Seattle Genetics Initiates Phase I Clinical Trial of SGN-35.

BOTHELL, Wash. -- Seattle Genetics, Inc. (Nasdaq: SGEN) announced today that it has initiated a phase I clinical trial of SGN-35 for patients with Hodgkin's disease 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.

"SGN-35 represents an exciting, emerging class of antibody-based therapeutics," commented Clay B. Siegall, Ph.D., President and Chief Executive Officer of Seattle Genetics. "Many antibodies lack sufficient potency on their own to have a therapeutic benefit. ADCs exploit an antibody's ability to target tumor cells to deliver a highly potent drug payload, sparing non-targeted cells many of the toxic effects of traditional chemotherapy. SGN-35 is the first auristatin-based ADC that Seattle Genetics has advanced into clinical trials, marking an important milestone for the company."

The single-agent, dose-escalation phase I study is designed to evaluate the safety, pharmacokinetic profile and antitumor activity of SGN-35 in patients with relapsed or refractory CD30-positive hematologic malignancies, including Hodgkin's disease. The trial is expected to enroll up to approximately 40 patients at multiple centers in the United States.

"SGN-35 has the potential to address a significant unmet clinical need in the relapsed setting of Hodgkin's disease and CD30-positive T-cell lymphomas," said Andres Forero-Torres, M.D., Associate Professor in the Division of Hematology-Oncology at the University of Alabama, Birmingham. "Currently, patients and their doctors have only limited therapeutic options and no approved antibody-based treatments."

About SGN-35

SGN-35 is an ADC comprised of an anti-CD30 antibody joined 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. Treatment with SGN-35 resulted in complete tumor regressions in preclinical models of Hodgkin's disease and anaplastic large cell lymphoma (ALCL).

About CD30-Positive Hematologic Malignancies

Lymphoma is a general term for a group of cancers that originate in the lymphatic system. The lymphomas are divided into two major categories: Hodgkin's disease and all other lymphomas, or non-Hodgkin's lymphomas. Hodgkin's disease 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, an estimated 7,800 cases of Hodgkin's disease will be diagnosed in the United States during 2006, and nearly 1,500 people will die of the disease. The American Cancer Society estimates that nearly 59,000 new cases of non-Hodgkin's lymphoma will be diagnosed in the United States during 2006. An estimated 15 percent of non-Hodgkin's lymphomas are of T-cell origin, many of which express CD30.

About Seattle Genetics

Seattle Genetics is a biotechnology company developing monoclonal antibody-based therapies for the treatment of multiple types of cancer, including non-Hodgkin's lymphoma, multiple myeloma, acute myeloid leukemia and Hodgkin's disease. The company has also 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 currently has license agreements for its ADC technology with a number of leading biotechnology and pharmaceutical companies, including Genentech, Bayer, CuraGen and MedImmune. 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-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.