Seattle Genetics Highlights Next Generation Antibody-Drug Conjugate SGN-CD33A At ASH Annual Meeting

Seattle Genetics, Inc. (Nasdaq: SGEN) today presented preclinical data from SGN-CD33A, an antibody-drug conjugate (ADC) in development for the treatment of acute myeloid leukemia (AML), at the 54 ^th American Society of Hematology (ASH) Annual Meeting and Exposition being held December 8-11, 2012 in Atlanta, GA. SGN-CD33A is a novel CD33-directed ADC utilizing Seattle Genetics’ next generation technology. The CD33 antibody is attached to a highly potent cytotoxic agent called a pyrrolobenzodiazepine (PBD) dimer via a proprietary site-specific conjugate technology to a monoclonal antibody with engineered cysteines (EC-mAb). Seattle Genetics expects to advance SGN-CD33A into a phase I clinical trial in 2013.

“Our SGN-CD33A program showcases our next generation ADC technology, including our latest highly potent cell-killing agent and our new engineered antibody technology,” said Jonathan Drachman, M.D., Senior Vice President, Research and Translational Medicine at Seattle Genetics. “Of approximately 30 ADCs in development, more than 50 percent utilize Seattle Genetics’ technology. Through our continued innovation we are leading the development of novel ADCs, and believe that ADCs can transform the way cancer is treated.”

ADCs are monoclonal antibodies that are designed to selectively deliver cytotoxic agents to tumor cells. This approach is intended to spare non-targeted cells and thus reduce many of the toxic effects of traditional chemotherapy while enhancing antitumor activity.

PBDs are a class of DNA-crosslinking agents that are significantly more potent than systemic chemotherapeutic drugs. Seattle Genetics has been working with PBDs since 2008 under an exclusive licensing arrangement with Spirogen Ltd. Over the past four years, Seattle Genetics has selected and optimized specific PBD molecules combined with novel linkers for use in ADCs, and has conducted process development and scale-up activities to create robust synthetic GMP manufacturing processes for these PBD drug-linkers.

SGN-CD33A: A Novel CD33-Directed Antibody-Drug Conjugate, Utilizing Pyrrolobenzodiazepine Dimers, Demonstrates Preclinical Antitumor Activity Against Multi-Drug Resistant Human AML (Abstract #3589)

Key findings from the preclinical evaluation of SGN-CD33A included:

• SGN-CD33A induced CD33-specific activity at low doses in a broad panel of AML cell lines and primary AML patient samples, including those resistant to multiple other anti-leukemic agents.
• SGN-CD33A yielded antitumor activity, durable remissions and improved survival in multiple preclinical AML models.
• Delivery of PBD via the SGN-CD33A ADC may overcome multi-drug resistance in AML patients.
• The data support a planned 2013 phase I clinical trial of SGN-CD33A for AML.

About Seattle Genetics

Seattle Genetics is a biotechnology company focused on the development and commercialization of monoclonal antibody-based therapies for the treatment of cancer. The U.S. Food and Drug Administration granted accelerated approval of ADCETRIS in August 2011 for two indications. ADCETRIS is being developed in collaboration with Millennium: The Takeda Oncology Company. In addition, Seattle Genetics has three other clinical-stage ADC programs: SGN-75, ASG-5ME and ASG-22ME. Seattle Genetics has collaborations for its ADC technology with a number of leading biotechnology and pharmaceutical companies, including Abbott, Agensys (an affiliate of Astellas), Bayer, Celldex Therapeutics, Daiichi Sankyo, Genentech, GlaxoSmithKline, Millennium, Pfizer and Progenics, as well as ADC co-development agreements with Agensys and Genmab. 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 initiation of future clinical trials with SGN-CD33A. 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 generate the appropriate data and information to support an investigational new drug submission to the FDA. More information about the risks and uncertainties faced by Seattle Genetics is contained in the company’s 10-Q for the quarter ended September 30, 2012 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.

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