Study to Evaluate More Frequent Dosing of SGN-35, as Well as Combination with Gemzar(R)

BOTHELL, WA, USA | March 28, 2008 | Seattle Genetics, Inc. (Nasdaq:SGEN) announced today that it has initiated a second phase I clinical trial of SGN-35 in patients with Hodgkin lymphoma and other CD30-positive hematologic malignancies. SGN-35 is an antibody-drug conjugate (ADC) that utilizes Seattle Genetics’ proprietary technology to empower antibodies by linking them to potent cell-killing drugs.

"We are investing in a robust SGN-35 clinical program in order to fully explore the therapeutic potential of this first-in-class ADC," said Thomas C. Reynolds, M.D., Ph.D., Chief Medical Officer of Seattle Genetics. "We are very encouraged by the multiple objective responses observed at well-tolerated doses in our ongoing phase I trial of SGN-35 utilizing an every three week dosing schedule, and we look forward to reporting additional data at the American Society of Clinical Oncology meeting in early June. In addition, we are initiating this second phase I trial to evaluate the tolerability, antitumor activity and pharmacokinetic profile of more frequent SGN-35 dosing, followed by an exploration of SGN-35 in combination with Gemzar, a chemotherapy agent commonly used in the treatment of Hodgkin lymphoma. During 2008, we are planning to move forward rapidly to define our development pathway for SGN-35, including late-stage clinical trials and registration strategy."

The phase I study will initially evaluate escalating single-agent doses of SGN-35 administered weekly to patients with relapsed or refractory CD30-positive hematologic malignancies, including Hodgkin lymphoma. Following the monotherapy portion of the study, the trial will continue with an investigation of SGN-35 in combination with Gemzar (gemcitabine). The trial is expected to enroll up to approximately 40 patients at multiple centers in the United States.

SGN-35 is an ADC comprising an anti-CD30 antibody attached by an enzyme cleavable linker to a potent, synthetic drug payload, monomethyl auristatin E (MMAE), using Seattle Genetics’ proprietary technology. The ADC is designed to be stable in the bloodstream, but to release MMAE upon internalization into CD30-expressing tumor cells, resulting in a targeted cell-killing effect.

About CD30-Positive Hematologic Malignancies

Lymphoma is a general term for a group of cancers that originate in the lymphatic system. There are two major categories of lymphoma: Hodgkin lymphoma and non-Hodgkin lymphoma. Hodgkin lymphoma is distinguished from other types of lymphoma by the presence of one characteristic type of cell, known as the Reed-Sternberg cell. A defining attribute of the Reed-Sternberg cell is its expression of the CD30 antigen.

According to the American Cancer Society, approximately 8,200 cases of Hodgkin lymphoma will be diagnosed in the United States during 2008, and an estimated 1,300 people will die of the disease. An additional 2,000 to 3,000 patients per year in the United States are diagnosed with anaplastic large cell lymphoma, a T-cell lymphoma that expresses the CD30 antigen. Based on market research, Seattle Genetics believes that there are several thousand newly relapsed or refractory lymphoma patients in the United States each year who would be eligible for treatment with SGN-35, and that the United States prevalence population of these patients is approximately 10,000 to 12,000 individuals. SGN-35 has received orphan drug designation from the FDA for the treatment of Hodgkin lymphoma.

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 has a worldwide collaboration with Genentech to develop and commercialize SGN-40. Seattle Genetics also has two other product candidates in ongoing clinical trials: SGN-33 and SGN-35. 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.

Certain of the statements made in this press release are forward looking, such as those, among others, relating to the therapeutic potential of SGN-35. 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 in this phase I clinical trial and the risk of adverse clinical results as SGN-35 moves into and advances in clinical trials. 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.

SOURCE: Seattle Genetics, Inc.