Abgenix and Sosei to Codevelop Antibodies against Allergic Inflammation Target.

Business Editors/Health/Medical Writers

FREMONT, Calif., TOKYO and LONDON--(BUSINESS WIRE)--March 31, 2003

Abgenix, Inc. (Nasdaq:ABGX) and Sosei Co., Ltd. announced today an agreement to discover, develop and commercialize fully human monoclonal antibody therapies against a promising allergic inflammation target. The target, CRTH2, is a receptor expressed on human T-helper type cells that are linked to the immune response in certain allergic reactions, such as asthma. Financial details of the agreement were not disclosed.

Under terms of the agreement, Abgenix and Sosei will codevelop antibodies against CRTH2 and additional target antigens that may be contributed to the collaboration. The companies intend to share development and manufacturing costs for antibody products advanced by the collaboration. Abgenix will utilize its XenoMouse(R) technology to generate, screen and characterize human monoclonal antibodies for the alliance. Under the agreement, Abgenix has responsibility for manufacturing monoclonal antibodies for use in clinical trials, and may offer early commercial manufacturing for antibodies stemming from the collaboration. Sosei will be responsible for commercialization in Japan, while Abgenix will lead commercialization of any potential antibody product in the United States. Future profits resulting from the collaboration will be shared equally on a worldwide basis.

"This collaboration leverages the full spectrum of our integrated antibody development platform, including state-of-the-art manufacturing capabilities, to explore a promising target for allergic inflammation," said Ray Withy, Ph.D., president and chief executive officer of Abgenix. "We look forward to applying our discovery, development and production expertise to commercialize antibody therapeutics with Sosei."

Shin-Ichi Tamura, president and chief executive officer of Sosei said, "We are pleased to collaborate with Abgenix. Adding human monoclonal antibodies to our R&D pipeline will significantly enhance the value of our business."

CRTH2 (chemoattractant receptor-homolog on Th2 cells) is a novel G protein-coupled receptor, which is selectively expressed in Th2 cells including allergen-responsive Th2 cells, eosinophils, basophils, but not Th1 cells. It has been suggested that CRTH2 occupies a pivotal role in development of Th2 type immune responses. Sosei has obtained from BML, Inc., one of the leading diagnostic companies in Japan, worldwide exclusive licenses for therapeutic uses under the patents and know-how relating to CRTH2 and monoclonal antibodies against this target.

Antibodies are naturally occurring proteins used by the body's immune system to combat many diseases. As therapeutic products, antibodies have several potential advantages over other therapies. The highly specific interaction between an antibody and its target may, for example, reduce unwanted side effects that may occur with other therapies. Fully human antibodies are desirable because they avoid the risk of rejection present with mouse or partial mouse antibodies.

About Sosei

Sosei is a leading private Japanese biopharmaceutical company focused on drug development. It enriches the core product pipeline via in-licensing compounds from Western and Japanese companies, by its distinctive drug re-profiling platform (DRP(R)) and through its internal R&D programs in collaboration with biopharmaceutical companies and universities both in Japan and the West. The company is capitalizing on its extensive global network established over the past 10 years in its successful technology transfer business. Sosei's investors include experienced US/EU funds and Japanese institutional investors. For further information about Sosei, please visit www.sosei.com.

About Abgenix

Abgenix is a biopharmaceutical company focused on the discovery, development and manufacturing of human therapeutic antibodies. The company's antibody development platform includes a leading technology and state-of-the-art manufacturing capabilities that enable the rapid generation, selection and production of high affinity, fully human antibody product candidates to a variety of disease targets. Abgenix leverages its leadership position in human antibody technology to build a diversified product portfolio through the establishment of collaborations with multiple pharmaceutical and biotechnology companies. For more information on Abgenix, visit the company's website at www.abgenix.com.

Statements made in this press release about Abgenix's technologies, product development activities, collaborative arrangements and manufacturing activities, other than statements of historical fact, and about its projected financial results, are forward-looking statements and are subject to a number of uncertainties that could cause actual results to differ materially from the statements made, including risks associated with the success of clinical trials, the progress of research and product development programs, product manufacturing, the regulatory approval process, competitive products, future capital requirements and the extent and breadth of Abgenix's patent portfolio. Please see Abgenix's public filings with the Securities and Exchange Commission for information about risks that may affect Abgenix.