Seattle Genetics to Present Advances in Preclinical Pipeline and ADC Technology at AACR Annual Meeting.

BOTHELL, Wash. -- Seattle Genetics, Inc. (Nasdaq:SGEN) announced today that it will report data from its preclinical programs and novel antibody-drug conjugate (ADC) technology during the 2005 Annual Meeting of the American Association for Cancer Research (AACR) to be held April 16-20, 2005, in Anaheim, California. In addition, Seattle Genetics' collaborators will make multiple presentations that highlight their preclinical programs utilizing Seattle Genetics' ADC technology.

"Our strong presence at AACR this year illustrates the substantial progress being made with Seattle Genetics' product candidates as well as the utility of our ADC technology in the development of antibody-based therapies for cancer," stated Clay B. Siegall, Ph.D., President and Chief Executive Officer at Seattle Genetics. "We will make presentations concerning our anti-CD70 programs, SGN-70 and SGN-75, which we have advanced significantly in the past year. There are also seven abstracts reporting continued progress with our industry-leading ADC technology from both us and our collaborators, Genentech and CuraGen. We are pleased by the impact that our science and drug development programs are making to cancer research."

Anti-CD70 Program

Seattle Genetics is developing two preclinical product candidates that target the CD70 antigen, which is expressed in high density on renal cancer and certain hematologic malignancies. SGN-70 is a humanized monoclonal antibody with intrinsic cell-killing ability and SGN-75 is an ADC comprised of the same antibody stably linked to a potent auristatin derivative using the company's ADC technology. The company will present preclinical data showing that SGN-70 possesses potent antitumor activity against hematologic and carcinoma tumors. Additionally, SGN-75 exhibits selective cytotoxicity in preclinical models of renal cell cancer. These findings demonstrate that both SGN-70 and SGN-75 have therapeutic potential in the treatment of CD70-expressing cancers. Seattle Genetics is continuing to evaluate both programs as potential IND candidates in 2006 or 2007. (Abstracts 6142 and 6143)

Antibody-Drug Conjugate (ADC) Technology

ADCs are monoclonal antibodies linked to potent cell-killing drugs. Due to the targeting ability of monoclonal antibodies, ADCs are capable of delivering a cell-killing drug directly to a tumor cell, while minimizing toxicity to normal cells. Seattle Genetics' ADC technology is composed of highly potent drugs and stable linker systems. Recent advances with the technology include generating new linkers aimed at further extending the therapeutic window of ADCs. The company will report data on a series of ADCs in which various linker components were altered or deleted. The findings suggest that modifications to the linker can have pronounced impacts on toxicity and lead to novel ADCs with improved therapeutic windows. In addition, preclinical studies have reinforced the importance of antigen density and utilization of an internalizing antibody to achieve robust cell-killing with an ADC. (Abstracts 1421 and 688)

ADC Collaborator Activities

Seattle Genetics' collaborators have also submitted multiple abstracts to AACR reporting progress with their internal programs utilizing the company's ADC technology. These include:

--Genentech, Inc. -- "Trastuzumab-Auristatin immunoconjugates inhibit growth and induce apoptosis in human breast cancer cells in vitro" (Abstract 6184)

--Genentech, Inc. -- "Tissue distribution, metabolism, and excretion of the antibody-drug conjugate Herceptin-monomethyl auristatin E in rats" (Abstract 5063)

--Genentech, Inc. -- "Targeting the ovarian cancer antigen, CA125/MUC16, with antibody-drug conjugates" (Abstract 562)

--Genentech, Inc. -- "Gene expression profiling of gliomas identifies candidate genes for the development of an antibody-based therapeutic" (Abstract 4868)

--CuraGen Corporation -- "CR011, a potent fully human monoclonal antibody-auristatin E conjugate for the treatment of melanoma" (Abstract 557)

About Seattle Genetics

Seattle Genetics discovers and develops monoclonal antibody-based therapeutics to treat cancer and other human diseases. The company has built a diverse portfolio of product candidates targeted to many types of cancer, including three being tested in multiple ongoing clinical trials, SGN-30, SGN-15 and SGN-40, and four in preclinical development, SGN-35, SGN-70, SGN-75 and SGN-17/19. The product candidates encompass three platform technologies: genetically engineered monoclonal antibodies, antibody-drug conjugates (ADCs) and antibody-directed enzyme prodrug therapy (ADEPT). Seattle Genetics has developed leading ADC technology comprised of highly potent synthetic drugs and stable linkers for attaching the drugs to monoclonal antibodies. The company currently has license agreements for its ADC technology with Genentech, UCB Celltech, Protein Design Labs, CuraGen and Bayer and for its ADEPT technology with Genencor International. More information about Seattle Genetics can be found at www.seattlegenetics.com.

Certain of the statements made in this press release are forward-looking, such as those, among others, relating to the potential therapeutic potential of Seattle Genetics' product candidates and ADC technology. Actual results or developments may differ materially from those projected or implied in these forward-looking statements. Factors that may cause such a difference include risks related to adverse clinical results as our product candidates move into and advance in clinical trials, risks inherent in early stage development and failure by Seattle Genetics to secure or maintain relationships with collaborators. More information about the risks and uncertainties faced by Seattle Genetics is contained in the Company's filings with the Securities and Exchange Commission. Seattle Genetics disclaims any intention or obligation to update or revise any forward-looking statements, whether as a result of new information, future events or otherwise.