six issued patents with a number of applications allowed or pending.

Rigel uses post-genomics combinatorial biology technology to discover novel drug targets. Post-genomics combinatorial biology technology is designed to identify molecules which 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 the cell’s response to the disease. Rigel currently has programs in asthma/allergy, autoimmunity, transplant rejection, rheumatoid arthritis, inflammatory bowel disease, chronic bronchitis, cancerous tumor growth and hepatitis C. In addition to its proprietary programs, Rigel has research and product development collaborations with Pfizer Inc., Cell Genesys, Inc., Janssen Pharmaceutica N.V. and Novartis Pharma A.G. Rigel is based in South San Francisco, California.

The information in this press release is current as of its release date. Rigel takes no responsibility to update this information.