Seattle Genetics Highlights Data on its Proprietary Antibody-Drug Conjugate Technology at AACR.

Multiple Presentations Demonstrate Potential of Company's Leading ADC Technology Platform to Empower Antibodies for the Treatment of Cancer

BOTHELL, Wash. & LOS ANGELES -- Seattle Genetics, Inc. (Nasdaq: SGEN) today announced data from fifteen presentations by the company and its collaborators at the 2007 Annual Meeting of the American Association for Cancer Research (AACR) demonstrating preclinical advances with its proprietary antibody-drug conjugate (ADC) technology and cancer product pipeline.

ADCs are monoclonal antibodies linked to cell-killing drugs. Seattle Genetics' ADC technology employs synthetic, highly potent drugs that are bound to monoclonal antibodies through proprietary linker systems. The linkers are designed to be stable in the bloodstream but to release the drug payload under specific conditions once inside target cells, thereby sparing non-target cells many of the toxic effects of traditional chemotherapy.

"Empowered antibodies utilizing our ADC technology have the potential to significantly impact the way cancer is treated," said Clay B. Siegall, Ph.D., President and Chief Executive Officer at Seattle Genetics. "Our strong presence at AACR this year reflects continued progress with our proprietary and partnered ADC programs. We expect to report clinical data from the ongoing phase I dose-escalation trial of SGN-35, our lead ADC, later this year, while both we and our collaborators continue to advance additional ADC programs into and towards the clinic."

ADC Programs

Researchers reported preclinical data at AACR describing how changes in the structure of the attached drugs can affect ADC efficacy and tolerability, the impact that linker structure and these modifications can have on ADC activity, and the ability of Seattle Genetics' ADC technology to effectively deliver active drugs to target tissue while avoiding non-targeted drug release. (Abstracts #4793, #916 and #4088) Further data from preclinical models of ADCs provide mechanistic insight into how and when drugs are released inside of target cells. (Abstracts #658 and #4086)

In addition, data were reported on preclinical studies of ADCs targeted to glypican-3, CD133/prominin-1 and Lewis-Y, which are expressed in a variety of carcinomas including melanoma, ovarian, brain, colorectal and pancreatic cancers. Data were presented in multiple poster sessions demonstrating that ADCs to these targets are active and thus have therapeutic potential in the treatment of multiple types of solid tumors. (Abstracts #656, #1332 and #3332)

Several of Seattle Genetics' collaborators presented additional preclinical data on programs utilizing the company's ADC technology. These included presentations by Progenics, Genentech, MedImmune and Celera, an Applera Corporation business. (Abstracts #4102, #1551, #4468, #5744 and #1324)

SGN-33 and SGN-30 Programs

Data presented during AACR also illustrate ongoing preclinical activities with SGN-33 and SGN-30 to further define their therapeutic potential.

SGN-33 is a humanized monoclonal antibody currently in a single-agent phase I clinical trial for acute myeloid leukemia (AML) and myelodysplastic syndromes (MDS). In preclinical studies, SGN-33 reduces the tumor-induced activity of macrophages, suggesting a novel mechanism that may contribute to its antitumor activity. Seattle Genetics plans to advance SGN-33 into combination clinical trials during 2007, including a phase I study in combination with Revlimid[R] for MDS and a phase II study in combination with low-dose chemotherapy for AML. (Abstract #4111)

SGN-30 is currently in three phase II clinical trials in combination with chemotherapy for the treatment of Hodgkin's disease and anaplastic large cell lymphoma (ALCL) that are sponsored by the National Cancer Institute. Preclinical findings presented by the company during AACR further characterize SGN-30's mechanism of action. (Abstract #661)

Downloadable copies of Seattle Genetics' posters are available from the "Technology" section of the company's website at www.seattlegenetics.com.

About Seattle Genetics

Seattle Genetics is a biotechnology company developing monoclonal antibody-based therapies for the treatment of multiple types of cancer, including lymphoma, multiple myeloma, leukemia and solid tumors. The company has an exclusive worldwide license agreement with Genentech to develop and commercialize SGN-40. Seattle Genetics also has three other proprietary programs in ongoing clinical trials: SGN-33, SGN-35 and SGN-30. In addition, the company has developed proprietary antibody-drug conjugate (ADC) technology comprised of highly potent synthetic drugs and stable linkers for attaching the drugs to monoclonal antibodies. Seattle Genetics has collaborations for its ADC technology with a number of leading biotechnology and pharmaceutical companies, including Genentech, Bayer, CuraGen, Progenics and MedImmune, as well as an ADC co-development agreement with Agensys. More information 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' collaborators to advance product candidates incorporating its technology. More information about the risks and uncertainties faced by Seattle Genetics is contained in Seattle Genetics' 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.