Seattle Genetics, Inc. (Nasdaq: SGEN) announced today that it has achieved a milestone under its antibody-drug conjugate (ADC) collaboration with Agensys, Inc., an affiliate of Tokyo-based Astellas Pharma Inc. The milestone was triggered by Agensys’ initiation of a phase I trial for AGS-16M8F, an ADC for the treatment of cancer utilizing Seattle Genetics’ technology.

“This trial initiation increases the total number of ADC programs utilizing our technology currently in clinical trials to eight, further extending the investigation of our technology for the treatment of multiple types of cancer,” said Eric L. Dobmeier, Chief Business Officer of Seattle Genetics. “Our ADC collaborations generate substantial financial resources for Seattle Genetics, including more than $130 million received to date. We also have the potential to receive approximately $2.7 billion in milestone payments if all collaborator ADCs are commercialized, as well as royalties on net sales of resulting ADC products.”

Under the terms of the ADC collaboration agreement, Agensys has rights to use Seattle Genetics’ ADC technology with monoclonal antibodies against targets selected by Agensys. Agensys is responsible for research, product development, manufacturing and commercialization activities. Seattle Genetics is entitled to progress-dependent fees, milestone payments and mid-single digit royalties on worldwide net sales of ADC products developed and commercialized solely by Agensys. The companies are currently co-developing ASG-5ME, which is in a phase I clinical trial for metastatic pancreatic cancer and a planned phase I trial for advanced prostate cancer. Seattle Genetics also has options under the collaboration to co-develop two additional ADC programs (excluding AGS-16M8F) at the time of Investigational New Drug submission in exchange for 50:50 cost and profit-sharing.

ADCs are monoclonal antibodies that selectively deliver potent anti-cancer agents to tumor cells. With over a decade of experience and knowledge in ADC innovation, Seattle Genetics has developed proprietary technology employing synthetic, highly potent cell-killing agents called auristatins (such as MMAE and MMAF) and stable linker systems that attach auristatin to the antibody. Seattle Genetics’ novel linker systems are designed to be stable in the bloodstream and release the potent cell-killing agent once inside targeted cancer cells. This approach is intended to spare non-targeted cells and thus reduce many of the toxic effects of traditional chemotherapy while enhancing the antitumor activity.

About Seattle Genetics

Seattle Genetics is a clinical-stage biotechnology company focused on the development and commercialization of monoclonal antibody-based therapies for the treatment of cancer and autoimmune disease. The company’s lead product candidate, brentuximab vedotin (SGN-35), is in a pivotal trial under a Special Protocol Assessment with the U.S. Food and Drug Administration. Brentuximab vedotin is being developed in collaboration with Millennium: The Takeda Oncology Company. In addition, Seattle Genetics has five other clinical-stage programs: lintuzumab (SGN-33), dacetuzumab (SGN-40), SGN-70, SGN-75 and ASG-5ME. Seattle Genetics has collaborations for its ADC technology with a number of leading biotechnology and pharmaceutical companies, including Bayer, Celldex Therapeutics, Daiichi Sankyo, Genentech, GlaxoSmithKline, MedImmune, a subsidiary of AstraZeneca, Millennium: The Takeda Oncology Company and Progenics, as well as an ADC co-development agreement with Agensys, an affiliate of Astellas. More information can be found at .

Certain of the statements made in this press release are forward looking, such as those, among others, relating to the therapeutic potential of Seattle Genetics’ ADC technology and the potential for its collaborators to advance their ADC product candidates. 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 the inability to show sufficient safety or activity as our or our collaborators’ ADC product candidates move into and advance in clinical trials. More information about the risks and uncertainties faced by Seattle Genetics is contained in the company’s 10-Q for the quarter ended June 30, 2010 filed 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.