Seattle Genetics Initiates Phase I Clinical Trial of Antibody-Drug Conjugate SGN-75

Seattle Genetics, Inc. (Nasdaq: SGEN) today announced that it has initiated a phase I clinical trial of SGN-75 for metastatic renal cell carcinoma and relapsed and refractory non-Hodgkin lymphoma. SGN-75 is an antibody-drug conjugate (ADC) targeting CD70 that utilizes the company’s proprietary technology.

“Our compelling preclinical data demonstrate that SGN-75 possesses potent antitumor activity in models of both CD70-positive renal cell carcinoma and hematologic malignancies,” said Clay B. Siegall, Ph.D., President and Chief Executive Officer of Seattle Genetics. “CD70 is expressed on a variety of solid tumors, including renal cell carcinoma, pancreatic, ovarian and lung cancers and glioblastoma as well as multiple myeloma and several types of lymphoma. This broad expression profile provides substantial and diverse therapeutic opportunities to address unmet needs for patients with these malignancies.”

The single-agent phase I study is designed to enroll up to 80 patients at multiple centers in the United States. The trial will evaluate the safety, tolerability, pharmacokinetic profile and antitumor activity of SGN-75 in order to identify a dose and schedule for future clinical trials.

SGN-75 is an ADC comprising an anti-CD70 antibody attached to a potent, synthetic drug payload, monomethyl auristatin F (MMAF), using Seattle Genetics’ proprietary technology. The ADC is designed to be stable in the bloodstream, but to release its payload upon internalization into CD70-expressing tumor cells, resulting in targeted cell-killing.

About Non-Hodgkin Lymphoma

Non-Hodgkin lymphoma represents a diverse group of cancers that develop in the lymphatic system and are characterized by uncontrolled growth and accumulation of abnormal lymphocytes. Lymphocytes are white blood cells that are responsible for defending the body against infection. The most common forms of non-Hodgkin lymphoma are follicular and diffuse large B-cell lymphoma. According to the American Cancer Society, approximately 66,000 cases of non-Hodgkin lymphoma are expected to be diagnosed in the United States during 2009 and approximately 19,500 patients will die from the disease.

About Renal Cell Carcinoma

Renal cell carcinoma (RCC) forms in the kidney, which filters and cleans the blood. Metastatic RCC occurs when the cancer has spread to other parts of the body. RCC is the most common type of kidney cancer in adults, representing approximately 90 percent of cases. The American Cancer Society estimates that there will be more than 57,700 new cases of kidney cancer in the United States during 2009, and about 13,000 people will die from the disease.

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 FDA. Brentuximab vedotin is empowered by Seattle Genetics’ proprietary ADC technology comprising highly potent synthetic drugs and stable linkers for attaching the drugs to monoclonal antibodies. In addition, Seattle Genetics has four other product candidates in ongoing clinical trials: lintuzumab (SGN-33), dacetuzumab (SGN-40), SGN-70 and SGN-75. Dacetuzumab is being developed under a worldwide collaboration with Genentech (a wholly-owned member of the Roche Group). Seattle Genetics has collaborations for its ADC technology with a number of leading biotechnology and pharmaceutical companies, including Genentech, Bayer, CuraGen, a subsidiary of Celldex Therapeutics, Progenics, Daiichi Sankyo, MedImmune, a subsidiary of AstraZeneca, and Millennium: The Takeda Oncology Company, as well as an ADC co-development agreement with Agensys, a subsidiary of Astellas Pharma. 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 therapeutic potential of SGN-75. 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 in this phase I clinical trial and the risk of adverse clinical results as SGN-75 moves into and advances 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 September 30, 2009 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.